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Electrical cable selection

(c)2001 Kevin Boone, all rights reserved. No part of this publication may bereproduced in material form (including photocopying or printing), or transmitted in electronic form, without theexpress permission of the copyright holder, except for the purposes of private study, review, and criticism, or in

accordance with the provisions of the Copyright, Designs and Patents Act (1988). So there.

OverviewOverviewOverviewOverview

This article describes the selection of cables for difficult'

domestic electrical installations. By difficult' I meaninstallations where cables are unusually long, currentsabnormally high, or shock protection requirements especially


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rigorous. I have written this article for competent, sensible DIYenthusiasts who may already be doing electrical work, but lackthe theoretical and regulatory knowledge to tackle these moredifficult jobs. I have assumed that the reader has a basicunderstanding of electrical theory and is prepared to do somesimple arithmetic.

Cable selection is guided by two main principles. First, thecable should be able to carry the current load imposed on itwithout overheating. It should be able to do this in the mostextreme conditions of temperature it will encounter during its

working life. Second, it should offer sufficiently sound earthingto (i) limit the voltage to which people are exposed to a safelevel and (ii) allow the fault current to trip the fuse or MCB in


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a short time.

To meet these requirements requires consideration of thecircuit load current, the ambient temperature, installation

technique, cable thickness and length, and the over-currentprotection device. In some cases you may need to considerfactors that are outside your control, like the external earth

loop impedance. Typical worst-case' values for these factorsare given in the article.

Scope of this articleScope of this articleScope of this articleScope of this article

In most domestic wiring scenarios, the principles andtechniques described in this article are simply not relevant. The


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materials and equipment currently available are designed tosimplify installation, and common sense and the ability to readthe manufacturer's instructions are all that is required. Ordinarydomestic power and lighting circuits do not require any specialskills or knowledge to install, beyond what you would find in aDIY handbook. This article covers the issues that DIY bookssteer clear of, like running long cables to outbuildings,

installing supplies for electrical showers, and electrical wiringin bathrooms. It assumes that the reader has sufficient time andenthusiasm to get to grips with the theory, which can be rather

technical in places.This article is intended for readers in the UK, and in placeswhere UK practices and regulations are followed.


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How this article is organizedHow this article is organizedHow this article is organizedHow this article is organizedThis article has three chapters and an appendix. Chapter 1describes the theory of over-current protection, and discussesthe properties of cables, fuses, MCBs and related devices.Chapter 2 describes principles of electrical shock protectionand the effect of cable length and thickness on shock voltage

and disconnection time. Chapter 3 describes a practicalcalculation based on the principles from the first two chapters.Finally, the appendix provides design tables for cable selection,based on theIEE Wiring Regulationsand variousmanufacturer's product data sheets. Please don't use the designtables without reading the text; it will be easy to misinterpret


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the information if you do.

Warning and disclaimerWarning and disclaimerWarning and disclaimerWarning and disclaimer

I would hate to think of anyone coming to harm as a result ofreading this article. It describes procedures which, if notcarried out competently, could lead to death or serious injury,

or substantial damage to property. Please be careful. Alwaysensure that before starting to work on an electrical system, therelevant circuit has been isolated from the supply, and youhave taken steps to ensure that it remains that way until you

have finished work. Ensure that you understand theconsequences and implications of any work you intend to carryout. While I have taken every effort to ensure that the


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information in this article is accurate, and will lead to a safeand reliable installation, I do not accept any responsibility forany adverse consequences arising from its use. Please note thatthe article is aboutmodern domesticinstallations; theprocedures, design tables, and calculations described may wellbe unsuitable for commercial or industrial premises andequipment. In particular, the article assumes the use of a

single-phase supply, and predominantly resistive loads. If youdon't know what these terms mean, I respectfully suggest youought not to be reading this yet. In addition, this article doesnot describe any procedures for dealing circuits protected bysemi-enclosed (re-wirable) fuses. Although they are allowedwith the terms of theIEE Wiring Regulations, they are


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obsolete and ought to be replaced.

Note on the textNote on the textNote on the textNote on the text

The symbol [IEE]' in the text indicates a guideline that shouldbe followed to ensure conformance with theIEE WiringRegulations, 16th edition. Where this symbol is followed by

numbers, e.g., [IEE 528-01], this refers to a specific regulationin that document. Note that theIEE Wiring Regulationsisequivalent in practice to British Standard 7671 Requirementsfor Electrical Installations''. To the best of my knowledge, this

article complies strictly to theIEE Wiring Regulations.

1. Over1. Over1. Over1. Over----current protectioncurrent protectioncurrent protectioncurrent protection


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1.1 Overloads and short1.1 Overloads and short1.1 Overloads and short1.1 Overloads and short----circuitscircuitscircuitscircuitsOver-current is one of the two major safety hazards that mustbe controlled in a wiring system. The other, of course, is

electric shock. Protecting against over-current provides ameasure of protection against electric shock as well, as weshall see. Over-currents are dangerous because they lead to a

risk of fire. In the UK every year about 50,000 fires areattributed to electrical faults. So it's worth paying a bit ofattention to this issue.

This chapter provides a fair amount of technical detail, whichyou won't always need to know. For many applications,provided that you choose a fuse or MCB (see below) that has


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the same current rating as the cable to which it is connected,this will work nicely. For example, if the current rating of thecircuit is 35 amps, say, then a 32 amp MCB (that's the nearestsize available below 35 amps) should do the trick for most

applications.

There will be situations, however, where this simple rule won't

work. This chapter explains what they are, and what to doabout them. It also explains why fuses sometimes blow whenthere's nothing wrong.

1.2 Types of over1.2 Types of over1.2 Types of over1.2 Types of over----currentcurrentcurrentcurrentAn over-current is any increase in the current in the electrical


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system, above the level for which it is designed.Electrical cables are intended to become warm in operation;heat is generated whenever a current flows in anything, and

this is perfectly normal. However, the level of heat generatedby electrical cables is only safe when it is kept withinreasonable limits. Standard PVC-insulated cables are designed

to run at temperatures up to 70 degrees Celsius; beyond thisthere is a risk of damage.

In practice, over-currents can be grouped into two types.

OverloadAn overload occurs when a current flows that is somewhat to high (usually 50% to100% too high) for the system. Overloads don't normally cause immediate, catastrophicdamage. Instead, the likelihood of damage increases gradually as the duration of the


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overload increases. If the fault is not resolved, cables will overheat and melt, exposingbare conductors. The heat generated may be sufficient to cause a fire.

In a domestic setting, overloads usually result from using too many appliances at thesame time, or plugging a heavy-duty appliance into a supply that isn't strong enough forit. An example of the latter is connecting an electric shower to a standard 13-amp plug,

and plugging it into a socket.

Short-circuitA short-circuit is a connection between live and neutral, or between live and earth, thatbypasses an appliance. The connection will probably have a low resistance, and the

current that can flow may be hundreds or thousands of times too high for the system.This current is usually called thefault currentor short-circuit current. A short-circuitwill by produced if, for example, the wires in a mains plug become loose and touch oneanother.

The ability to handle short-circuits is not just important toprotect cables, it is part of the protection against electric shock.If a live conductor in, say, an electric kettle becomes loose and


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touches the metal case, we hope that a large fault current willflow. This current will flow from the live, through the case,and back to earth via the earth wire. The fault current will blowthe fuse or trip the MCB, thus rendering the circuit dead. If this

does not happen, then we have a potentially very dangeroussituation: a metal casing with a live voltage on it.

In practice, in domestic installations overload protection andshort-circuit protection are both provided by the same device:either a fuse of an MCB. Additional shock protection may beprovided by an RCD. Whether a fuse or an MCB is used, when

the current exceeds a certain limit for a certain time, the fusewill blow' (break) or the MCB will trip'. In both cases thiswill open the circuit and prevent the flow of further current.


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For simplicity, I will use the term trip' for both these events.

1.3 Over1.3 Over1.3 Over1.3 Over----current protection devicescurrent protection devicescurrent protection devicescurrent protection devices

1.3.1 Fuses1.3.1 Fuses1.3.1 Fuses1.3.1 Fuses

A fuse is a simple device that will limit the current flowing inan electrical circuit. In practice a fuse normally consists of a

piece of wire of exactly the right length and thickness tooverheat and break when the current gets to a particular level.

There are two sorts of fuse normally used in houses. Cartridgefuseshave the wire enclosed in a sealed cylinder, with acontact at each end. You should be familiar with this kind of


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fuse: it's the kind that goes in a plug. Larger versions areavailable for distribution boards as well.

