electrical systems safety

NUE 505ANUE 505A

Electrical systems safetyElectrical systems safety

Assessment preparationAssessment preparation

Protection devicesProtection devices

10-1

1

10

102

103

4.5

Time (sec)

Current (x IRATED)

BMagnetic Section

Thermal Section

Circuit Breaker Types

10-1

1

10

102

103

Time (sec)

Current (x IRATED)7.5

C

Thermal Section

New Magneticoperation


10-1

1

10

102

103

Time (sec)

Current (x IRATED)12

D

Thermal Section



RCD,sRCD,s

Residual Current Device RCDSafety Switch

LOAD

NA/s

Supply

When the circuit is in good condition.

(No Earth Faults)

(I active = I neutral) = No Flux = No Induced EMF

Supply to the Load is Maintained

IA

IN

RCD with Earth Fault

LOAD

NA/s

Supply

When an Earth Faults occurs.

(I active I neutral) = Flux in Core = Induced EMF

The fault is Isolated

Safety SwitchNA/s

Supply

Not Earthed

A person can receive a shock with a “Safety Switch” installed

Another point to consider: An RCD rated at 40Amps will not trip (like a C/B) if say 52Amps will to flow through the device.

3 RCD 3 LOAD

NL1

Supply

LOAD

L2 L3

3 RCD 1 LOAD

N

Supply

LOAD

A

Max Demand Max Demand CalculationsCalculations

Domestic LightingDomestic Lighting


Domestic PowerDomestic Power


Domestic AppliancesDomestic Appliances


Domestic Multi-phaseDomestic Multi-phase

Max Demand Max Demand CalculationsCalculationsNon-Domestic LightingNon-Domestic Lighting

A Small motel installation contains the following


Non-Domestic PowerNon-Domestic Power

A factory has the following loading, calculate the maximum

demand consideration.

Max Demand Max Demand CalculationsCalculationsNon-Domestic AppliancesNon-Domestic Appliances

A factory has the following three-phase loads,what would be the loading for a maximum

demand calculation.

Cable selectionCable selection

Cable selection Cable selection based on voltage based on voltage

dropdrop

Voltage dropVoltage drop

Single-phaseSingle-phase

Voltage dropVoltage drop

Multi-phaseMulti-phase

Conductor size Conductor size based on voltage based on voltage

dropdrop

Overall multi-phase Overall multi-phase voltage drop voltage drop calculationcalculation

Fault loop Fault loop impedanceimpedance

AS/NZ 3000:2000 - 1.7.4.3.3AS/NZ 3000:2000 - 1.7.4.3.3

The The earth system impedanceearth system impedance, and the , and the Trip Trip characteristic of the protective devicecharacteristic of the protective device must be such must be such that if;that if;

A fault of negligible impedance occurs (active to A fault of negligible impedance occurs (active to earth - short circuit) earth - short circuit)

Automatic disconnect of the supply (high enough fault Automatic disconnect of the supply (high enough fault current to operate the protective device)current to operate the protective device)

Within a specified timeWithin a specified time6.3.3.2.1

6.3.3.2.2AmendmentAS/NZS 3000:2000

The Fault Loop

1. The impedance needs to be low enough, to allow a high enough fault current, to operate the protective device, within a given time period. (6.3.3.2.2)

2. The Earth Loop Impedance is matched to the Protective Device Tripping Characteristics.

3 Phase Supply N/L

Earth SystemEarth System There are three ways to determine whether There are three ways to determine whether

fault loop impedance is OK:fault loop impedance is OK: 1.1. Measure fault loop impedance at the Measure fault loop impedance at the

load.load.

(Compare values against Table B4.1 of (Compare values against Table B4.1 of AS3000)AS3000)

2. Calculate the maximum length allowable 2. Calculate the maximum length allowable for the FSC, and come in for the FSC, and come in under that.under that.

3.3. Measure the A/A-E impedance.Measure the A/A-E impedance.

(Compare it to Table 3.2 AS3017)(Compare it to Table 3.2 AS3017)

Earth SystemEarth System

1 Measure the Fault Loop Impedance…

There are three ways to determine whether fault loop impedance is OK:


♦ ZLOOP = ZACTIVE + ZEARTH + Z NEUTRAL + Z TX

♦ All these will limit current and dictate the fault current that will flow.

