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Arrange circuits, control
and
protection
for
general electrical installations
Week 3: Prospective Fault Current and Short Circuit Temperature Rise
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All writing in BLUE is examinable
All writing in RED is
NOT examinable.
Slide 2 of 41
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Prospective Fault Current
To cover:
1) What is it?
2) How to calculate at terminals of Tx:
3) How to calculate at MSB:
4) How to calculate at DB:
Slide 3 of 41
FLI worst case, not enough I.PFC worst case, too much I. Both Ω’s law
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Prospective Fault Currents (PFC) (also
known as prospective short-circuit current) is a calculation or measurement ofhow high thecurrent could be inthe event of ashort circuit.
Reference: 2.5.4AS/NZS 3000
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When an arc is initiated, the live conductors are in close enough proximity to create sparking. The arc itself is caused by uncontrolled conduction of electrical current from phase to phase, or from phase to earth/neutral, and this ionises the surrounding air.
When conductive metal is vaporized, a pressure wave develops. A phase to phase, or phase to earth/neutral arc fault can escalate into a three phase arc within a millisecond. The heat energy and intense light produced at this stage is known as the arc flash.
Reference NHP Technical newsletter
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The energy released as a transformer dissipates it’s magnetic field energy can cause enough heat to instantly vaporise copper.
(Heated so hot it bypassesthe liquid phase and goesstraight to a gas causinga fatal explosion)
Slide 6 of 41
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Slide 7 of 41
H= I2 R t
The heat generated (in Joules) is to the square of the current times the resistance times the time it is there.
Double the current, quadruple the heating effect
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If we put in adequately rated Circuit Breakers and/or Fault Current Limiters (HRC fuses) we can interrupt fault currents before the switchboards explodes
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Common size CBs1.5 kA3 kA4.5 kA6 kA9 kA
A 6 kA breaker can interrupt faults up to 6000A
Cheap
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The fault current must be interruptedhere before it is reaches it’s maximum.
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Slide 11 of 41
3 Feb 2015 fatality in Western Australia
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EnergySafety (WA) has imposed new safety precautions for the type of high-voltage (HV) switches involved in the 3 February Morley Galleria Shopping Centre explosion.
The HV oil-insulated combined-fuse switches produced by Long & Crawford Manchester must be completely disconnected from the electricity supply before any person may open the switch’s lid.
Announcing this today the Director of EnergySafety, Ken Bowron said that the Morley Galleria accident wasstark evidence of the inherent danger associated with these switches.”
Slide 12 of 41
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Transformers (Tx) have a high PFC
As do batteries,however Solarpanels don’thave high PFCs
A commonIndustrial1 MVA Tx willlikely have a PFC close to 30 kA
Slide 13 of 41
A fault like 3000 10A welders
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0.01Ω
400V0.02Ω
=20kA230V0.01Ω
=23kA
PFC is highest phase to Earth (when at close to the terminals of the transformer)
230V, 0.01Ω0.01Ω
0.01Ω400V, 0.02Ω
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0.01Ω
400V0.04Ω
=10kA230V0.03Ω
=7.67kA
PFC is highest phase to phase (when far from the transformer)
230V, 0.03Ω0.01Ω
0.01Ω
400V,0.04Ω
0.01Ω
0.01Ω
0.01Ω
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Slide 16 of 41
The PFC at at the terminals of a TX (I
PFCTX)
Can be calculated
(or measured live using a Fault Loop Impedance tester)
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To calculate IPFCTX
The kVA rating, voltage and % impedance of the TX must be know.
Line current in a three phase system can be calculated using the formulaP = √3 V I (Using line V and I: P = √3 V
L I
L)
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PT – Transformer kVA rating
VL – Line voltage (400 volts)
IL – Line current
PT=√3VLILPT
√3×VL=√3×VL×IL√3×VL
IL=PT
√3×VL
IL=250,000
√3×400V(250KVATX)
=360.85A
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Transformer name plates are stamped with the % Impedance which was measured using the following test...
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At 1%of 6.35 kV=63.5V=90.2 AAt 2%of 6.35kV=127.04V=180.4 AAt 3%of 6.35kV=190.5V=270.6 AAt 4%V6.35kV=254V=360.8A
This is a 4% impedance TX
Full line current
V ADead short in secondary with Ammeter
VariableVoltageSupply
Tx360.8 A per phase at 6.35KV−11kV phase−phase
VL=VP×√3
VP=VL√3
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If a fault causes 360A to flow with only 4% of the voltage, what will 100% of the supply voltage cause?
