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Topic Title: PROTECTIVE CONDUCTORS
Topic Summary: Cross-sectional areas 543.1.1
Created On: 10 December 2012 10:15 PM
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 10 December 2012 10:15 PM
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andrew331

Posts: 110
Joined: 21 December 2008

543.1.1 is telling us to (i) calculate using the adiabatic equation or to (ii) select using Table 54.7.
It goes on to tell us that calculation is necessary '.....if the choice of line conductors has been determined by considerations of short-circuit current...'
I am not sure what is meant by 'determined by considerations of short-circuit current'
Can someone explain this please?
 10 December 2012 10:43 PM
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OMS

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Joined: 23 March 2004

Generally what it says on the tin - if you've selected a cable based on short circuit withstand - and the earth fault current is lower, then you need to test if the CPC is going to hold against the circuit protective device selected on the short circuit withstand.

Effectively, what it means is that you could end up with quite small line conductors based on short circuit currents determined adiabatically (say a motor circuit with seperate overload protection) - if you then pick a CPC based on the line conductor size and you have a lower earth fault current, you'll have trouble with the CPC (and line conductor) getting pretty damn hot

Regards

OMS

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Failure is always an option
 11 December 2012 11:18 AM
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andrew331

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Thanks OMS.

Nearly all my work for the past 25 years has been domestic, so mostly 'standard' circuits, twin and earth,and not much design, so please bear with me.

If 'designing' rather than using a 'standard' circuit I would start by considering the load, installation method, external influences, and maybe voltage drop, and then selecting a cable type and size. At this stage I would not be thinking about fault currents.

I would then move on to selecting protective devices. Now I would be thinking about fault currents, the adiabatic equation and checking the cpc size, but the line conductor csa would already have been determined.

But, the wording, in 543.1.1, '...if the choice of cross-sectional area of line conductors has been determined by consideration of short circuit current...', seems to suggest a design where fault current is the crux of the cable selection.

I understand what you are saying with your motor example but would this be a case of selecting a cable to suit the cpd characteristics rather than the other way round? If so is this something often done in industrial installations?

Andrew
 11 December 2012 12:20 PM
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Parsley

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Originally posted by: andrew331

Thanks OMS.



Nearly all my work for the past 25 years has been domestic, so mostly 'standard' circuits, twin and earth,and not much design, so please bear with me.



If 'designing' rather than using a 'standard' circuit I would start by considering the load, installation method, external influences, and maybe voltage drop, and then selecting a cable type and size. At this stage I would not be thinking about fault currents.



I would then move on to selecting protective devices. Now I would be thinking about fault currents, the adiabatic equation and checking the cpc size, but the line conductor csa would already have been determined.



But, the wording, in 543.1.1, '...if the choice of cross-sectional area of line conductors has been determined by consideration of short circuit current...', seems to suggest a design where fault current is the crux of the cable selection.



I understand what you are saying with your motor example but would this be a case of selecting a cable to suit the cpd characteristics rather than the other way round? If so is this something often done in industrial installations?



Andrew


If you've got circuit breakers as protective devices, high fault current and disconnection in less than 0.1 secs you need to refer to manufactuers energy let through data (I2t) to ensure the CPC'S K2S2 is greater than the I2t see 434.5.2. so you may need a larger CSA.

Regards
 11 December 2012 01:44 PM
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OMS

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Originally posted by: andrew331

Thanks OMS.



Nearly all my work for the past 25 years has been domestic, so mostly 'standard' circuits, twin and earth,and not much design, so please bear with me.

Although as you are using reduced CPC's then you would be checking the CPC withstand anyway wouldn't you

If 'designing' rather than using a 'standard' circuit I would start by considering the load, installation method, external influences, and maybe voltage drop, and then selecting a cable type and size. At this stage I would not be thinking about fault currents.

OK - its iterative, and there is a degree of interdependence - you can't change the CPC size on an armoured for example - it's fixed by the cable CSA and number of cores you select - so if the earth fault dominates, you could end up with a line size bigger than you need.

typically, you would go through the sequence in roughly the order below - but it can and does change depending on the scheme stage.