Semi-enclosedor re-wirablefuses are the kind that can be

rewired with fuse wire. Although they are still widely used,they are discouraged by most authorities for two reasons: acommon-sense reason and a technical reason. The technicalreason will be discussed later. The common-sense reason issimply that it is very easy to rewire it with the wrong size offuse wire, so that we end up with, for example, a 30-amp fuseprotecting' a 5-amp circuit. This is exceptionally dangerous.

Fuses are an effective basic method of over-current protection,but they have a practical disadvantage: if a fuse blows and you


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haven't got one of the right rating, what do you do? Of courseyou won't use a fuse of the wrong rating, or wrap a bit oftinfoil around a blown one, but someone will.

Because of this failing, and for technical reasons that will bediscussed, permanently-installed equipment (particularly mainsdistribution panels) often have electrical over-current protectorsrather than fuses.

1.3.2 MCBs1.3.2 MCBs1.3.2 MCBs1.3.2 MCBs

The most popular of these electrical protection devices is the`miniature circuit breaker' (MCB). An MCB can usually act asan ordinary switch as well as an over-current circuit breaker,


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and so has a lever on the front for manual operation. This isvery convenient, and MCBs are universally used in newdomestic distribution boards (and most industrial ones as well).

MCBs are available in various types1.1: 1', 2', 3', B' and C'.

Each has different characteristics, and is appropriate for aparticular application. In a domestic system, we will normallyuse a type 1' or a type B' device, as these are general-purposeunits.

If you have a distribution board with re-wirable fuses, anddon't want to replace it (yet), you can get adapters that will letyou plug in an MCB in place of the fuse. If you are replacing


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wiring with a system that is rated on the basis that you willeventually be using MCBs and not fuses, this is a very sensiblething to do.

1.4 Fuse and MCB characteristics1.4 Fuse and MCB characteristics1.4 Fuse and MCB characteristics1.4 Fuse and MCB characteristicsFuses and MCBs are rated in amps. The amp rating given onthe fuse or MCB body is the amount of current it will passcontinuously. This is normally called therated currentornominal current. We normally assume that if the current in thecircuit is lower than the nominal current, the device will not

trip, however long the current is maintained. This isn'tquitetrue, but it's a reasonable design assumption.


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Many people think that if the current exceeds the nominalcurrent, the device will trip, instantly. So if the rating is 15amps, for example, a current of 15.00001 amps will trip it,right? This is not true. There isn't any reason why it should be

true: the MCB or fuse is designed to protect the circuit cabling,and a current of 15.00001 amps won't damage a 15-amp cable.So when will it trip?

This is where things start to get interesting. It turns out thatthere's a rather complex relationship between the trippingcurrent and the time for which an over-current is maintained.

As an example, the relationship between time and level ofover-current that will trip either a 32-amp type-1 MCB or a 30-amp cartridge fuse are shown in figure1.1.


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The horizontal axis of this graph shows the current flowing inthe fuse/MCB and the circuit it is protecting. The vertical axisshows the duration for which the device can stand this currentbefore it trips. There are a few things to note about this graph.

z The fuse and the MCB, even though their nominal currents are similar, have very

different properties. For example, to be sure of tripping in 0.1 seconds, the MCBrequires a current of 128 amps, while the fuse requires 300 amps. The fuse clearlyrequires more current to blow it in that time, but notice how much biggerboththesecurrents are than the 30 amps' marked current rating.

z Neither device will trip at 30 amps, in any length of time shown on the graph, but thelines get closer and closer to 30 amps as the time increases. There is a small likelihoodthat in the course of, say, a month, a 30-amp fuse will trip when carrying 30 amps. If thefuse has had a couple of overloads before (which may not even have been noticed) this

Figure 1.1:Figure 1.1:Figure 1.1:Figure 1.1: time for which a 32-amp MCB or 30-amp fuse will standan over-current before tripping


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is much more likely. This explains why fuses can sometimes blow' for no obvious

reason.

z Both fuse and MCB will stand currents of over 40 amps for an hour or so.

If the fuse is marked 30 amps', but it will actually stand 40amps for over an hour, how can we justify calling it a 30 amp'fuse? The answer is that the overload characteristics of fusesare designed to match the properties of modern cables. For

example, a modern PVC-insulated cable will stand a 50%overload for an hour, so it seems reasonable that the fuseshould as well.

In fact it would be very impractical to use a fuse or MCB thattripped at a current very close to the nominal value. This is


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because many electrical devices take higher currents for thefirst fraction of a second after they are switched on, comparedto normal running. Take an ordinary lightbulb, for example.The resistance of all metals increases as they heat up. When the

lightbulb is first switched on, its filament is cold, and it has avery low resistance. As it heats up, the resistance increases, sothe current decreases. For the first tenth of a second or so, the

current flowing in a lightbulb may be 5-10 times higher than its

normal running current1.2.

So we have to allow some margin for start-up currents, or thefuse or MCB will tend to trip by accident, which isinconvenient.


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Because the MCB trips very quickly once a particular thresholdis reached, the concept of an instantaneous trip current' isappropriate for MCBs. This is the current that will trip thedevice in 0.1 seconds. For type 1 MCBs the instantaneous trip

current is guaranteed to be between 2.7 and 4 time the nominalcurrent; for type B it is 3 to 5 times the rating.

1.5 Fuse/MCB selection1.5 Fuse/MCB selection1.5 Fuse/MCB selection1.5 Fuse/MCB selectionWhen selecting the correct MCB or fuse to use, we have toconsider its role in both over-current protection, and short-

circuit protection. The basic principles are as follows.

Nominal current ruleNominal current ruleNominal current ruleNominal current ruleThe nominal current of the fuse/MCB must be less than the current rating of the cable it


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is protecting, but higher than the current that it will carry continuously [433-02-01]. For

example, a 32-amp MCB is suitable for a current of 30 amps in a 35-amp cable circuit.

Tripping ruleTripping ruleTripping ruleTripping ruleA current of 1.45 times the nominal current must cause the device to trip in less than 1

hour1.3. In practice you haven't normally got to worry about this, it's the job of the MCBdesigner. All modern devices meet this requirementexcept re-wirable fuses. This is whyre-wirable fuses are discouraged. These fuses normally require about twice the nominalcurrent to blow them in one hour.

Disconnection time ruleDisconnection time ruleDisconnection time ruleDisconnection time rule

In a short-circuit condition, the fuse/MCB must trip in less than a specified short time(see below).

The disconnection time' rule is the most awkward to ensure

compliance with in a domestic installation; it will be discussedlater. In practice it doesn't affect what ratingof fuse or MCB touse, but it often affects whether to use a fuse or an MCB, and


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may impose the use of additional protective devices.

1.6 Example1.6 Example1.6 Example1.6 Example

In this example we will determine which MCB to use toprotect a circuit.

Assume we are installing a lighting circuit, which will

nominally have 8 light fittings of 100 watts each. The currentis 800 / 230 or about 3.5 amps. 1 mm cable appears to beappropriate, as it has a rating of at least 11 amps (see

tableA.1), even when concealed in a plastered wall1.4.


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So the MCB must have a nominal current (that is, the currentmarked on its body) of at least 3.5 amps, and less than 11amps. Furthermore, its tripping current must be less than 1.45times 11 amps, in order to protect the cable. Looking in the

manufacturer's catalogue, I find a 6 amp MCB, that has a tripcurrent of 8.7 amps. This appears to be just right. Thesecurrents are shown in figure1.2.


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Figure 1.2:Figure 1.2:Figure 1.2:Figure 1.2: Relationship between the fuse nominal and tripping currents, and the currentcarrying capacity of the cable, for the example given in the text


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Note that in this example, the MCB trip current is not onlybelow the short-term current capacity of the cable (which itmust be), but it is even below the nominal current rating of thecable. This means that the MCB will prevent the cable

reaching even it normal working current. Of course that's finein this case, because we know exactly what the load will be:eight 100 watt bulbs. The tripping current does not have to be

below the nominal current of the cable, but itdoeshave to bebelow 1.45 times the nominal current of the cable.

Note that we need also to check the disconnection time in the

event of a live-earth fault, but doing so requires moreinformation than has been supplied; see below.


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1.7 Common examples of bad design1.7 Common examples of bad design1.7 Common examples of bad design1.7 Common examples of bad designSome wiring configurations can lead to overload currents thatcannot be detected.