Load

N/L

16A

MEN Link must be left intact

10-1

1

10

102

103

4

Time (sec)

Current (x IRATED)

BMagnetic Section

Thermal Section


1x

10-1

1

10

102

103

Time (sec)

Current (x IRATED)

7.5

C

Thermal Section



1x

10-1

1

10

102

103

Time (sec)

Current (x IRATED)

12.5

D

Thermal Section



1x


Load

16A

N/L



Load

16A

N/L

Active

Earth


•Total maximum allowable Fault Loop Impedance = 1.92Ω

• Cold Fault Loop Impedance = 1.92x0.8 = 1.54Ω

• If supply is 240V per phase, then multiply Z by 240/230, or 1.04: Fault Loop Impedance = 1.54 x 1.04

= 1.60Ω

• If RCD protection is used, AS3000, rule 6.3.4.2.1 (2) states that if the RCD operates during the FLI test, the test result is considered satisfactory.




2. Calculate the maximum length allowable for the Final Sub Circuit and come in underthat.


Load

16A

N/L

With 80% of voltage drop here in a fault, the FSC must have 80% of the total Fault Loop Impedance.

Where the length and CSA of the mains is not known, we may assume that 80% of the voltage drop under fault conditions will occur in the final sub-circuit (B5.2.1b).

0.8 x VNOM x CSAACTIVE x CSA EARTHLMAX = ITRIP x x (CSAACTIVE + CSA EARTH)

= 22.5 x 10-3 ohm-mm2/metre for Copper = 36 x 10-3 ohm-mm2/metre for Aluminium

• This gives the maximum length allowable for any given circuit.

•Maximum length allowable for any given circuit can be calculated from (B5.2.2):

(Page 237 AS3000)




2. Calculate the maximum length allowable for the Final Sub Circuit and come in underthat.

3. Measure the A/A-E impedance.(Compare it to Table 3.2 AS3017)

AS3017:Testing

andInspectionGuidlines

Notes:1. It is cold resistance.2. It is for FSC only.3. There are still holes…

MAIN SWITCHBOARD

Load

But what if the cable/CB size is not there on that

table?

Say, 300mm2 orange circ. cable supplying a 415V, 350A, 3-phase motor through underground conduit 140mtrs away. C/B is 400A type “C”.

1. CCC of 300mm2 cable = 415A

2. Determine the current required to cause instantaneous operation of the C/B.• 7.5 x 400 = 3000A

3. Determine the (hot) fault loop impedance• 240/3000 = 0.08

4. Total circuit Cold FLZ = 0.08 x .8 = 0.064

5. Final Sub-Circuit FLZ (cold) = 0.064 x 0.8 = 0.0512

Say, 300mm2 orange circ. cable supplying a 415V, 350A, 3-phase motor through underground conduit 140mtrs away…

7. Measure the actual loop impedance.

MAIN SWITCHBOARD

Load

A

V

I 5A

Z = V/I

But what if you are wanting to install the cable, and you want to know if everything is OK?

1. Size of earth: 120mm2 (from AS3000 table 5.1)

2. Find Impedance actual of A-E loop on FSC.


Z

R

XL

Note that including XL of the cable only comes into play above 120mm2

But what if you are wanting to install the cable, and you want to know if everything is OK?

1. Size of earth: 120mm2 (from AS3000 table 5.1)

2. Find Impedance actual of A-E loop on FSC.


Active: XL = 0.0732/km (Table 30, AS3008)= 0.010248 for 140mtrs

R@750C = 0.0778/km (Table 35, AS3008)R@200C = 0.0778 x 0.8 x 0.14 = 0.0087136 for 140mtrs

•Earth: XL = 0.0743/km = 0.010402 for 140mtrs

R@750C = 0.188/km R@200C = 0.188 x 0.8 /km =0.1504R for 140mtrs = 0.021056


• Active XL = 0.010248 R = 0.0087136

• Earth XL = 0.010402 R = 0.021056


RA + RE = 0.0298

XA + XE = 0.02065Z = 0.0362

RACTIVEREARTH

XACTIVE

XEARTH

4. Is Loop Z < Maximum Allowable Z?

Z = 0.0362 Z = 0.0512

Last Step…:

• Remember that a type C breaker trips at 7.5 times its rating? This figure is only an approximation. It can be anywhere between 5 and 10 times its rating…

• Remember too, that we took the FSC as having 0.8 of the total circuit impedance? This is an approximation too.