First establish Ib, the deisgn current and select a probable cable type to reflect the installation

Determine what factors of influence you have - temperature grouping, insulation, buried etc etc

Determine the probable CPD type

Determine the minimum conductor CSA for the load

Test that for voltage drop

Estimate the limiting Zs

Test that against earth fault current withstand

Check the cable is OK for short circuit faults


I would then move on to selecting protective devices. Now I would be thinking about fault currents, the adiabatic equation and checking the cpc size, but the line conductor csa would already have been determined.

It may well have - but earth fault also flows in the line conductor so you are actually checking both line and CPC


But, the wording, in 543.1.1, '...if the choice of cross-sectional area of line conductors has been determined by consideration of short circuit current...', seems to suggest a design where fault current is the crux of the cable selection.

Yup - if I was looking at cables coming from a large high fault current source over a long distance but with low load (say a ring main in a hospital) then I m,ay well check the cable short circuit withstand first and then test the CSA against load - short circuit may dominate.

I understand what you are saying with your motor example but would this be a case of selecting a cable to suit the cpd characteristics rather than the other way round? If so is this something often done in industrial installations?

For sure - if you need a type D breakler or a MCCB with an adjustable short time pick up under fault to deal with aggressive loads then you have to pick a cable to reflect that - load and voltage drop may be irrelevant at that point.

Andrew


Quick example:

single phase circuit feeding fixed equipment, 30m run, 4.0mm2 cable selected, PVC insulated 3 core flexible. Aggresive starting load (sump motor perhaps) needs a 63A BS 88 fuse. PSCC at DB is 8kA, EFLI 0.45 Ohm. The fuse offers short circuit an indirect contact protection

OK

Circuit conductor impedance is 11.06/1000 x 30 = 0.332 Ohm

Impedance L-N at DB is 230/8000 = 0.029 Ohm

Fault current Short Circuit = 230/)0.332 + ).029) = 637A

Checking against the fuse characteristics disconnection is about 0.17 seconds

Checking adiabatically - t = K2S2/I2 = 0.52 seconds

So we are adequately protected for short circuit

For earth fault, I = 230/(0.45 + 0.332) = 294A

Disconnection time is about 4 seconds

Testing adiabatically, t = k2S2/I2 = 2.45 seconds

Obviously, the circuit is not thermally protected under earth fault even though it complies with table 54G !!

Does that help ?

Regards

OMS

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Failure is always an option
 11 December 2012 03:14 PM
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andrew331

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Great help, thanks for your time.
 11 December 2012 03:48 PM
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OMS

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No worries -

OMS

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Failure is always an option
 26 September 2013 03:33 PM
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ccte5

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Hi OMS,

I have some question regards your quick sample when checking the thermal constraint of CPC.
Why not we use the PSCC 8kA as the short circuit current and put into the adiabatic equation? (this would result in bigger size of CPC)
How about for Three phase supply at 415V. If three phase short to earth what should we consider in term of earth fault loop impedance?

I'm confuse to sizing up the CPC by using the adiabatic equation, because some handbook using the same method as you shown.

Some other use the ultimate short circuit current (start from supply source up to the design point) to size up the CPC by checking the let through energy of protective device is smaller than the thermal constraint of CPC I2t<S2k2

Please help to advice me. is it both also can be use under IEE wiring regulation?

Thank you very much

Best Regards

Edited: 26 September 2013 at 04:12 PM by ccte5
 26 September 2013 05:15 PM
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OMS

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Originally posted by: ccte5

Hi OMS,



I have some question regards your quick sample when checking the thermal constraint of CPC.

OK
Why not we use the PSCC 8kA as the short circuit current and put into the adiabatic equation? (this would result in bigger size of CPC)

But 8kA is not the earth fault current it's the short circuit current. I gave an EFLI value that allowed you to determine the much smaller earth fault current

How about for Three phase supply at 415V. If three phase short to earth what should we consider in term of earth fault loop impedance?

OK - if you've shorted the 3 phases and then put them to earth, you have an indeterminate short circuit condition but you could estimate that the short circuit current is 8kA x 1.73 and that the earth fault current hasn't increased, but now current is being shared. If it's just one phase of the system to earth, then the EFLI reflects that scenario

I'm confuse to sizing up the CPC by using the adiabatic equation, because some handbook using the same method as you shown.

