protective conductor sizing calculations

to the 16th Edition IEE Regulations

chapter 1The IEE

Regulations

chapter 2Installation Requirement

s and

Characteristics

chapter 3Installation

Controland

Protection

chapter 4Cables,

Conduitsand

Trunking

chapter 5Earthing

chapter 6Circuits

chapter 7Special

Installations

chapter 8Testing

and

Earthing

5.1 - The earthing principle5.6 - Protective multiple earthing (PME)

5.2 - Earthing Systems 5.7 - Earthed concentric wiring

5.3 - Earth fault loop impedance

5.8 - Other protection methods

5.4 - Protective conductors5.9 - Residual current devices (RCDs)

5.5 - Earth electrodes 5.10 - Combined functional and protective--------- earthing

5.4.1 - Earthing conductors

5.4.2 - Protective conductor types

5.4.3 - Bonding conductors

5.4.4 - Protective conductor cross-section assessment

5.4.5 - Protective conductor cross-section calculation

5.4.6 - Unearthed metalwork

5.4.5 - Protective conductor cross-section calculation

The c.s.a. of the circuit protective conductor (c.p.c.) is of great importance since

the level of possible shock in the event of a fault depends on it (as seen in {5.4.4}).

Safety could always be assured if we assessed the size using {Table 5.7} as a basis.

However, this would result in a more expensive installation than necessary because we would often use protective conductors which are larger than those found to be acceptable by calculation. For example. twin with cpc insulated and sheathed cables larger than 1 mm² would

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Inspection

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and Networks

Contents

be ruled out because in all other sizes the CPC is smaller than required by {Table 5.7}.

In very many cases, calculation of the CPC size will show that a smaller size than that detailed in {5.4.4} is perfectly adequate. The formula to be used is:

S = Ö(Ia²t) k

whereS is the minimum protective conductor cross-sectional area (mm2)Ia is the fault current (A)t is the opening time of the protective device (s)k is a factor depending on the conductor material and insulation, and the initial and maximum insulation temperatures.

This is the same formula as in {3.7.3}, the adiabatic equation, but with a change in the subject. To use it, we need to have three pieces of information, Ia, t and k.

1) To find Ia

Since Ia = Uo we need values for Ia = uo and Zs

Zs

Uo is simply the supply voltage, which in most cases will be 240V.

Zsis the earth-fault loop impedance assuming that the fault has zero impedance.

Since we must assume that we are at the design stage, we cannot measure the loop impedance and must calculate it by adding the loop impedance external to the installation (Ze) to the resistance of the conductors to the furthest point in the circuit concerned. This technique was used in {5.3.6}.

Thus, Zs = Ze + R1 + R2 where R1 and R2 are the resistances of the phase and protective conductors respectively from {Table 5.5}.

2) To find t

We can find t from the time/current characteristics of {Figs 3.13 to 3.19} using the value of Ia already calculated above. For example, if the protective device is a 20 A miniature circuit breaker type I and the fault current is 1000 A, we shall need to consult {Fig 3.16}, when we can read off that operation will be in 0.01 s (10 ms). (It is of interest here to notice that if the fault current had been 80 A the opening time could have been anything from 0.04 s to 20 s,so the circuit would not have complied with the required opening times).

3) To find kk is a constant, which we cannot calculate but must obtain from a suitable table of values. Some values of k for typical protective conductors are given in {Table 5.8}.

It is worth pointing out here that correctly installed steel conduit and trunking will always meet the requirements of the Regulations in terms of protective conductor impedance.

Although appearing a little complicated, calculation of acceptable protective conductor size is worth the trouble because it often allows smaller sizes than those shown in {Table 5.7}.

Table 5.8 - Values of k for protective conductors

Nature of protective conductor

Initial temp.(°C)

Final temp(°C)

Conductormaterial

K

p.v.c. insulated, not in cable

or bunched30 160 Copper 143

- 30 160 Aluminium 95

- 30 160 Steel 52

-

p.v.c. insulated, in cable



-

Steel conduit or trunking

50 160 Steel 47

-

Bare conductor 30 200 Copper 159


- 30 200 Steel 58

Example 5.2A load takes 30 A from a 240 V single phase supply and is protected by a 32 A HBC fuse to BS 88. The wiring consists of 4 mm² single core p.v.c. insulated cables run in trunking, the length of run being 18 m. The earth-fault loop impedance external to the installation is assessed as 0.7 Ohms. Calculate the cross-sectional area of a suitable p.v.c. sheathed protective conductor.

