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Choosing best rating circuit breaker to protect 16A commando socket

Alan B

I was installing a 16A commando socket today which I protected with a 30mA rcd followed by a 16A MCB.

Other breakers nearby were not heavily loaded so derating was not an issue. But if I say wanted to connect to a heavily loaded consumer unit my initial reaction would be to put in something like a 20A breaker and suitable cable.
But then I thought the socket and plugs are not fused and I have no control over the cable connected to the socket. Therefore do I have to assume I am protecting a cable that is only rated at 16A and never use a breaker with a rating that exceeds this?

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  • 0

    Follow-on question: If I've got a fixed 230:110V transformer feeding a 16A tool socket, am I right in thinking the MCB feeding the transformer will need to be rated at about 8A? 

  • 0 in reply to Stephane English

    yes, transformer action means that as voltage rises by the turns ratio the current falls. but actually, you will want rather more than that to handle the inrush. so it may need to be C or even D type ane probably 10 amps or more.
    Mike.

  • 0 in reply to Stephane English

    Follow-on question: If I've got a fixed 230:110V transformer feeding a 16A tool socket, am I right in thinking the MCB feeding the transformer will need to be rated at about 8A? 

    Well the transformer should draw around 8A steady state at 230V ... but an MCB of that rating would be problematic ... partly because they don't make 8A MCBs but mostly because steady-state is only happens once things have settled down. Most appliances draw rather more than their rated current when they start up (filament lamps and heaters have a lower resistance when cold, motors need extra power to accelerate to their working speed, capacitors need current to charge if switched on when the a.c. cycle isn't at zero, likewise inductors). That's why you'd often see 13A fuses supplied with fridges/freezers even though they're only rated at a couple of hundred watts. It's not that protective devices open instantly the moment the current exceeds their rating even by a fraction - but there is a trade-off between the size of the overload and the time the protective device will remain operational. Small overload will take some time to trip, but very large currents will open it much more quicky.   Transformers have especially large starting currents as they establish their magnetic fields (plus any starting currents from downstream loads if they happen to be connected) often many many times their running current, so can easily trip circuit breakers rated for only the running current. With MCBs some compensation can be had by selecting a higher "type" - e.g. D type rather than B or C, but still some care is needed to co-ordinate things properly.

    More generally, there's usually little need to tie the rating of protective device too tightly to the size of the load. The protective device is mostly there to protect the conductors feeding the appliance, rather than the appliance itself (unless the manufacturer specifies otherwise). For instance a 16A MCB feeding 20A cables feeding a 1A load is usually perfectly fine.

       - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    they don't make 8A MCBs

    FWIW, Eaton do.

    https://datasheet.eaton.com/datasheet.php?model=EMBH108&locale=en_GB

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