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Figure 1.3:Figure 1.3:Figure 1.3:Figure 1.3: Extending a spur with a second spur: not a good idea


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Consider, for example, the system shown in figure1.3.Suppose we have a standard ring circuit wired using 2.5 mmcable and protected by a 30 amp MCB. This is a perfectlystandard, reasonable configuration. Now suppose we extend

this circuit with a spur ( spur 1' on the diagram). What is themaximum current that can flow in the spur cable? Assume thatwe have used standard single socket outlets. We can plug an

appliance rated up to 13 amps into the outlet, so in the worstcase the current load could be 13 amps in the spur cable. Thisis well within the current carrying capacity of the spur cable, sono problem.

Now suppose we extend the system further by connecting asecond spur ( spur 2') to the first spur. Now the maximum


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current that could be carried by spur 2 is 13 amps, and themaximum current that could be carried by spur 1 istwice13amps: 13 amps from each appliance. That's 26 amps. The cableusedmightcarry 26 amps, in ideal circumstances, but we

shouldn't rely on it. TheIEE Wiring Regulationsthe capacity ofthis cable as between 18.5 amps - if it is concealed in a wall -

to 30 amps if it is exposed to air all around1.5.

If we plug several heavy duty appliances into the outlets in themain ring, we may cause an overload that will trip the MCB.

But this isn't the problem: the MCB will always trip at a lowercurrent than will damage the cable. In the spur, the currentcarried could be too high for the cable, but too low to trip the


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MCB. This is a potentially dangerous situation.

It's worth noting that we could, in principle, avert thisdangerous situation by using a heavier cable for spur 1. 4 mm

would probably do the job. However, spur 1 was probably putin before the need for another spur was recognized, and itwould probably have been constructed from the same sizecable as the ring.

The simple, robust solution to this problem is to connect spur 1to the ring using a fuse unit, as shown in figure1.4.


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Figure 1.4:Figure 1.4:Figure 1.4:Figure 1.4: extending a ring with a fused spur


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The spur is connected to the ring using a fuse or MCB. Therating of the fuse or MCB should be no higher than 13 amps,which means that the total current taken by the spur can neverbe higher than the current that would be taken by a single

outlet attached to the ring by an unfused spur. Since we havealready established that this current is insufficient to damagethe cable, the fused spur allows the ring to be extended in a

safe way. Note also that we can wire the fused spur in lightercable (1.5 mm should do) because the fuse limits the nominalcurrent to 13 amps.

The circuit shown in figure1.3only becomes at risk of dangerfrom overload if two high-current appliances are plugged intothe spur. You could argue that this won't ever happen: this spur


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will only ever be used to supply a pair of desk lamps, forexample. The problem with this logic is thatyoumay knowthis, but not everyone else does. What's to stop someone (avisitor to your house, for example) from plugging an electric

fire into each one? The example described above crops up intwo common situations.

The first is that of powering a large, complex hi-fi/TV system.Suppose we have ten electronic appliances of this sort, allstacked one on top of the other in a small area. It might beconvenient to fit a row of, say, six double socket outlets just

behind the system. We might do this by running a spur fromthe main power ring. One could reasonably argue that theseappliances actually have a small power consumption. A 100-


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watt amplifier is staggeringly loud in a domestic lounge, but ittakes less current on average than a lightbulb. The complete hi-fi/TV system probably takes less than 5 amps. In thecircumstances one might be tempted to use an unfused spur.

The problem, as before, is that althoughyouonly plug hi-fiequipment in here, who's to say what the next owners of yourhouse will do?

A related problem is this one: I am commonly asked whether itis safe to fit an extra mains outlet in a room, by connecting it toan existing one on the other side of the wall. This is a very

handy way to add an extra socket, if there's an existing one inposition. In a modern house, wired to comply with theIEEWiring Regulations, then this almost certainly is safe. Such


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houses are wired to a standard scheme, where all the poweroutlets on each floor (perhaps excluding the kitchen) are wiredinto a simple ring system. If you tap one of the socket outletsthen you will be effectively installing an unfused spur. As there

will only be one new socket on the spur, this is fine.

However, if there's any possibility that the socket you want totap is itself a spur, then you should probably not do it. If the

existing socket has only one cable entering it, this is probablywhat it is. Even if the socket has two cables, you can't becertainit's part of a ring. It might be part of radial system, or

even a dodgy unfused spur fitted by someone else. The olderyour house is, and the more haphazard the wiring is, the morelikely this is to be the case. There is no straightforward


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solution to the problem: you need to determine how the varioussockets are connected to one another before you can decidewhether the extra socket is safe, or not.

1.8 Disconnection times1.8 Disconnection times1.8 Disconnection times1.8 Disconnection timesWhen an overload occurs, the protective device (fuse or MCB)must cut off the supply within a time short enough to prevent

damage to the installation. This time may, in somecircumstances, be as much as an hour. However, when a short-circuit fault occurs, it may be because a live part has come into

contact with a metal casing. In this case lives are at risks unlessthe supply is cut off very quickly.


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Short-circuits may be between live and earth, or live andneutral. In the case of live-neutral shorts, the current flowcould be enormous (thousands of amps). It is limited only bythe resistance of the cable between your house and the supply

system, which is usually less than an ohm. In this kind of fault,the MCB or fuse will trip in its shortest possible time: usuallyabout 0.1 seconds for a fuse and 0.01 seconds for an MCB. In

domestic installations we usually don't have to worry aboutthis, because if we can meet the requirements for disconnectiontime in the case of a live-earth fault, we will also meet therequirements for a live-neutral fault.

The notion of disconnection time in the event of a live-earthfault will be discussed in much more detail on page , when


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we consider the effect of disconnection time on electric shockprotection.

2. Shock protection2. Shock protection2. Shock protection2. Shock protection

This chapter describes the measures that are taken to protectagainst electric shock. For most simple jobs, ensuring that theearth conductors are properly connected is all that is necessary

to ensure adequate shock protection. The more detailedinformation in this chapter is intended for use in moresubstantial work, like supplying power to a garden or

outbuilding, or adding a new power circuit.

2.1 The nature and risk of shock2.1 The nature and risk of shock2.1 The nature and risk of shock2.1 The nature and risk of shock


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Most electric shocks that are received in the home aredangerous because of the effects they have on the heart orrespiratory system. Relatively small currents may be sufficientto kill or injure. Larger currents can also cause burns and tissue

damage. Shocks occur when electric currents flow through thebody between points at different voltage. For example, if youtouch a live electrical terminal in a lampholder (230 volts)

while standing on the ground (assumed 0 volts), a voltage of230 volts is developed between your hands and feet. Thisvoltage causes a current to flow through the body, includingthe heart and lungs. The current causes stimulation of the

muscle mass of the heart, and of the nervous systemcontrolling the lungs. Clearly this is a bad thing.


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in summary, the risk and severity of injury depends on twofactors:

z the duration of the shock, andz the amount of current that flows in the body tissue.

Increasing either of these risk factors increases the risk ofinjury or death. Later in this chapter we will discuss howknowledge of the shock risk factors is used in specifying the

performance of the shock protection system.

2.2 Types of shock2.2 Types of shock2.2 Types of shock2.2 Types of shock

Electric shocks are of two types: direct contact, and indirectcontact.


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2.2.1 Direct contact2.2.1 Direct contact2.2.1 Direct contact2.2.1 Direct contact

Direct contact' occurs when a body part touches a live partdirectly. This type of shock is particularly dangerous, as the

full voltage of the supply can be developed across the body. Ina well-designed electrical installation there should be little orno risk of direct contact; in most cases it arises out ofcarelessness (e.g. changing a lightbulb with the outlet switched

on). However, it can sometimes arise from wear and tear, suchas the breakdown of insulation on a flexible cable that is badlystressed. RCDs (see below) provide some measure of

protection against direct contact.

2.2.2 Indirect contact2.2.2 Indirect contact2.2.2 Indirect contact2.2.2 Indirect contact


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Indirect contact occurs when a live part touches a piece ofmetal, and the body comes into contact with the live metal.Indirect contact can occur as a result of faults in electricalappliances, particular with metal casings.

Your main protection against indirect contact is earthing,combined with an overcurrent cut-out device. This worksbecause the large current that will flow to earth in the event of

a fault should activate the overcurrent device.

A fault in an appliance where a live part comes into contactwith an earthed enclosure is called a live-earth or live-to-earthfault.


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2.3 Earthing and bonding2.3 Earthing and bonding2.3 Earthing and bonding2.3 Earthing and bondingThese terms are often confused, and it is important tounderstand the distinction.