This is one of those cases where we need to make an assumption of the answer to the problem before we can solve it. Assume that a 2.5 mm² protective conductor will be acceptable and calculate the combined resistance of the phase and protective conductors from the origin of the installation to the end of the circuit. From {Table 5.5}, 2.5 mm² cable has a resistance of 7.4 mohms/m and 4 mm² a resistance of 4.6 mOhms/m. Both values must be multiplied by 1.2 to allow for increased resistance as temperature rises due to fault current.

Thus, R1 + R2 =

(7.4 + 4.6) x 1.2 x 18

Ohms =

12.0 x 1.2 x 18

= 0.26 Ohms

1000 1000 This conductor resistance must be added to external loop impedance to give the total earth-fault loop impedance.

Zs = Ze + Rl + R2 = 0.7+ 0.26 Ohms = 0.96 Ohms

We can now calculate the fault current:

la =

Uo = 240 = 250A

Zs 0.96 Next we need to find the operating time for a 32 A BS 88 fuse carrying 250 A. Examination of {Fig 3.15} shows that operation will take place after 0.2 s.

Finally, we need a value for k. From {Table 5.8} we can read this off as 115, because the protective conductor will be bunched with others in the trunking.

We now have values for Ia, t and k so we can calculate conductor size.

S = Ö(Ia²t) = Ö(250² x mm² = 0.97

0.02) mm² k 115

This result suggests that a 1.0 mm² protective conductor will suffice. However, it may he dangerous to make this assumption because the whole calculation has been based on the resistance of a 2.5 mm² conductor. Let us start again assuming a 1.5 mm² protective conductor and work the whole thing through again.

The new size protective conductor has a resistance of 18.1 mOhms/m, see {Table 5.5}, and with the 4 mm² phase conductor gives a total conductor resistance, allowing for increased temperature, of 0.491 Ohms. When added to external loop impedance this gives a total earth-fault loop impedance of 1.191 Ohms and a fault current at 240 V of 202 A. From {Fig 3.15} operating time will be 0.6 s. The value of k will be unchanged at 115.

S = Ö(Ia²t) =Ö(202² x

0.6)mm² = 1.36 mm²

k 115 Thus, a 1.5 mm² protective conductor can be used in this case. Note that if the size had been assessed rather than calculated, the required size would be 4 mm², two sizes larger. A point to notice here is that the disconnection time with a 1.5mm² protective conductor is 0.6 s, which is too long for socket outlet circuits (0.4 s max.).

Example 5.3A 240 V, 30 A ring circuit for socket outlets is 45 m long and is to be wired in 2.5 mm² flat twin p.v.c. insulated and sheathed cable incorporating a 1.5 mm² cpc. The circuit is to be protected by a semi-enclosed (rewirable) fuse to BS 3036, and the earth-fault loop impedance external to the installation has been ascertained to be 0.3 Ohms. Verify that the 1.5 mm² cpc enclosed in the sheath is adequate.

First use {Table 5.5} to find the resistance of the phase and cpc conductors. These are 7.4 mOhms/m and 12.1 mOhms/m respectively, so for a 45 m length and allowing for the resistance increase with temperature factor of 1.2.

R1 + R2 =

(7.4 + 12.1) x 1.2 x 45

Ohms =

19.5 x 1.2 x 45

Ohms = 1.05 Ohms

1000 1000

Zs = Ze +

R1+ R2

= 0.3 +

1.05 Ohms = 0.3 + 0.263 Ohms = 0.563 Ohms

4 4

The division by 4 is to allow for the ring nature of the circuit.

Ia =

Uo = 240 A = 426A

Zs 0.563 We must then use the time/current characteristic of {Fig 3.13} to ascertain an operating time of 0.10 s.

From {Table 5.8} the value of k is 115.

Then S =

Ö(Ia²t) =Ö(426² x

0.10) mm² = 1.17 mm²

k 115 Since this value is smaller than the intended value of 1.5 mm², this latter value will be satisfactory.