Remember the last question?

Q: Why do we want a low resistance earth wire?To CREATE a high enough fault current to trip the protective device.

To ensure that we do create a high enough fault current, fault loop impedance must be low enough.

AS3017:Testing

andInspectionGuidlines

A Simplified CircuitWe need to look at a complete loop (circuit)

3 Phase Supply

The current path includes the:

Supply Transformer, Distribution System, Mains, Protection Device, Final Sub-Circuit including the Load

N/L

B5.1 Maximum Circuit Length

• This process is used to calculate the maximum circuit length (FSC) taking into consideration:– The rating and type of Protection device– The CSA of the Active conductor– The CSA of the Earthing conductor– The route length of the Final sub-circuit.

• A variation of this process may be used to determine an exact value of the FSC’s impedance, and prove the earth continuity.

3 Phase Supply

There will always be more than 80% of the Nominal Voltage AS/NZ3000:2000 B5.2.1 b

Zext Zint

Zint = 0.8 Uo

Ia

B5.2.1

Isolation, Isolation, disconnection disconnection

and and reconnection reconnection proceduresprocedures

Safe Isolation 1. Assess the need for isolation

2. Notify others

3. Determine how to isolate the circuit

4. Test the supply is present & test meter

5. Isolate the supply. (switch, fuse, C/B etc)

6. Attach danger tags or locks

7. Test that you have isolated the correct circuit

8. Test your meter again

MEN MEN ConnectionConnection

Recent studies have shown that 95% of electricians do not fully understand the MEN system.

This figure comes from the Electrical Contractors Association of Queensland who were conducting training sessions throughout Queensland during 1998.

They asked electricians to draw a MEN system and explain it. Only 5% could!!!

MEN System

MENConnection

ConsumerMains MAIN SWITCHBOARD

Circuit ProtectiveDevices

Earth LinkNeutral Link

Main Switch

MENConnection


ConsumerMains MAIN SWITCHBOARD

Neutral Link

Main Switch

So why do we earth the neutral at the board?

Load

Direct Earthing System16A

Q: Will the fuse blow?

0.3

0.2

23

FAULT

RTOTAL= 23.5 IFAULT = V/R= 240/23.5 10A

Load

MEN Earthing System

0.3

0.2

16A

RTOTAL= 1 Q: Will the fuse blow?

0.5

IFAULT = V/R= 240/1= 240A

23

N/L

FAULT

RRETURN = 0.5//23 0.5

Q: Why do we want a low resistance earth wire?

To CREATE a high enough fault current to trip the protective device.

MAIN SWITCHBOARD

MENConnection



NORMALLOAD CURRENT

Typical Earth stake to Earth

resistance = 30 - 2k

Main Switch

ConsumerMains

Load

Low R

MAIN SWITCHBOARD

MENConnection



FAULT CURRENTMain Switch

Load

A Low Resistance earth CREATES

A high fault current.

Low R

MAIN SWITCHBOARD

MENConnection


ConsumerMains


What are the values given by AS3000on earth resistance?

Main Switch

Load

MAIN SWITCHBOARD

MENConnection


ConsumerMains


2 maximum2 maximum

Main Switch

Load

MAIN SWITCHBOARD

MENConnection


ConsumerMains


Main Switch

< 0.5(6.3.3.2.2)

“The resistance of the protective earthing conductors shall be

low enough to permit the passage of current

necessary to operate the overcurrent protective device”

(6.3.3.2.2)

Load

133133

Why TestWhy Test

134134

MAIN SWITCHBOARD


Main

Sw

itch


LoadOPEN CIRCUIT

MENConnectio

n

135135

MAIN SWITCHBOARD


Main

Sw

itch


NORMALLOAD

CURRENT

Everything operates OK!!!

Load

136136

THEREFORE FAULT CURRENT WILL BE

VERY LOW

MAIN SWITCHBOARD

MENConnection


Main

Sw

itch


FAULT CURRENT

ButRESISTANCE TO

EARTH IS USUALLY HIGH.

Load

137137

MAIN SWITCHBOARD

MENConnection


Main

Sw

itch


FAULT CURRENT

AND PROTECTION WILL NOT TRIP.

Load

138138

MAIN SWITCHBOARD

MENConnection


Main

Sw

itch


FAULT CURRENT

NOTE THAT IF ACTIVE IS SHORTED TO EARTH,ALL EARTHS ARE LIVE!!!

