What fault condition are you trying to prevent ? If the load is internally fused, it is only if there is cable damage that the upstream supply breaker should operate. For a general sort of circuit length, almost any breaker will protect a 10A mains flex against a dead sort. Or have I misunderstood.

Thanks for your reply Mike.


I am trying to protect for any faults between the Commando socket and IEC C13 inlet i.e. the lead, which will be 20m in length.  I will most likely specify H07 cable.


It's going to be in a lab so unlikely to be subject to any rough handling.

It sounds to me no worse than being plugged into a domestic outlet in most places other than the UK.

Most other countries have domestic outlets on 16 amp or 20 amp circuits, no fuses in the plugs, and millions of appliances with 10 amp connectors are used.

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.

Thanks again Mike for your detailed reply.


The excess cable will be managed and cable guards used where it is likely to be walked over so, there is minimal risk of damage to the cable.  The only risk might be if the cable hasn't been terminated properly on the plug and socket at either end of the cable however, a direct short should trip the upstream overcurrent device as you say.


I do intend to specify an RCD as I am not sure if the Commando socket is protected by one.


Do you know if there is a latching inline version of the one that you linked to?

Generally, like RCD plugs, the in-line RCDs tend to be non-latching, so the mechanism is tested each time it is unplugged and re-plugged.

The only latching versions I have seen in the UK, but it is far from exhaustive, have been theatrical distro boxes and similar items ( like this ) that are a re-purposing of a fixed wiring RCD .

Provided the commando plug is a 16 A one, and the socket-outlet it is to be connected to is protected by a 16 or 20 A device, then there's definitely not a problem with flexible cable 1.0 mm² and above.


The only thing that I would caution is that the cable at 20 m is rather long ... I'd be inclined to use 1.5 mm² to avoid volt-drop. Rewirable connectors to BS EN 60320 should be suitable for flexible cables up to 1.5 mm² according to the standard.


As you stated, H07RN-F is probably a good choice.