Loop impedance and equipment cables

Duncan1980 · Jan 25, 2023

Two related questions in 1, if anyone is up for helping! (i) When electricians install supplies, is there anything in the regs that says how many milliohms they need to include for equipment cables? I have been told "no", that the loop impedance is only up to the outlet. Now I am an R&D engineer who routinely uses 5 metre lengths of 4 mm^2 SWA to 32 A equipment - which is several tens of milliohms. I was told by the electricians who installed the wiring that this would be OK, but only because they had been kind enough to include a large factor of safety on top of what the regs require, which according to them is absolutely nothing......but surely loop impedance calcs should not only include Ze external to the building, but also reasonable lengths of equipment cable connected to the outlets too? And for my second question.....this is hypothetical because we actually have an on-site transformer....but there must be lot of people up and down the land working in tin sheds with TN-S earthing, and therefore a max Ze of 0.8 ohms. How on earth to they run 32 A or even 63 A supplies, when you cannot get safe disconnection times with 0.8 ohms? I feel there is something seriously wrong with my understanding here!

timhoward · Jan 25, 2023

What actually matters in all these cases is the protective device, and whether enough fault current will flow to operate it quickly enough.
If you had a Ze of 0.8 ohms and a 32amp supply protected by a B32 breaker, then that leaves 0.3 ohms for the final circuit as the adjusted max loop impedance for a B32 breaker is 1.1 ohms.
230 / 1.1 = 209 amps which is enough to disconnect the supply within the 0.4 seconds for a TN earthed final circuit (there are graphs in the regs to show this)

If this isn't possible then using larger cables can help. In some circumstances (e.g. distribution circuits) considering different protective devices e.g. fuses can help.
Ultimately having exhausted everything else then RCD's can be used for fault protection and volt-drop becomes the most significant consideration.

Any circuit design should take into account how the supply will be used.
Does that help?