Load

139139

MAIN SWITCHBOARD

MENConnection


Main

Sw

itch


FAULT CURRENT

NOTE ALSO THAT UNDER NORMAL CONDITIONSEVERYTHING ELSE WILL STILL WORK OK!!!

Load

140140

Open CircuitMEN

Connection

MAIN SWITCHBOARD


Neutral Link

Sub Mains

Sub Mains

Neutral Link

DISTRIBUTION BOARD 3

CircuitProtective

Devices

Earthing Bar



Neutral Link

Earthing Bar

Main Earthing

Conductor

Sub Mains


Neutral Link

Earthing Bar


Main Switch

What happens on Distribution Boards when the MSB MEN link open-circuits?

AS30005.6.6b(iv)

141141

What happens when What happens when incoming Active and incoming Active and

Neutral are swapped?Neutral are swapped?

142142

MAIN SWITCHBOARD

MENConnectio

n


Main

Sw

itch

ConsumerMains


A N

LIVEN UP: -Earth stake-Water pipes-Taps-Sink-Cases of appliances

Load

143143

MAIN SWITCHBOARD


Neutral Link

Sub Mains

Sub Mains

`Neutral Link


CircuitProtective

Devices

Earthing Bar



Neutral Link

Earthing Bar

Main Earthing

Conductor

Sub Mains


Neutral Link

Earthing Bar


Main Switch

What happens on Distribution Boards when Main Active and Main Neutral

are swapped?

NA

144144

What happens when What happens when the Main Neutralthe Main Neutral

goes Open Circuit?goes Open Circuit?

145145

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains

Earth LinkNeutral LinkOPEN CIRCUITNeutral

Load

146146

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


LOAD CURRENT

Load

147147

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


LOAD CURRENT

Load

148148

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


Lo load Resistance

Hi stake - earthResistance:30 - 2k

High Voltage

Low Voltage

High Voltage on Earth System

Load

Voltages...

149149

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


Lo load Resistance

Hi stake - earthResistance:30 - 2k

High Voltage

Low Voltage

High Voltage on Earth System

Livens:-Taps,-Sinks-Water pipes-Metal cases of appliances

Load

Voltages...

150150

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


AllLoads

Q: What causes “tingles” on taps?

151151

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


AllLoads


VDN=4V

152152

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


AllLoads


4V

0V

TEST EQUIPMENT TEST EQUIPMENT REQUIRED REQUIRED (AS3017 1.6.2)(AS3017 1.6.2)

• High Voltage Insulation Resistance Tester• Low reading ohmmeter (0.5-5) • Voltage Present Indicator• Trailing lead (of known resistance).• Fault Loop Impedance tester• RCD Tester

TEST EQUIPMENT TEST EQUIPMENT REQUIRED REQUIRED (AS3017 1.6.2)(AS3017 1.6.2)

• Test for dead device.– Category 1: electronic– Category 2: Single Phase boards– Category 3: Three Phase DB / MSB– Category 4: Three Phase power lines

TESTSTESTS

• Visual• Earth Continuity• Insulation Test• Polarity• Correct circuit connections• Fault Loop Impedance check• RCD test

1. General

2. Consumer Mains

3. Switchboards

4. Wiring Systems

5. Electrical Equipment

6. Earthing

Step 1: Visual Examination

158

General RequirementsClause No. What to look for

2.9.6 No exposed live parts. E.g. no

excessive removal of insulation at terminations, terminal covers in place etc.Double insulation maintained where required. E.g. no single insulation in ceiling above light fittings, no more than 100mm single insulation in wall

behind accessories, insulating shrouds installed where required.1.9

All equipment is approved/compliant with Australian Standards and in good condition. E.g. no unsafe/damaged or non-compliant equipment installed.

159

Consumer MainsClause No. What to look for

3.4.1 Consumers Mains should be able to carry the maximum demand current of the installation with some capacity to spare. As a guide: 16mm2 (or parallel 6mm2) for overhead mains, 10mm2 for underground mains.

160

SwitchboardClause No. What to look for

Switchboard is in a suitable location. E.g. At appropriate height2.4 Correct over-current protection is installed. E.g. Correct circuit breaker ratings for each sub-circuit conductor and fault level at switchboard.