Earth' is that big lump of rock and mud we all live on. Mostelectrical power systems currently in use are connected,somewhere, to earth. This helps to keep the voltage at a well-

defined level. For convenience we treat the voltage of the earthas zero', and everything else is measured with respect to it.When we talk about a 230-volt supply, we mean 230 volts

higher than earth. Earthing' is the process of connecting partsof the electrical system to earth.


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Bonding', on the other hand is the connecting of metalequipment together. It doesnotnecessarily mean connecting toearth, although in practice there usually is an earth connectionsomewhere.

The main purpose of bonding is to keep all metalwork that cancome into contact with an electrical potential at the samevoltage. If two pieces of metal are at the same voltage, then it

is impossible to get a shock by touching them simultaneously.No current will flow. Earthing ensures that if a fault doesoccur, the current that flows to earth is sufficient to activate an

overcurrent device and shut off the supply.

So earthing and bonding have complementary functions, and in


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domestic practice are always used together2.1.

2.4 Types of primary earth connection2.4 Types of primary earth connection2.4 Types of primary earth connection2.4 Types of primary earth connection

Your house will (or at least should) contain, in or near themain distribution board, a primary earth terminal. This is themain point to which all circuit earths will be run back. Of

course, there may beotherpaths to earth for current elsewherein the premises. It stands to reason that the main earth terminalshould provide averylow resistance path to true earth.

There are three main ways that this earth terminal may beconnected to a true earth. These are identified by the


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abbreviations2.2shown in table2.1.

Table 2.1:Table 2.1:Table 2.1:Table 2.1: typical methods of provision of the main earthing

terminal

Supply type code Meaning

TN-SSupplier provides a separate earthconnection, usually direct from thedistribution station and via the metalsheath of the supply cable

TN-C-S

Supplier provides a combined

earth/neutral connection; your main earthterminal is connected to their neutral


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Most houses have TN-S' supplies at present, but the TN-C-S'method is increasing in popularity because of its lower cost andsuperior earth contact resistance. TN-C-S is also called

protective multiple earthing', or PME, because of theadditional earthing required inside the house. If your powersupply is from an overhead cable, then you may well have aTT arrangement. The earth spike will usually be located in asmall pit, with a cable run into the house. TT earthing presentssignificant challenges for the electrical installer, as its earth

TTSupplier provides no earth; you have anearth spike near your premises


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resistance is uncomfortably high (see below), and may varywith the weather. When it is very wet, the contact with earthwill be better than when it is dry.

2.5 The earth loop2.5 The earth loop2.5 The earth loop2.5 The earth loopTo understand how earthing requirements are to be met, it isimportant to understand where current flows in the event of a

live-earth fault (figure2.1).


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54/142

If a live part in the appliance comes into contact with thecasing, a current will flow from the live of the supplycompany's apparatus to the premises, along the live conductorto the appliance, through the fault to the casing, from the

casing to the earth terminal via the earth conductor, and fromthe earth terminal back to the supply company's apparatus viaits earth connection.

This circuit is called the earth loop'. Note that part of the earthloop is outside your premises, and in the supply company'scables and apparatus. You have no control over that part of the

loop. The part of the loop inside your house has a resistancewhich can be calculated, because we know the resistances permetre of the various cables that are likely to be used.


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If an earth fault occurs (that is, a short-circuit between live andearth), the path for current includes that supply company's liveconductor into your premises, the live part of your cabling, theearth part of your cabling, and the earth part of the supplier's

system. You can calculate the resistance of your part thissystem, or measure it, but you may need to approach thesupply company for the resistance of their part. Suppliers arelegally obliged to tell you this; it is, after all, very important forensuring safety.

When a short-circuit from live to earth occurs, the earth loop is

the path that the current will flow in. This current could bevery large; it should certainly be large enough to blow the fuseor trip the MCB before serious injury occurs. This suggests


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that the earth loop resistance should be as low as possible.

For initial and approximate design calculations, you can usethe worst case' values of earth loop resistance given in

tableA.11. If your installation appears safe with these worstcase figures, it will almost certainly prove to be safe with thetrue figures. However, the earth loop resistance figures dependon your knowing the supply type of your premises. If you don't

know this, you will need to ask the supply company anyway.

2.6 Main and supplementary bonding2.6 Main and supplementary bonding2.6 Main and supplementary bonding2.6 Main and supplementary bonding

Normally all electrical appliances with metal cases will beearthed. The case will be connected to the main earth terminal


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via the appliance cable and the power circuit itself. If a faultoccurs in the appliance, and a live part touches the case, theearthing prevents injury to the user.

However, what happens if a fault occurs in, say, a mains cable,and a live conductor comes into contact with a central heatingpipe? In theory, this could result in the whole system becominglive, as the metal pipework will carry the live potential through

the house. We avoid this problem by ensuring that thepipework is sufficiently well earthed to prevent a potentialbeing developed. The same considerations apply to water

mains, gas mains, structural metalwork, and metal ventilationducts. These items should all be earthed.


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Normally gas, water and ventilation systems will be earthed at

at least one point by a connection direct to the main earthingterminal. This is calledmain bonding. In a house, you willnormally see a heavy earth cable running from the earth

terminal to clamps on the incoming service pipes. If you have alightning conductor, this must be earthed as well.

You could argue that the service pipes are earthed

automatically, by virtue of being buried in the ground. Theproblem with this argument is that the earthing is uncertain. Insome district, plastic water pipes are used for the mains, and

these provide no significant earth contact at all. In others, theremay be a contact of uncertain resistance. Main bondingeliminates this uncertainty.


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We don't have to earth every piece of metal in the house, but

everything that may be in contact with earth should be well-bonded. We need to avoid the situation where metalwork isable to carry live voltage between rooms, but is not sufficiently

well earthed to protect the occupiers. The basic principle isthis: Any earth should be a good earth.

We don't need to earth doorhandles, or filing cabinets, or

window frames, as these don't present a significant risk ofcarrying a potential. However, we do have to earth pipeworkand structural metal. By structural' is meant central support

beams and joists, not metal window frames. In practice, mosthouses do not have a steel frame, and it won't be necessary toearth the structure.


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In some places, main bonding is not considered to be

sufficient, and we need to employsupplementary bondingaswell. This is the connection of metalwork together in a smallarea, to prevent voltages being developed between different

parts of a room. In a house, this is only likely to be necessaryin a bath or shower room.

Where supplementary bonding is used, it should be used

thoroughly. In particular, you need to take the trouble to bondallmetal parts that have a connection outside the room. In abathroom, this includes pipework, tubs and sinks, taps,

radiators, etc. In older houses it may also include waste pipes,which may be of cast iron. The bonding connections need to be


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made with a stout earth cable, typically2.34 mm , connected toclamps with integral warning plates.

Note that the supplementary bonding doesnothave to be runback to the main earth terminal. A connection to, say, the earthterminal of a lighting outlet would be fine. It is thebondingthat is important, rather than theearthing.

The benefits of supplementary bonding have always beencontested; many authorities believe that in some circumstancesit reduces electrical safety rather than improving it. The wholediscussion has recently been re-opened, with the widespreaduse of plastic plumbing. Clearly, plastic pipes don't conduct

l t i it d di t t db l ti i i t bl


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electricity, and a radiator connected by plastic pipes is not able

to carry a current out of the room. This issue is discussed inmore detail later in this chapter.

2.7 Shock voltage: the limitation of earthing2.7 Shock voltage: the limitation of earthing2.7 Shock voltage: the limitation of earthing2.7 Shock voltage: the limitation of earthingIt might be thought that if a metal appliance is soundly earthed,then it is impossible for a person touching it to receive a shock,

regardless of the nature of the fault. This is an untrue anddangerous misapprehension. Consider the following example.

A live-earth fault occurs in an electrical appliance. Theappliance is connected by its own dedicated cable to the maindistribution board. The cable is 2.5 mm two-core-and-earth,

10 t l Thi t f bl h 15 th


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10 metres long. This type of cable has a 1.5 mm earth

conductor, which has a resistance of 0.015 ohms per metre at

70 degrees Celsius2.4. The live conductor has a resistance of

0.009 ohms per metre. So the total resistance of the earthconductor is 10 x 0.015 ohms, or 0.15 ohms. The totalresistance of the live conductor is 10 x 0.009, or 0.09 ohms.Assume that the resistance of the supply company's part of thisloop has a resistance of 0.3 ohms. (see tableA.11for typicalfigures for various types of earthing).

