Firstly, ADS for safety of life needs to be by RCD or RCBO, so any fault causing 30mA or more to flow to earth will operate that. So we are now only interested in overload, so L-N loop, and no time limit other than that set by cable damage.

Assuming the commando socket is installed to BS767a, it can only be either one of a a group on a 20A circuit or less, or a single socket on a 16A circuit less,  the test is to ask is there a credible fault condition to cause the cable to draw more than it's rating, and overheat, but not enough to be smartly protected by the breaker or fuse?


Unless the voltage drop is very high so the installation is already marginal, I do not think so.  Again if to BS 7671, at the socket the PSSC will be no less than perhaps 300A (or there would be be volt drop issues, so we can assume less than perhaps 1 ohm external L-N loop, probably a lot less.)

your 20m of (assume worst case 1mm2) cold will be 16milliohms per metre per core (scale for other cross-sections) more like 18 when hot.

Hot is worst case, so 20* 36millioms= 0.64 ohms, (There and back!) for the flexible cable.

So total worst case loop is say 1.64 oms, for any commando socket that is installed to BS7671, and almost all sockets on the planet will be lower impedance, so higher L-N PSSC, so an L-N short at the load end of the cable will draw (230/1.64) 140 A. No problem tripping a 16, 20 or 32A breaker. Maybe not always on the near instant magnetic part, but within 10 seconds, and the cable will not even blink in terms of temperature rise. Higher short circuit currents just operate the trip or fuse faster, so the cable is still protected.


So long as the socket you plug into is wired to BS7671, and the type of load cannot draw more current or has internal fusing, no further protection is required, at least for my example of a 1mm2 cable.

The only thing you may wish to add in-line, if the quality/ age of the commando socket wiring is not known to have one, is an in-line RCD near the commando plug.

example of the sort of thing.

Firstly, ADS for safety of life needs to be by RCD or RCBO, so any fault causing 30mA or more to flow to earth will operate that. So we are now only interested in overload, so L-N loop, and no time limit other than that set by cable damage.

Assuming the commando socket is installed to BS767a, it can only be either one of a a group on a 20A circuit or less, or a single socket on a 16A circuit less,  the test is to ask is there a credible fault condition to cause the cable to draw more than it's rating, and overheat, but not enough to be smartly protected by the breaker or fuse?


Unless the voltage drop is very high so the installation is already marginal, I do not think so.  Again if to BS 7671, at the socket the PSSC will be no less than perhaps 300A (or there would be be volt drop issues, so we can assume less than perhaps 1 ohm external L-N loop, probably a lot less.)

your 20m of (assume worst case 1mm2) cold will be 16milliohms per metre per core (scale for other cross-sections) more like 18 when hot.

Hot is worst case, so 20* 36millioms= 0.64 ohms, (There and back!) for the flexible cable.

So total worst case loop is say 1.64 oms, for any commando socket that is installed to BS7671, and almost all sockets on the planet will be lower impedance, so higher L-N PSSC, so an L-N short at the load end of the cable will draw (230/1.64) 140 A. No problem tripping a 16, 20 or 32A breaker. Maybe not always on the near instant magnetic part, but within 10 seconds, and the cable will not even blink in terms of temperature rise. Higher short circuit currents just operate the trip or fuse faster, so the cable is still protected.


So long as the socket you plug into is wired to BS7671, and the type of load cannot draw more current or has internal fusing, no further protection is required, at least for my example of a 1mm2 cable.

The only thing you may wish to add in-line, if the quality/ age of the commando socket wiring is not known to have one, is an in-line RCD near the commando plug.

example of the sort of thing.