Ok, so here's a challenge. Can you devise an electrical arrangement which is fully compliant with BS 7671:2018 (apart from, maybe, the fuzzy aspirations in Part 1), yet which is highly dangerous?
yes, at least in the sense of single fault to danger, rather than double fault or more as we are hoping to achieve.
In practice most of these are likely to be spotted by anyone competant and avoided, and require some sort of hapless user..
1 ) Consider a metal CU on a TT supply with outdoing RCBOs, and a simple main switch as incomer. How is the busbar protected, against a falling paperclip or a fault from case to a meter tail ?
Permitted apparently.
2) plug in a full legal length extension lead into a circuit with Zs already near the limit - an LN short at the far end just sets fire to the wiring, trip is not guaranteed.
3) IPXXD is not an adequate enclosure spec for trunking with single insulted wire inside - now you are at single fault to danger...
4) Mixing low impedance and high impedance earths - BS 7671 makes no distinction, but there is.
a) plug in any appliance via flex that has its own path to earth that is better than the DNO one, and watch the CPC suffer.
b) Consider a private genset or small LV transformer, feeding a metal frame outbuilding as a TT island, and some other location as TNS or PME.
Allow the outbuilding to have steel pile foundations, and an effective electrode resistance a small fraction of that of the electrode at the genset, and then have a live to building fault.