So the total resistance in the current path (the earth loop) is 0.3+0.09 +0.15 ohms, or 0.54 ohms. With a supply voltage of230 volts, the fault current will be (230 / 0.54) amps, or about

426amps Thiscurrent flo ingintheearthconductor(15


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426 amps. This current flowing in the earth conductor (1.5

ohms) will develop a voltage of (426 x 0.15) volts, or about 64volts.

This 64 volts is called the prospective shock voltage' orprospective touch voltage', because this is the voltage at thecasing of the appliance when the fault occurs, that is, thevoltage that a person will experience if he or she touches the

appliance in the fault condition. The term prospective' is usedbecause, in practice, the use of an overcurrent device (e.g.,MCB) or an earth fault device (e.g., RCD) may stop this shock

voltage being reached. For example, if an MCB will alwaysinterrupt the supply when the current reaches 20 amps, the trueshock voltage will be resistance of the earth conductor

multipliedby20amps(asdiscussedlater)


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multiplied by 20 amps (as discussed later).

In summary, a fault in this appliance could cause the casing todevelop a voltage of 64 volts, which is potentially dangerous.Note that the only practical ways to reduce the shock voltageare (i) to shorten the cable, (ii) to reduce the earth resistance, or(iii) to shut off the supply before the fault current reaches thecalculated figure.

There are two ways to ensure that the supply is disconnectedautomatically. The traditional' approach is to ensure that thecurrent that flows to earth is enough to trip the overcurrentdevice for the circuit. The modern approach is to use an RCD.

Withoutautomaticsupplydisconnection theshockvoltagewill


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Without automatic supply disconnection, the shock voltage will

be given by multiplying the fault current by the earth conductorresistance, as calculated above. In that example, the shockvoltage would have been 64 volts. However, suppose the cable

had been protected by a 20 amp type-1 MCB. Theinstantaneous trip voltage' of the MCB will be approximatelyfour times the rating, or 80 amps. This means that as soon asthe current rises to 80 amps, the supply will be cut off. With 80amps flowing, the shock voltage is 80 multiplied by the earthconductor resistance (0.15 ohms), which is 12 volts. So theoverall shock voltage is set by the trip current of the

overcurrent device, and the resistance of the cable.

Of course, this approach will only work if the overcurrent

deviceactuallycutsoff thecurrent Inthecasedescribed the


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device actually cuts off the current. In the case described, the

fault current of 426 amps will trip the MCB very quicklyindeed, probably in less than a hundredth of a second.However, if the fault current is low, it may not trip the device

at all, or may not do so quickly enough to prevent danger. Sowe need to consider the notion of disconnection time' in anearth fault.

2.8 Disconnection time and shock voltage2.8 Disconnection time and shock voltage2.8 Disconnection time and shock voltage2.8 Disconnection time and shock voltage

We have already seen that the use of earthing alone will not

necessarily protect against electric shock, because in a fault thecurrent flow may be so large that it still develops a dangerousvoltage between earth and the fault. To prevent injury, a

protectivesystemmustbeavailabletoshutoff thesupply in


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protective system must be available to shut off the supply in

the event of a fault. Obviously, a quick disconnection is to bepreferred.

It is difficult to specify the performance of an electric shockprotection system in terms of tissue current, since this is notreadily measurable. It is relatively straightforward to specify interms of shock voltage', which is the voltage at the point

where the live part touches the body, as described above.Because the shock voltage and the tissue current are notdirectly related, large safety margins are specified.

The disconnection time given by regulations [IEE 413-02-09]will depend on the level of risk to which a user of the circuit is

exposed; for thisdiscussionI will dividecircuitsinto high'


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exposed; for this discussion I will divide circuits into high,

medium' and low' risks categories. Note that low risk' heremeanscomparativelylow risk; no mains electrical system canreally be classified as low-risk.

high' risk circuitshigh' risk circuitshigh' risk circuitshigh' risk circuitsThese are circuits for which the shortest disconnection time must always be maintained:0.4 seconds. According to theIEE Wiring Regulations, this category consists of fixedbathroom equipment (shower pumps, electric heaters), outdoor and garden mains socketoutlets, and other places where the users are likely to be exposed to wet conditions. It isassumed that exposure even to the full supply voltage is not likely to be dangerous if itis restricted to 0.4 seconds. Other high-risk areas are recognized by the Regulations, butthese are unlikely to be found in the home (the possible exception is a swimming pool,if you're fortunate enough to have one).

It is commonly thought that domestic kitchens are a high risk' area, and are thereforeregulated in the same way as bathrooms; this is, in fact, not true (at least according totheIEE Wiring Regulations). If you wish to protect your kitchen more strictly than other

parts of the house, this is sensible, but not a regulatory issue.


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p , , g y

medium' risk circuitsmedium' risk circuitsmedium' risk circuitsmedium' risk circuitsThese are all circuits supplying socket outlets and hand-held appliances. These circuitsalso require disconnection times less than 0.4 seconds, unless the maximum shockvoltage is less than 50V. It is assumed that an exposure to 50V is unlikely to bedangerous, even for an extended time. In fact, a person with clean, dry skin standing ona dry floor can tolerate much higher voltages that this. The 50V limit includes a safetymargin to account for damp skin. These circuits must disconnect within 5 seconds; thistime is really rather arbitrary, but there has to be some sort of limit. In practice, if theshock voltage is less than 50 V, the disconnection time will be adequate anyway, as willbe discussed.

low' risk circuitslow' risk circuitslow' risk circuitslow' risk circuitsThis group includes everything else: lighting circuits, cabling between distributionboards, and fixed appliance circuits (except bathroom). These must disconnect in 5seconds.

2.9 Checking disconnection time2.9 Checking disconnection time2.9 Checking disconnection time2.9 Checking disconnection time

Thissectiondescribeshowtocalculatedisconnectiontimes


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This section describes how to calculate disconnection times,

and the circumstances in which you need to do this.

2.10 When do we need to be concerned about2.10 When do we need to be concerned about2.10 When do we need to be concerned about2.10 When do we need to be concerned about

disconnection times?disconnection times?disconnection times?disconnection times?Even in substantial projects you may not need to be concernedabout disconnection times. The following points should beconsidered.

z You only need to be concerned if you are making an addition, or a substantial extension,to a circuit. Adding a new socket to a power ring, for example, is unlikely to lead to a

problem.

z If your house has a TT' earth supply, that is, the earth is provided locally, the earth loopresistance will almost certainly be too large to meet any disconnection time regulation.

For example, suppose the external earth loop resistance is 20 ohms. The maximum


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current that can flow to earth in your house is 230/20 amps, or 11.5 amps. This is notlarge enough to trip even a 5-amp MCB in a reasonable time. Thus an earth fault couldremain in place for hours. Houses with a TT supply need RCD protection on all circuits,or an overall RCD for the main distribution board. With this in place, the issue ofdisconnection time becomes moot: you won't have to calculate it.

z Using an RCD is a sensible option for other installations, even where the disconnectiontimes can be met by earthing. The RCD provides protection against a greater range offaults that earthing alone. In addition, it makes it unnecessary to calculate thedisconnection time.

z If the external earth loop resistance is 0.8 ohms (a conservative estimate), the maximum

acceptable length for a standard' lighting (1.5 mm radial) circuit, when protected by a6-amp, type-1 MCB is 303 metres. In order words, disconnection time is very unlikelyto be an issue in domestic lighting circuits.

z The maximum acceptable length for a standard' power (2.5 mm ring) circuit, whenprotected by a 30-amp type-1 MCBs is 56 metres. This is based on consideration ofshock voltage, and does not depend on earth loop resistance. If you don't know the

external earth loop resistance, you should take 56 metres as your maximum length. Note


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that this length is for thewhole ring, from the distribution board to the furthest point andback. If you plan to run the ring at full capacity, voltage drop requirements limit thelength to 36 metres anyway.

Bear in mind that in some cases you will need to know the

external earth loop resistance to do these checks.

2.10.1 Disconnection time in high risk circuits2.10.1 Disconnection time in high risk circuits2.10.1 Disconnection time in high risk circuits2.10.1 Disconnection time in high risk circuits

In these circuits, disconnection must occur in 0.4 seconds,whatever the shock voltage. Therefore we don't need tocalculate the shock voltage. The check procedure is a follows.

1. Calculate the total loop resistance of your cable (live and earth conductors in series), bymultiplying its length by the appropriate figure from tableA.5, column 3. This gives thetotal resistance of your part of the circuit.