Which bit are you confused about - if you have an EFLI value or an earth fault current, all you are comparing is energy I2t (ampere^2 seconds) with cable withstand K2S2 (again in ampere^2 seconds)

Some other use the ultimate short circuit current (start from supply source up to the design point) to size up the CPC by checking the let through energy of protective device is smaller than the thermal constraint of CPC I2t<S2k2

Yes, but all you are doing is determining the fault energy at the relevant point and assuming back up protection is operating - effectively, you are substituting fault current and time for the stated I2t at that point in the system - and potentially you are ignoring discrimination within the system


Please help to advice me. is it both also can be use under IEE wiring regulation?

Both methods can be use, but you would be examining different aspects with either method.

Thank you very much

Best Regards


Regards

OMS

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Failure is always an option
 27 September 2013 02:36 AM
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gnuh

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I came across a very funny argument that an engineer asking me to use came material of cable insulation type of the circuit as CPC. For eg. I'm i'm using XLPE/PVC cable as the circuit conductor, my CPC shall be XLPE/PVC type as well, and If i'm using FR cable as the circuit conductor, the CPC shall be FR as well.

As far as I know all CPC cables are Pvc insulated in green and yellow. I cannot think of any reason supporting the engineer argument.
 29 September 2013 02:17 PM
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ccte5

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Hi OMS,

Thank for your time. I still need your further advice as follow by quoting your quick example :

Quick example:



single phase circuit feeding fixed equipment, 30m run, 4.0mm2 cable selected, PVC insulated 3 core flexible. Aggresive starting load (sump motor perhaps) needs a 63A BS 88 fuse. PSCC at DB is 8kA, EFLI 0.45 Ohm. The fuse offers short circuit an indirect contact protection



OK



Circuit conductor impedance is 11.06/1000 x 30 = 0.332 Ohm



Impedance L-N at DB is 230/8000 = 0.029 Ohm



Fault current Short Circuit = 230/)0.332 + ).029) = 637A

The fault current value is calculate at load side. Why don't we use the 8kA to size and check the cable size at the DB side? because I feel that is much more safer to take precaution for the fault happen close to the DB(cable short at the output of DB). Please advice me is it both method also acceptable (one consider Short circuit current at the load side or consider Short circuit current at the source side?)and comply to IEE regulation. (I prefer to use your method because CPC conductor size can be much more smaller)

Checking against the fuse characteristics disconnection is about 0.17 seconds



Checking adiabatically - t = K2S2/I2 = 0.52 seconds



So we are adequately protected for short circuit



For earth fault, I = 230/(0.45 + 0.332) = 294A

Why don't we use the short circuit current of 637A as earth fault current as well? because is single phase, L short to E isn't it same as short circuit? In my mind the short circuit current for single phase is mean earth fault as well.


Disconnection time is about 4 seconds



Testing adiabatically, t = k2S2/I2 = 2.45 seconds



Obviously, the circuit is not thermally protected under earth fault even though it complies with table 54G !!



Does that help ?


Regards

OMS



Beside above, May i know what is the 543.1.2 trying to tell us.

543-01-02 Where a protective conductor is common to several circuits, the cross-sectional area of the protective conductor shall be:

(i) Calculated in accordance with Regulation 543-.1-03 for the most onerous fo the values of fault current and operating time encountered in each of the various circuits, or

(ii) Selected in accordance with Regulation 543-01-04 so as to correspond to the cross-sectional area of the largest phase conductor for the circuit.

After I read for the above. I feel that the (TT or TNS system) CPC conductor which serve from Main Switch Board to DB can be just follow the largest conductor which I have at the outgoing circuit of the DB with refer to table 54G.
Example, A 40A TPN DB is serve from 100A TPN MSB with cable size of 4x10mm2 PVC/PVC + 4mm2 PVC/PVC (E) since the largest phase conductor at the 40A TPN DB outgoing is 4mm2 PVC/PVC

Please help to advice me am I wrong?



Thank you very much.

Best Regards,
ccte

Edited: 29 September 2013 at 02:27 PM by ccte5
 30 September 2013 04:46 AM
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gnuh

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Hi CCTE5,

According to Clause 543.1.2, you can either determine the fault current and operating time as per formula given in clause 543.1.3 or you may just refer to clause 543.1.4 (table)
 30 September 2013 10:20 AM
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AJJewsbury

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The fault current value is calculate at load side. Why don't we use the 8kA to size and check the cable size at the DB side? because I feel that is much more safer to take precaution for the fault happen close to the DB(cable short at the output of DB).