Example 5.4A 240 V single-phase circuit is to be wired in p.v.c. insulated single core cables enclosed in plastic conduit. The circuit length is 45 m and the live conductors are 16 mm² in cross-sectional area. The circuit will supply fixed equipment, and is to be protected by a 63 A HBC fuse to BS 88. The earth-fault loop impedance external to the installation has been ascertained to be 0.58 Ohms. Calculate a suitable size for the circuit protective conductor.

With the information given this time the approach is somewhat different. We know that the maximum disconnection time for fixed equipment is 5 s, so from the time/current characteristic for the 63 A fuse {Fig 3.15} we can see that the fault current for disconnection will have a minimum value of 280 A.

Thus, Zs =

Uo = 240 Ohms = 0.857 Ohms

Ia 280 If we deduct the external loop impedance, we come to the resistance of phase and protective conductors.

R1 +R2 = Zs-Ze = 0.857-0.58Ohms = 0.277 Ohms

Converting this resistance to the combined value of R1 and R2 per metre,

(R1 + R2) per metre =

0.277 x 1000

mOhms/m = 5.13 mOhms/m

45 x 1.2

Consulting {Table 5.5} we find that the resistance of 16 mm² copper conductor is 1.15 mOhms/m, whilst 10 mm² and 6 mm² are 1.83 and 3.08 mOhms/m respectively. Since 1.15 and 3.08 add to 4.23, which is less than 5.13, it would seem that a 6 mm² protective conductor will be large enough. However, to be sure we must check with the adiabatic equation.

R1 + R2 =

(1.15+3.08) x 1.2 x 45

Ohms = 0.228 Ohms

1000

Zs = Ze+(R1 +R2) = 0.58+0.228 Ohms = 0.808 Ohms

Ia = Uo = 240 A = 297 A Zs 0.808

From {Fig 3.15} the disconnection time for a 63 A fuse carrying 297 A is found to he 3.8s.

Table 5.7 - Main earthing and main equipotential bonding conductor----------------- sizes for TN-S and TN-C-S supplies

Phase conductor (or

neutral for PME supplies )

Earthing conductor (not buried or

protected against mechanical damage)

Main equipotential bonding conductor for PME supplies

Main equipotential

bonding conductor

csa mm² csa mm² csa mm² csa mm²

4 4 6 10

6 6 6 10

10 10 6 10

16 16 10 10

25 16 10 10

35 16 10 10

50 25 16 16

70 35 16 25

Extracted from The Electricians Guide Fifth Editionby John Whitfield

Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations

3.7.3 -Operating time

Not only must the short-circuit protection system open the circuit to cut off a fault, but it must do so quickly enough to prevent both a damaging rise in the conductor insulation temperature and mechanical damage due to cable movement under the influence of electro-mechanical force. The time taken for the operation of fuses and circuit breakers of various types and ratings is shown in {Figs 3.13 to 3.19}. When the prospective short circuit current (PSC) for the point at which the protection is installed is less than its breaking capacity there will be no problem.

When a short circuit occurs there will be a high current which must be interrupted quickly to prevent a rapid rise in conductor temperature.

The position is complicated because the rise in conductor temperature results in an increase in resistance which leads to an increased loss of energy and increased heating (W = l²Rt), where W is the energy (J) and R is the resistance W. The Regulations make use of the adiabatic equation which assumes that all the energy dissipated in the conductor remains within it in the form of heat, because the faulty circuit is opened 50 quickly. The equation is:

t = k²S² I²

where t = the time for fault current to raise conductor temperature to the highest permissible level

k = a factor which varies with the type of cable

S = the cross-sectional area of the conductor (mm²)

I = the fault current value (A) - this will be the PSC

Some cable temperatures and values of k for common cables are given in {Table 3.7}.