2. Add this to the external earth loop resistance obtained from the supply company to give


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the total earth loop resistance. This figure gives the total resistance of the path thatcurrent will flow around if there is a live-earth fault.

3. Calculate the maximum fault current by dividing the mains voltage (230 V) by the loopresistance.

4. Check whether this fault current is appropriate for a 0.4 second disconnection, with theprotection device you have selected. Maximum acceptable values of earth loopresistance are given in tableA.8.

5. If the earth loop resistance is too high to allow disconnection, fit an RCD, reduce the

current limit of the protective device, or improve the earth path using supplementaryearth bonding.

Alternatively, use the figures in tableA.12, columns 4 or 5;

this gives maximum lengths that will allow a disconnection in0.4 seconds for a range of popular cable/MCB/fuse

combinations.


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2.10.2 Disconnection time in medium risk circuits2.10.2 Disconnection time in medium risk circuits2.10.2 Disconnection time in medium risk circuits2.10.2 Disconnection time in medium risk circuits

In these circuits, disconnection must occur in 0.4 seconds,

unless the shock voltage is less than 50 volts. In the latter case,we are allowed a disconnection time of 5 seconds. So weshould first calculate the shock voltage, and then check the

disconnection time according to whether the shock voltage isless than 50 volts or not.

The check procedure is as follows.

1. Calculate the earth conductor conductor resistance by multiplying the appropriate figurefor your selected cable size from tableA.5, column 4, by the length of the cable fromthe supply earth terminal to the furthest point.

2. Determine the maximum current that will trip the selected overcurrent device in 5


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seconds (tableA.7).

3. Work out the shock voltage by multiplying the current by the cable resistance. If it isless than 50 volts, then the circuit is OK; no more checks are required.

4. If the shock voltage is greater than 50 V, test for a 0.4 second disconnection time usingthe procedure described for high risk' circuits described above. Alternatively, use asupplementary earthing connection at the far end of the circuit to reduce the earthresistance. If you use metal pipework for the supplementary earth, you might need tomeasure the resulting resistance using a meter rather than calculating it.

Alternatively, use the figures in tableA.12, column 3 todetermine what length of cable will give an acceptable shockvoltage.

2.10.3 Disconnection time in low risk circuits2.10.3 Disconnection time in low risk circuits2.10.3 Disconnection time in low risk circuits2.10.3 Disconnection time in low risk circuits

In these circuits, a disconnection time of 5 seconds is allowed.


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The check procedure is as follows.

1. Calculate the earth loop resistance of your cable, by multiplying its length by theappropriate figure from tableA.5, column 3.

2. Add this to the external earth loop resistance obtained from the supply company to givethe total earth loop resistance. This figure gives the total resistance of the path thatcurrent will flow around if there is a live-earth fault.

3. Check whether this figure is appropriate for a 5 second disconnection, with the

protection device you have selected. Maximum values of earth loop resistance are givenin tableA.9.

4. If the earth loop resistance is too high to allow disconnection, fit an RCD or improve theearth path using supplementary earth bonding.

Alternatively, use the figures in tableA.12, columns 6 or 8;

this gives maximum lengths that will allow a disconnection in


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5 seconds for a range of popular cable/MCB/fusecombinations.

The process of checking disconnection times is complicated bythe fact that the external earth loop resistance cannot easily bemeasured or calculated: it must be obtained from the supplycompany.

2.10.4 General guidelines on disconnection time2.10.4 General guidelines on disconnection time2.10.4 General guidelines on disconnection time2.10.4 General guidelines on disconnection time

z A type-1 MCB will trip at a slightly lower current than a type-B, with the same nominalrating. This means that it will make it slightly easier to meet the disconnection time

requirements and the shock voltage requirements with a type-1 device.

z Except for very low current circuits (less than 6 amps), an MCB will offer better

disconnection time performance than a fuse, and make it easier to select and installcable For currentsover 40amps afusewill notoffer 04seconddisconnectioninany


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cable. For currents over 40 amps, a fuse will not offer 0.4 second disconnection in anycircumstances.

z A thicker cable can be longer than a thin one, to achieve the same disconnection times.

z A ring circuit will allow adequate disconnection for a longer cable than a radial circuit,because the overall resistance is lower.

z If a cable is carrying a much lower load than its rating, consider using a lower-ratedMCB to improve shock protection (if you aren't using an RCD). For example, suppose

you are running a 3-amp freezer on a dedicated 2.5 cable. Although this cablemay allow a maximum current of 20 amps, fitting a 6-amp MCB will allow the cablerun to be about twice as long while still maintaining the same shock voltage anddisconnection times.

2.11 RCDs2.11 RCDs2.11 RCDs2.11 RCDs

We have seen that we can't always rely on the use of earthing


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to disconnect the mains supply in a fault. Even if it doesdisconnect, it may not do so quickly enough, or keep thevoltage at a safe level. Increasing awareness of these problems

has led to the widespread uptake of other devices that candetect and isolate earth faults. The most popular at present istheresidual current device.

The residual current device measures the difference in currentbetween the live and neutral conductors of a system. In allnormal circumstances these should be equal. If the live current

and the neutral current are different, this indicates that somecurrent is flowing somewhere other than the live and neutral.There are only a few places that the extraneous current can

flow, all of them bad. In many cases the difference will


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indicate a current flow to earth, via a fault.

RCDs are available with different sensitivities. That is, theytrip at different levels of current leakage. In the UK, the mostwidely used are 30 mA (milliamp) and 100 mA devices. Acurrent flow of 30 mA (or 0.03 amps) is sufficiently small thatit makes it very difficult to receive a dangerous shock. Even

100 mA is a relatively small figure when compared to thecurrent that may flow in an earth fault without such protection(hundred of amps).

An RCD does not necessarily require an earth connection itself(it monitors only the live and neutral). In addition it detects

current flows to earth even in equipment without an earth of its


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own. This means that an RCD will continue to give shockprotection in equipment that has a faulty earth. It is theseproperties that have made the RCD more popular than its

rivals. For example, earth-leakage circuit breakers (ELCBs)were widely used about ten years ago. These devices measuredthe voltage on the earth conductor; if this voltage was not zerothis indicated a current leakage to earth. The problem is that

ELCBs need a sound earth connection, as does the equipmentit protects. As a result, the use of ELCBs is no longerrecommended.

RCDs are now available in all sorts of guises. For example, anRCD plug replaces a standard three-pin plug, but has a built in

RCD. For permanent installations, you can get RCDs built into


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socket outlets (figure2.2). Of course, RCDs are available to fitstandard distribution boards alongside MCBs.

2.11.1 Where to use RCDs2.11.1 Where to use RCDs2.11.1 Where to use RCDs2.11.1 Where to use RCDs

Figure 2.2:Figure 2.2:Figure 2.2:Figure 2.2: a socket with integral RCD protection.

Houses built in the last few years probably have an overall


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RCD for the main supply. This means that no further RCDprotection is necessary, or helpful, from a safety perspective.Additional RCDs may, perhaps, have a practical benefit, as

will be discussed below.There are a few places around the home where RCD protectionis stipulated by theIEE Wiring Regulations: fixed equipment

in bathrooms, socket outlets in rooms that contain a showercubicle (see below), gardens, and outbuildings.

Remember that you can provide RCD protection in various

places: a distribution board, a spur unit, or a single socketoutlet.

2.12 Where not to use RCDs2.12 Where not to use RCDs2.12 Where not to use RCDs2.12 Where not to use RCDs


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Remember that RCDs are only available for mains-voltagesupplies. 12-volt and 24-volt systems do not require RCD

protection, and none is available.You may want to consider avoiding RCD protection onlighting circuits, with the possible exception of kitchen and

bathroom lighting. You need to weigh up the advantages anddisadvantages quite carefully. The risk of electrocution from alighting system is actually quite small, and the victim isunlikely to be grasping the faulty equipment strongly (contrastthis to the use of, say, an electric drill or a hairdryer). Ifsomeone does receive a shock from a light fitting, it may not

be to the victim's benefit if the house is plunged into darkness.


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You decide.

Another place to avoid RCDs is in the supplies for equipmentthat really must run without interruption. In the home thisapplies particularly to freezers, but you may want to consideraquarium pumps and some kinds of computer equipment. Itwould not be in your best interests if an electrical fault in the

garden caused your freezer to shut down. In addition, RCDs dosometimes trip by accident, as will be discussed.