Main switch/es is/are appropriate for installation. E.g. current raringNeutral bars/links marked for identification. E.g. Main N/L, RCD N/L

Switchboard wiring correctly fixed. E.g. to hinged panel and at back of board to minimise flexing at terminations.Consistency of switchboard layout and marking. E.g. Correct marking of main switch/es, circuit breakers and corresponding neutral link connections.

161

Wiring SystemsClause No. What to look for

Correct conductor sizes. E.g. adequate current carrying capacity for circuit/load, 2.5mm2 minimum for socket outlets etc.Adequate support and fixing of cables where required. E.g. surface wiring.

Wiring fixed or passing through holes within 50mm of underside of floor is protected by RCD or mechanical protection.Wiring fixed or passing through holes within 50mm of ceiling material or ceiling fixing support is protected of by RCD or mechanical protection.

162

Wiring fixed or passing through holes within 50mm of underside of roof material is protected by RCD or mechanical protection.Wiring fixed or passing through holes within 50mm of surface of wall is protected by RCD or mechanical protection if outside nominated ‘zones’.

Wiring in location where it is ‘deemed likely to be disturbed is fixed to prevent undue sagging.Single insulated wiring is enclosed in conduit/trunking or junction box, except for switchboard wiring and in wall cavities for up to 100mm behind accessories at which the wiring is terminated.

Connection and terminations are correctly made. E.g. stranded conductors twisted, single conductors doubled back, no excess insulation removed, damaged insulation reinstated, no undue mechanical stress on any connection.

163

Electrical EquipmentClause No. What to look for

4.3.11 Stove circuit has a ‘functional switch’ installed in appropriate location.Equipment and accessories installed in accordance with safe and sound practice. E.g. accessible for operation, adequately fixed and supported.

Equipment is suitable for the conditions to which it is likely to be exposed. E.g. weatherproof fittings on external walls.

164

EarthingClause No. What to look for

MEN link correctly installed.MEN link is correct size.

2.9.4.4 MEN link terminated at extremity of Main Neutral Link and Main Neutral terminated in next adjacent terminal or both clearly marked.Correct location of Earth Electrode. i.e. outside, exposed to the weather.

Correct termination and protection of Main Earth at electrode.Correct size of Main Earth and Protective Earthing Conductors.

Correct size Equipotential Bonding Conductor.5.5.4.2 Earth conductors protected against ‘becoming displaced, damaged or cut’ as appropriate to ‘expected conditions’.

All earth conductor terminations comply with clause 3.7.Correct use of 1 or 2 screw terminal connections as required.

Initial Procedures

• Open all Main Switches.

• Turn C/B’s off / remove fuse wedges.

• Disconnect the Main Neutral, and Main Earth from the Neutral link.

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains

Earth Link

Neutral Link

Light

• From Main Earth to Earth Electrode: <0.5• Make sure all FSC earth resistance values

are under the value of table 3.2 of AS 3017.

• OR• Make sure all FSC are under the maximum

length allowable and have good continuity…

STEP 2: Earth Continuity

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains

Earth Link

Neutral Link

Light

Known resistance

STEP 3: Insulation Resistance

• Test between Main Earth and• 1. Main Active with main switch ON.• 2. Main Neutral

• Result: >1M

1. Consumer Mains:

Disconnect any service bonding conductor:


• Test between Main Earth at Switchboard and:

• The Active Conductors of circuits which require testing:>1M

• The Neutral Conductors of circuits which require testing: >1M

2. Final Subcircuits:

With all C/B’s ON or fuse wedges IN, and all switches in the installation ON:

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


Light

MAIN SWITCHBOARD


Main

Sw

itch

ConsumerMains


Light

Switched Active


• Where heater elements are tested: >10k

• Checks:• 1. Polarity of mains or submains.• 2. C/B’s and single pole switches operate

in Active.

• 3. Polarity socket outlets

STEP 4: Polarity Tests

STEP 5: Correct Circuit Connections

• Check that under normal operation earths do not carry current.

• There is no interconnection of conductors between different circuits.

MEN Connection

MAIN SWITCHBOARD


Neutral Link

Sub Mains

Sub Mains

`Neutral Link


CircuitProtective

Devices

Earthing Bar



Neutral Link

Earthing Bar

Sub Mains


Neutral Link

Earthing Bar


Main Switch

AN

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