Table 3.7 Cable temperatures and k values (copper cable)

Insulation materialAssumed initial

temperature (°C) Limiting final

temperature(°C) k

p.v.c 70 160 115

85°C p.v.c 85 160 104

90°C thermosetting 90 250 143

Mineral, exposed to touchor p.v.c. covered

70 160 115

Mineral not exposed to touch

105 250 135

As an example, consider a 10 mm² cable with p.v.c. insulation protected by a 40 A fuse to BS 88 Part 2 in an installation where the loop impedance between lines at the point where the fuse is installed is 0.12 W. If the supply is 415 V three phase, the prospective short circuit current (PSC) will be:

I = UL A = 415 A = 3.46 kA Z 0.12 from {Table 3.7}, k = 115

t= k²S²

=115² x 10² = 0.110s

-I² 34602 {Figure 3.15} shows that a 40 A fuse to BS 88 Part 2 will operate in 0.1 5 when carrying a current of 400 A. Since the calculated PSC at 3460 A is much greater than 400 A. the fuse will almost certainly clear the fault in a good deal less than 0.1 s. As this time is less than that calculated by using the adiabatic equation (0.11 s) the cable will be unharmed in the event of a short circuit fault.

It is important to appreciate that the adiabatic equation applies to all cables, regardless of size. Provided that a protective device on the load side of a circuit has a breaking capacity equal to or larger than the PSC of the circuit then that circuit complies with the PSC requirements of the Regulations (see {Fig 3.22} and see also the note in {7.15.1} concerning the use of dual rated fuses for motor protection).

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to the 16th Edition IEE Regulations

chapter 1The IEE

Regulations


Requirements and

Characteristics


Controland Protection

chapter 4Cables, Conduits

and Trunking

chapter 5Earthing

chapter 6Circuits

chapter 7

Earthing

5.1 - The earthing principle

5.6 - Protective multiple earthing (PME)

5.2 - Earthing Systems 5.7 - Earthed concentric wiring

5.3 - Earth fault loop impedance

5.8 - Other protection methods

5.4 - Protective conductors

5.9 - Residual current devices (RCDs)

5.5 - Earth electrodes 5.10 - Combined functional and protective--------- earthing

5.3.1 - Principle

5.3.2 - The importance of loop impedance

5.3.3 - The resistance/impedance relationship

5.3.4 - Earth-fault loop impedance values

5.3.5 - Protective conductor impedance

5.3.6 - Maximum circuit conductor length

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Special Installations

chapter 8Testing

and Inspection

chapter 9Data cabling

and Networks

Contents

5.3.6 - Maximum circuit conductor length

The complete earth-fault loop path is made up of a large number of parts as shown in {Fig 5.7}, many of which are external to the installation and outside the control of the installer. These external parts make up the external loop impedance (Ze). The rest of the earth-fault loop impedance of the installation consists of the impedance of the phase and protective conductors from the intake position to the point at which the loop impedance is required.

Since an earth fault may occur at the point farthest from the intake position, where the impedance of the circuit conductors will be at their highest value, this is the point which must be considered when measuring or calculating the earth-fault loop impedance for the installation. Measurement of the impedance will he considered in {8.6.2}. Provided that the external fault loop impedance value for the installation is known, total impedance can be calculated by adding the external

Table 5.5 - Resistance per metre of copper conductors at 20°C for---------------- calculation of R1 + R2

Conductor cross-sectional area

(mm²)

Resistance per metre run

(m ohms / m)

1.0 18.1

1.5 12.10

2.5 7.41

4.0 4.61

6.0 3.08

10.0 1.83

16.0 1.15

25.0 0.727

Note that to allow for the increase in resistance with increased temperature under fault

conditions the values of {Table 5.5} must be multiplied by 1.2 for p.v.c. insulated cables (see

{Table 8.7})

impedance to that of the installation conductors to the point

concerned. The combined resistance of the phase and protective conductors is known as R1+ R2. The same term is sometimes used for the combined resistance of neutral and protective conductors (see {8.4.1}). In the vast majority of cases phase and neutral conductors have the same cross-sectional area and hence the same resistance.

For the majority of installations, these conductors will be too small for their reactance to have any effect (below 25 mm² cross-sectional area reactance is very small), so only their resistance's will be of importance. This can be measured by the method indicated in {Fig 5.9}, remembering that this time we are interested in the combined resistance of phase and protective conductors, or can be calculated if we measure the cable length and can find data concerning the resistance of various standard cables. These data are given here as {Table 5.5}.