In theory you should check both ends (and indeed anywhere in the middle) for the worst-case situation and base your design on that. In practice, it's known that the worst case energy let-through for standard fuses is at the lowest fault current - i.e. at the load end (work out a few examples if you like). For Circuit Breakers the situation is different and the overall energy let-though typically increases with fault current - so worst case is typically at the supply end - although you still need to check Zs at the load end to verify that sufficient current will flow to cause disconnection in the required time (typically by magnetic operation of the MCB).

As far as I know all CPC cables are Pvc insulated in green and yellow. I cannot think of any reason supporting the engineer argument.

Non PVC greeen/yellow is available - e.g. http://www.drakauk.com/products/saffire/saffire1.php
While different insulation will affect the current carrying capacity of the conductor, that'll make no difference to a c.p.c. as it's use will normally be adiabatic (i.e. doesn't depend on being able to loose heat from the conductor during use), so depends on the conductor material, c.s.a. and initial & final temperatures alone. Using a PVC c.p.c. with a non-PVC live conductors could cause some problems though:

1. The initial temperature of the c.p.c. could be higher than the tables for PVC insulated c.p.c. would presume - e.g. might be 90-degrees rather than 70-degrees.
2. The final temperature would still be limited by PVC's characteristics (e.g. 160-degrees rather than 250 degrees that might be accepted for thermosetting insulation).
3. Heat from the live conductors (especially during a fault) might exceed what the PVC insulated c.p.c. could withstand.

None of that would necessarily prevent you using a 70-degree PVC c.p.c. alongside (say) 90-degree live conductors, but you'd have to design out a number of possible problems - all in all it's probably easier just to use the same type throughout.

Plus of course, if you're using non-PVC live conductors for their fire performance characteristics (e.g. low smoke, low halogen etc) - then you're defeating that by drawing in a ordinary PVC conductor alongside them (or indeed putting them in PVC trunking or conduit).

- Andy.
 02 October 2013 04:44 AM
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gnuh

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As far as I know all CPC cables are Pvc insulated in green and yellow. I cannot think of any reason supporting the engineer argument.


Non PVC greeen/yellow is available - e.g. Link removed/products/saffire/saffire1.php

While different insulation will affect the current carrying capacity of the conductor, that'll make no difference to a c.p.c. as it's use will normally be adiabatic (i.e. doesn't depend on being able to loose heat from the conductor during use), so depends on the conductor material, c.s.a. and initial & final temperatures alone. Using a PVC c.p.c. with a non-PVC live conductors could cause some problems though:



1. The initial temperature of the c.p.c. could be higher than the tables for PVC insulated c.p.c. would presume - e.g. might be 90-degrees rather than 70-degrees.

2. The final temperature would still be limited by PVC's characteristics (e.g. 160-degrees rather than 250 degrees that might be accepted for thermosetting insulation).

3. Heat from the live conductors (especially during a fault) might exceed what the PVC insulated c.p.c. could withstand.



None of that would necessarily prevent you using a 70-degree PVC c.p.c. alongside (say) 90-degree live conductors, but you'd have to design out a number of possible problems - all in all it's probably easier just to use the same type throughout.



Plus of course, if you're using non-PVC live conductors for their fire performance characteristics (e.g. low smoke, low halogen etc) - then you're defeating that by drawing in a ordinary PVC conductor alongside them (or indeed putting them in PVC trunking or conduit).



- Andy.


Hi,

thanks Andy. But aren't CPC don't carrying any current during ordinary operation? So should I use pvc insulated CPC along with XLPE/PVC phase and neutral conductor?
 02 October 2013 09:11 AM
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AJJewsbury

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But aren't CPC don't carrying any current during ordinary operation? So should I use pvc insulated CPC along with XLPE/PVC phase and neutral conductor?

I'd say you should use the same insulation on the c.p.c. (e.g. XLPE or PVC) as on the live conductors, unless you can show that no problems (e.g. as listed above) can occur.
- Andy.
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