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Sizing of protective conductors - what am I missing here

Evening all. As ever I'm wrestling with a conundrum that I've hit a bit of a brick wall with. I could use a few pointers as to what I am missing/fill in some blanks.

Sizing of protective conductors:

If we take say a circuit wired in 1mm cable, protected by a suitable device, lets say a Type C 6A 60898. By selection from 54.7 we can use a 1mm cpc. From the design guide it is stated that if the current carrying capacity of the protective conductor is the same as the line conductor then the adiabatic equation will be satisfied (so long as the circuit has been designed for protection against overload) but if it is of reduced current capacity then we have to either select (54.7) or calculate (adiabatic/verify energy let through) to check the conductor is suitably protected. As a 1mm cpc is the same as the line conductor apparently no issues...

But...

Table B7 of the onsite guide states that the minimum CPC size for a class 3 energy limiting device Type C 16A or less, at a sub 3kA fault level is 1.5mm2. Presumably because the A2S according to BSEN60898 is permitted to exceed the K2S2 of a 1mm conductor for this device. I've checked through a number of manufacturers energy let through charts and indeed for a 6A Type C breaker at certain fault current levels (some around 2.0kA) - the I2T does exceed the 13225 K2S2 for a 1mm conductor. Surely though this would apply to a line-neutral fault as well?

So...

Why is it ok that potentially I could have a conductor with a K2S2 less than the I2T of the protective device in the case of a 1mm2/1mm2 cable just because it has been selected rather than calculated?

Furthermore... if that is truly ok - why (if fault current levels are at the aforementioned) does that 1mm conductor potentially become unsuitable if it is partnered with a 1.5mm2 line conductor instead?

I have a suspicion that the impedance/resistance of the line conductor (being the same size) is coming into play here and this will be the difference effecting the thermal stress on the protective conductor. But I could do with a bigger brain to point me in the right direction!

I hope that made sense! Thanks as ever for tolerating my questions!

0 David Stone over 3 years ago in reply to Scrambled

That is a good explanation, Mike. There is a problem in this area with both the regulations and the design guide, added to the additional complexity that a number of manufacturers circuit breakers are significantly faster (less energy let through) than the BS7671 graphs. It can lead to very significant overdesign that a zero-length (or very short) short circuit fault (although a limiting case) could produce adiabatic heating outside the normal overload limits, but this kind of very short transient heating does not generally have enough energy content to present a significant fire risk. It might be useful to have a discussion on this one day, particularly as cables are becoming very expensive, and the short circuit risk at the source is very low. Typical short circuit faults have a far from zero resistance as Mike said above, and very rarely take place other than at joints or due to severe mechanical accidents. It is probably impossible with T&E cables protected by RCDs, and I feel that the regulations should take these things into consideration, as should the design guide. I do not like theoretical "what ifs" without any evidence of outcome and circumstances, if so cars and even airplanes would probably be entirely mechanical, not fly or drive by electronics.
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Sizing of protective conductors - what am I missing here

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