The resistance values given in {Table 5.5} are for conductors at 20°C. Under fault conditions the high fault current will cause the temperature of the conductors to rise and result in an increase in resistance. To allow for this changed resistance, the values in {Table 5.5} must be multiplied by the appropriate correction factor from Table {8.7}. It should be mentioned that the practice which has been adopted here of adding impedance and resistance values arithmetically is not strictly correct. Phasor addition is the only perfectly correct method since the phase angle associated with resistance is likely to he different from those associated with impedance, and in addition impedance phase angles will differ from one another. However, if the phase angles are similar, and this will be so in the vast majority of cases where electrical installations are concerned, the error will be acceptably small.

It is often assumed that higher conductor temperatures are associated with the higher levels of fault current. In most cases this is untrue. A lower fault level will result in a longer period of time before the protective device operates to clear it, and this often results in higher conductor temperature.

Example 5.1A 7 kW shower heater is to be installed in a house fed with a 240 V TN-S supply system with an external loop impedance (Ze) of 0.8 Ohms. The heater is to be fed from a 32 A BS 88 cartridge fuse. Calculate a suitable size for the p.v.c. insulated and sheathed cable to be used and determine the maximum

possible length of run for this cable. It may be assumed that the cable will not be subject to any correction factors and is clipped direct to a heat conducting surface throughout its run.

First calculate the circuit current.

I = P = 7000

A = 29.2 A

U 240 Next, select the cable size from {Table 4.7} (which is based on [Table 4D2A]) from which we can see that 4 mm² cable of this kind clipped direct has a rating of 36 A. The 2.5 mm² rating of 27 A is not large enough. We shall assume that a 2.5 mm² protective conductor is included within the sheath of the cable.

Now we must find the maximum acceptable earth-fault loop impedance for the circuit. Since this is a shower, a maximum disconnection time of 0.4 s will apply, so we need to consult {Table 5.1}, which gives a maximum loop impedance of 1.09 Ohms for this situation. Since the external loop impedance is 0.8 Ohms, we can calculate the maximum resistance of the cable.

R cable

= max. loop impedance - external impedance

=1.09 - 0.8 Ohms = 0.29 Ohms

This assumes that resistance and impedance phase angles are identical, which is not strictly the case. However, the difference is unimportant.

The phase conductor (4 mm²) has a resistance of 4.6 m Ohms/m and the 2.5 mm² protective conductor 7.4 mOhms/m, so the combined resistance per metre is 4.6 + 7.4 = 12.0 m Ohms/m. The cable will get hot under fault conditions, so we must apply the multiplier of 1.2.

Effective resistance of cable per metre

= 12.0 x 1.2 m Ohms/m

= 14.4 m Ohms/m

The maximum length of run in metres is thus the number of times 14.4 mOhms will divide into 0.29 Ohms.

Maximum = 0.29 x 1000 20.1

length of run m = m 14.4

This is not quite the end of our calculation, because we must check that this length of run will not result in an excessive volt drop. From {Table 4.7} based on [Table 4D2B] a 4 mm² cable of this kind gives a volt drop of 11 mV/A/m.

Volt drop =

11 x circuit current (A) x length of run (m) divided by 1000

Volt drop =

11 x 29.2 x 20.1

V = 6.46 V

1000 Since the permissible volt drop is 4% of 240 V or 9.6 V, this

Table 5.5 - Resistance per metre of copper conductors at 20°C for---------------- calculation of R1 + R2

Conductor cross-sectional area(mm²)

Resistance per metre run (m ohms / m)

1.0 18.1

1.5 12.10

2.5 7.41

4.0 4.61

6.0 3.08

10.0 1.83

16.0 1.15

25.0 0.727

Note that to allow for the increase in resistance with increased temperature under fault conditions the values of {Table 5.5} must be

multiplied by 1.2 for p.v.c. insulated cables (see {Table 8.7})





Nature of protective conductor

Initial temp.(°C)

Final temp(°C)

Conductormaterial

K

p.v.c. insulated, not in cable



- 30 160 Steel 52

-

p.v.c. insulated, in cable



-

Steel conduit or trunking

50 160 Steel 47

-



- 30 200 Steel 58



Nature of protective conductorInitial temp.(°C)

Final temp(°C)

Conductormaterial

K

p.v.c. insulated, not in cable or bunched

30 160 Copper 143


- 30 160 Steel 52

-

p.v.c. insulated, in cable or bunched

70 160 Copper 115


-

Steel conduit or trunking 50 160 Steel 47

-



- 30 200 Steel

protective conductor sizing calculations

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residual current devices

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