Remember that RCDs detect an imbalance in the live and

neutral currents. A current overload, however large, cannot bedetected. It is a frequent cause of problems with novices to

replace an MCB in a fuse box with an RCD. This may be done

i i h k i f li l


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in an attempt to increase shock protection. If a live-neutralfault occurs (a short circuit, or an overload), the RCD won'ttrip, and may be damaged. In practice, the main MCB for the

premises will probably trip, or the service fuse, so the situationis unlikely to lead to catastrophe; but it may be inconvenient.

It is now possible to get an MCB and and RCD in a single unit,

called an RCBO (see below). Replacing an MCB with anRCBO of the same rating is generally safe.

2.12.1 RCD discrimination2.12.1 RCD discrimination2.12.1 RCD discrimination2.12.1 RCD discrimination

Common sense, and theIEE Wiring Regulations, suggests that

if a system has multiple RCDs, then when a fault does occur

h RCD h f l h ldb h i Thi i


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the RCD nearest the fault should be the one to trip. This iscalleddiscrimination. If we have, for example, an RCD socketin a garage (with protect on the individual socket), and this is

fed by a distribution board with overall RCD protection, thenthe two RCDs need to have different sensitivities. Specificallythe one in the garage needs to be more sensitive than the one inthe distribution board. This ensures that a fault in the garage

will shut off the garage socket, not the distribution board. If thegarage socket has 30 mA sensitivity, then using a 100 mAdevice in the board will ensure the correct discrimination.

However, there is no safety benefit in this approach: two RCDsaren't safer than one. The only reason for installing a system

with multiple RCDs in the same circuit is to localize the

i l ti i th t f f lt F l h


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isolation in the event of a fault. For example, suppose you havea mains power ring (with RCD protection) feeding a singlesocket in the garden. If you provide the garden socket with its

own RCD, this will prevent faults in the garden equipmenttripping the RCD in the power ring. Of course, for this towork, you would need to use two different sensitivities.

Bear in mind that if you have a 30mA RCD as the overallprotection for your house, then there is absolutely no advantagein using any other RCD anywhere in the premises. If you wantto fit additional RCDs, you need first to change the main RCDfor a 100 mA device.

2.12.2 Nuisance tripping2.12.2 Nuisance tripping2.12.2 Nuisance tripping2.12.2 Nuisance tripping


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In my experience, modern RCDs on modern wiring systemswith modern appliances are not prone to trip by accident. Anytripping of an RCD should be taken seriously. However, there

are well-known reasons for nuisance tripping.

Sudden changes in electrical load can cause a small, briefcurrent flow to earth, especially in old appliances. RCDs arevery sensitive and operate very quickly; they may well tripwhen the motor of an old freezer switches off. Someequipment is notoriously leaky', that is, generate a small,

constant current flow to earth. Some types of computerequipment, and large television sets, are widely reported to

cause problems.


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Persistent nuisance tripping, or an RCD that won't switch on atall, are causes for concern. Either can indicate an intermittentor permanent live-earth fault somewhere in the circuit (e.g., a

faulty appliance flex).

2.12.3 RCD hazards2.12.3 RCD hazards2.12.3 RCD hazards2.12.3 RCD hazards

RCDs are an extremely effective form of shock protection.When properly selected and installed in a system that isgenerally sound, they render it almost impossible to receive a

dangerous shock. This degree of protection is not entirelywithout problems, as it is very easy to overestimate the

protection that RCDs provide. Increasingly RCDs are seen as a

cureforanykindof electrical problem Manypeopleevidently


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cure for any kind of electrical problem. Many people evidentlythink that by having and RCD somewhere in the house thisgives license to use shoddy and poorly-planned wiring and

equipment. This can be very dangerous.RCDs don't offer protection against current overloads. Thewidespread, dangerous practise of taking long spurs from a

power ring will be just as dangerous. Neither does an RCDprotect against long-term overloads that are just below thetripping current of the protective device. Both these problemsare consequences of poor design. The latter, for example, oftenarises when the same ring circuit is used for a kitchen and therest of the socket outlets on one floor of a building.

An RCD will not protect against a socket outlet being wired

withits liveandneutral terminalsthewrongwayround


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with its live and neutral terminals the wrong way round.

An RCD will not protect against the overheating that resultswhen conductors are not properly screwed into their terminals.

An RCD will not protect against live-neutral shocks, becausethe current in the live and neutral is balanced. So if you touchlive and neutral conductors at the same time (e.g., bothterminals of a light fitting), you may still get a nasty shock.

In summary, an RCD is intended to shorten the disconnection

time in the event of a live-earth fault, and to give a measure ofprotection against earth failure in an appliance. If you rely on it

for anything else, you're living on borrowed time.


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2.13 RCBOs2.13 RCBOs2.13 RCBOs2.13 RCBOs

As discussed above, an RCD does not provide overcurrent

protection. If you rely on an RCD to trip in an overload it willprobably be damaged and stop working completely. To make itpossible to replace an MCB with an RCD, manufacturers have

introduced a new range of equipment called RCBOs. AnRCBO combines an RCD with an RCB, in an enclosure thatwill fit into the same distribution board as the RCD. Whenchoosing an MCBO, remember that its overload and its earth-fault protection capabilities are completely separate. Youshould apply exactly the same design and selection procedures

as you would for two separate units.

2 S i l id i f b hi l id i f b hi l id i f b h2 S i l id i f b h


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2.14 Special considerations for bathrooms2.14 Special considerations for bathrooms2.14 Special considerations for bathrooms2.14 Special considerations for bathrooms

As far as possible, mains-operated equipment should be kept

away from a tub. In general, there should not be electricalsocket outlets in a bathroom. Fixed equipment should beprotected by an RCD, unless specifically designed for

bathroom use. Any electrical equipment in a bathroom(including light fittings) should be of a type that is specificallydesigned to be safe for a bathroom. Shaver sockets can befitted, provided they're the type with an isolating transformerthat are specifically designed for bathrooms. Strictly speaking,[IEE 601-08] allows socket outlets that are placed at least 3

metres from the tub, so if your bathroom is large enough you

canhavesocketoutletsandcomplywiththeRegulations


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can have socket outlets and comply with the Regulations.However, common-sense suggests that it's probably better toavoid this. Under-floor heating grids should be protected by an

earthed metal screen.2.14.1 What constitutes a bathroom?2.14.1 What constitutes a bathroom?2.14.1 What constitutes a bathroom?2.14.1 What constitutes a bathroom?

TheIEE Wiring Regulationsnot mention bathrooms, it dealswith locations containing a bath or shower' [IEE 601, 3rdamendment]. Specifically, the Regulations recognize thatrooms other than a bathroom' may contain a bath or shower.

For example, if you are fitting a shower cubicle in a bedroom,then you will want to take some care with electrical fittings in

the vicinity of the shower. For example, you need to be careful

about locatingitwithrespecttomainsoutletsand


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about locating it with respect to mains outlets andlightswitches. Anyelectrical outlet in a room that contains abath or shower should be protected by an RCD [IEE 601-08-

01], even if outside the 3-metre zone.

2.15 Special considerations for gardens and2.15 Special considerations for gardens and2.15 Special considerations for gardens and2.15 Special considerations for gardens andoutbuildingsoutbuildingsoutbuildingsoutbuildings

In general, extra-low voltage (12V) equipment is to bepreferred for garden use. It is now possible to get 12V lighting,

pumps, lawn sprayers, among others. Where you must usemains power, you will need to use RCD protection for socket

outlets.

Thereisawidespreadmisconceptionthattheuseof RCD


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There is a widespread misconception that the use of RCDprotection outside the house is because the environment maybe wet, and increase the electrical contact between person and

ground. Clearly this is likely to be a problem in a garden.However, it is less obvious why we should need to provideRCD protection in outbuildings.

TheIEE Wiring Regulationsto something called the mainequipotential zone'. This is, essentially, the part of the premisesthat has a very low resistance contact to the main earthing

terminal. Any part of the site that is outside this zone should beconsidered a candidate for RCD protection, as it will be

difficult to meet disconnection time regulations otherwise. The

equipotential zone'probablydoesnot includeaseparated


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equipotential zone probably does not include a separatedgarage, and certainly does not include the garden. The reasonfor using an RCD in a garage is therefore not that it may get

wet, but because it may be difficult to control shock voltageand disconnection time otherwise.

2.16 Special considerations for kitchens2.16 Special considerations for kitchens2.16 Special considerations for kitchens2.16 Special considerations for kitchens

You may well feel that special consideration needs to be givento electrical equipment in kitchens; there is, in fact, no legal orregulatory requirement for this. However, it is probablysensible to use RCD protection on kitchen power outlets, andto use supplementary bonding for metal sinks and taps where

the pipework is metallic.