360A×100%4%
=9kA
( 3600.04
=9kA)
Will also give you the same answer
IPFCTX=IL×100%% impTx
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Slide 22 of 41
TXCM
PEN
MENP/E
MSBSM DB
Conductive pipes
Building Materials
Ground
The return path could be any number of parallel paths to earth
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TXCM
0Ω
MSBSM DB
Therefore we take a conservative approach and say that the return path = 0Ω (Estimating a low resistance means we allow for more current and put in better circuit protection devices)
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In the ACT:30,000A in commercial 10,000A in domestic 3Φ 6,000A in domestic 1Φ
SM – Sub mainCM – Consumer mainTX – transformerMSB – Main switch board
(Yet another application of ohms law)
ZTX=Volts (230V)
Perspective fault Current
IPFCMSB=V
ZTX+ZCM
IPFCDB=V
ZTX+ZCM+ZSM
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CM MSB SM1 DB1
ZTX=230V30,000A IPFCMSB=
VZTX+ZCM
SM2
DB2
IPFCDB1=V
ZTX+ZCM+ZSM1
IPFCDB2=V
ZTX+ZCM+ZSM2
TX
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TXMSB DB1
DB2
0.002 0.02
0.04
Example
ZTX=23030,000
=0.00767
IPFCMSB=230
(0.00767+0.002)=23.78KA
IPFCDB2=230
(0.00767+0.002+0.04)=4.63kA
IPFCDB1=230
(0.00767+0.002+0.02)=7.75kA
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To find the values of Zcm, Zsm etc.Look up the value in Tables 34 to 39 AS/NZS 3008.
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Use worst case assuming cold copper
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ZCM=Table 34 0.00703Ω/kM÷1000×60m=0.0042Ω
Example:
What is the resistance of 60m of 300mm2 single core cable?
1) Look up value on Table 352) Divide by 1000 to change Ωs per km to Ωs per m.3) Times value by length
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Slide 31 of 41
Select appropriately rated CB
If the calculatedPFC is 4.8kA, Select the next
size up.
A CB or Fuse that is rated to interrupt the maximum (prospective) fault current that could be present at the SWB.
1.5 kA3 kA4.5 kA6 kA9 kA
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Cascading of CB’s may be used if the same brand of CB is installed upstream (at the MSB)
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Example: With a 25 kA CB upstream 6kA breakers can have a rupturing capacity of 14kA
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This is because both circuit breakers start to open together increasing the overall breaking capacity of the pair.
MSB DB
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Short Circuit Temperature Rise (SCTR)
To cover:
1) What is it?
2) How to Select cables
Slide 35 of 41
SCTR is “Prospective Fault Current for cables”
All currents caused by a short-circuit shall be interrupted before the temperature of the conductors reaches the permissible limit. (Cables must withstand fault current)
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(111 PVC) (143 XLPE)
I2 t<K2S2
Factor dependant on the material
Time CB to trip
CSA
“Let throughenergy”
“Energy the cable can withstand”
<
PFC
I2t is found off manufactures specifications
Reference 5.3 AS/NZS 3008
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(111 PVC) (143 XLPE)
I2 t<K2S2
Factor dependant on the material
Time CB to trip
CSA
“Let throughenergy”
“Energy the cable can withstand”
<
PFC
I2t is found off manufactures specifications
Reference 5.3 AS/NZS 3008
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Slide 38 of 41
Look up I2t
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Slide 39 of 41
CSA
40kA2S
10kA
20A CB
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Example X-90
Start 90° (table 1)Finish 250° (table 53)
K rating = 143 (table 52)
Look up k value using Tables 1, 53 and 52 AS/NZS 3008
Example V-90
Start 75° (table 1)Finish 160° (table 53)
K rating = 111 (table 52)
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I2 t<K2S2
Will a 2.5mm2 cable withstand temperature rise when subjected to a 10kA fault and protected by a 20A CB?
40kA2S(specs)
111(V-90)
2.5mm2
(CSA)
40k<111x 2.5
40k<77k
YES