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?

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

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

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

Great help, thanks for your time.

No worries -

OMS

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Failure is always an option