217Effectof plasticwaterpipesonearthing217Effectof plasticwaterpipesonearthing217Effectof plasticwaterpipesonearthing217Effectof plasticwaterpipesonearthing


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2.17 Effect of plastic water pipes on earthing2.17 Effect of plastic water pipes on earthing2.17 Effect of plastic water pipes on earthing2.17 Effect of plastic water pipes on earthing

There has been a lot of controversy lately about the benefits

and hazards of plastic plumbing in kitchens and bathrooms.Remember that traditional metal pipework is a very goodconductor of electricity, and can be used to supplement

earthing. Plastic piping is a very good insulator, but the water itcontains may conduct, to a degree. As has already beensuggested, if the basic principle any earth should be a goodearth' is applied, this implies that metal equipment (e.g.,radiators and sinks) supplied by plastic piping should beearthed by bonding cables.

The electrical resistance of a 15 mm pipe containing tap water

isabout100kilohmspermetre;withcorrosioninhibitor (in


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is about 100 kilohms per metre; with corrosion inhibitor (inradiator pipes) it may be as low as 20 kilohms per meter(source: Electrical Research Association). While water purityand content varies from place to place, it is unlikely that therewill be much variation from the above figure for any tap water.Even in the worst case (radiator pipes), the amount of currentthat can be carried by the water in the pipes will be about 11

milliamps (230/20,000) divided by the length of the pipe to thenearest good earth. So even if a radiator is connected by aplastic pipe to a metal pipe at as little as 1 metre away, it

cannot carry enough current to present a significant hazard.This means that earthing the radiator will increasethe risk of

electric shock, rather than reduce it.

It is increasinglycommon therefore totreatplasticpipe- even


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It is increasingly common, therefore, to treat plastic pipe - evencontaining water - as an insulator. It makes no more sense toearth a radiator with plastic piping than it does to earth a filing

cabinet or window frame, even in the bathroom. There is noparticular requirement to earth bath or sink taps that areconnected by plastic pipe, any more than you would earth a

metal shelf or towel rail.

It is increasingly common to find that radiators are plumbedusing plastic pipe, except for the part that is visible above floor

level. There is no need to use supplementary bonding on thevisible part.

2.18 Special considerations for PME supplies2.18 Special considerations for PME supplies2.18 Special considerations for PME supplies2.18 Special considerations for PME supplies

R b th t if h PME l ( d h


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Remember that if you have a PME supply (and you have nocontrol over that, except by moving house), that your mainearth is the supply company's neutral. There is no earthconnection brought into the house. This is fine so long as thesupply neutral remains intact. If the neutral connection breaks,then you could be left with no effective earth. A fault in these

conditions could be extremely dangerous.

While it is good practice with all forms of supply to enhancethe earth connection by connecting the main earth terminal tothe incoming service pipes (e.g., gas and water), with a PMEsupply this is absolutely essential. Moreover, the use of

supplementary bonding within the premises, is even more

important than with other supplies. In fact, you - theh h ld t th l l ll bli t dt


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p pp , yhouseholder, not the supply company - are legally obligated toensure the integrity of supplementary bonding in this case. Themain bonding conductors (from the earth terminal to incomingservices) should normally be 10 mm with a PME supply, butcheck with the supply company because larger sizes arerequired in some cases.

If you have a PME supply, there may be a label near theincoming cables indicating that fact, but it is probably safer tocheck with the supply company or the local authority.

3. Example of selecting cable size3. Example of selecting cable size3. Example of selecting cable size3. Example of selecting cable size

Thisappendixdescribeshowtoselect theappropriatecable


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This appendix describes how to select the appropriate cablesize for a given application, to be in accordance with theIEEWiring Regulations. For your convenience I have extracted theinformation from the tables given in theRegulationsthat arelikely to be applicable to typical domestic installations andsummarized it in appendixA. However, this appendix makes

certain assumptions about your installation, and if theseassumptions are not true, you shouldnot use this information.Instead, you should refer to the approriate sections of theIEE

Wiring RegulationsIEE appendix 4] or, better still, calculatethe appropriate figures yourself.

The assumptions are as follows.

z The over-current device which is protecting the cable is an MCB or a cartridge fuse (not


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a rewirable fuse). This is important: rewirable fuses blow at a current level about 50%higher than an MCB.

z The cable is general-purpose PVC two-core-and-earth, with a maximum temperature

rating of 70 degrees celcius.z The protective device provides overload protection, and not just short-circuit protection

(this is nearly always the case in domestic installations; we expect the MCB to trip if thecurrent load is too high for the cable, not just if it is catastophically high).

z Where cables are bunched together, they are all the same size, carrying the same current,and have the same maximum temperature. In practice it has not been found to be aproblem when the cables are not the same size or at the same current, but they must beof the same type, and with the same maximum temperature, for these guidelines (andthose of theIEE Wiring Regulations) to be applied.

Note that in domestic wiring you will hardly ever need to usethis procedure. It is nearly always appropriate to use 2.5 mm

cable for power circuits and 1.5 mm cable for lighting. You

willneed to apply this procedure if you are planning long runsofcable (eg toanoutbuildingwithapowercircuit)orhigh


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of cable (e.g., to an outbuilding with a power circuit) or highcurrent appliances (e.g., electric showers or cookers).

The procedure to be described can be followed by anyone whohas a pocket calculator and can follow instructions no morecomplicated than multiply this number by the one you

calculated in step 2''. However, it will make more sense if youhave a basic understanding of electrical theory and Ohms's law.

3.1 Outline of the procedure3.1 Outline of the procedure3.1 Outline of the procedure3.1 Outline of the procedure

The basic procedure is to select a cable whose nominal current

rating appears appropriate for the load it will have to carry - or

the overcurrent device fitted, then check it meets the currentcarryingcapacity' requirements the voltagedrop'


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carrying capacity requirements, the voltage droprequirements, and the disconnectiont time' requirements, afterapplying various correction factors. If any requirement is notmet, you should try again with the next largest cable. If youcan't meet the disconnection time requirements with the largestcable, then you will need to use an RCD to protect against

earth faults.

3.1.1 Step 1: calculate the nominal current3.1.1 Step 1: calculate the nominal current3.1.1 Step 1: calculate the nominal current3.1.1 Step 1: calculate the nominal current

The nominal current will be rating of the MCB selected toprotect the system. Of course this must be higher than the

current required by the appliance(s) connected, otherwise it

will trip immediately. If we don't know the current required,wecalculateit fromtheappliancepower whichwill usuallybe


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we calculate it from the appliance power, which will usually bemarked on it (or ask the manufacturer). Calculate the currentby dividing the power in watts by the supply voltage. Nowchoose an MCB whose current rating is a close as possibleabove the calculated figure. This is the nominal current.

3.1.2 Step 2: pick a cable size that appears appropriate for the3.1.2 Step 2: pick a cable size that appears appropriate for the3.1.2 Step 2: pick a cable size that appears appropriate for the3.1.2 Step 2: pick a cable size that appears appropriate for thenominal currentnominal currentnominal currentnominal current

To do this, refer to tableA.1. In this table, enclosed in a wall'

refers to a cable that is concealed in plasterwork, either directlyor in a protective conduit. Enclosed in conduit' means fully

enclosed in conduit or trunking that is fastened to the surface

of a wall or ceiling. Clipped to a surface' means fastened atintervalstoaflatsurfacesuchasbrickworkor joists Free'


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intervals to a flat surface such as brickwork or joists. Freemeans completely surrounded by air. A cable that is passedacrossa line of joists, running through ample holes, couldprobably be classed as free' (but see discussion of thermalinsulation below). Free' also applies to cables that are laid onmetal cable trays, but this is unusual in domestic installations.

There will be some cases where your installation does notconform to any of the installation methods shown in tableA.1.In these circumstances you should usually pick the closest,

worst case. For example, lighting cables will often be runpartly free, partly along joists, and partly in plasterwork. In

such a case you would be well advised to assume that it will all

be in plasterwork, for purposes of cable selection.


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One case that requires special mention is that of cables that areclipped or free on one side, and in contact with thermal

insulation on the other. This is often the case in loft wiring,because cables will run along joists which are in contact withinsulation, or on the top of the insulation. In these

circumstances the IEE recommends that we use the same figureas for cable enclosed in a wall.

Make a note of the current carrying capacity of the cable you

have selected.

3.1.

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