Consumer unit

[Capt T]

Thanks - sounds like a box over the hole and connector blocks inside is the way to go. I assume you mean the ordinary brown chunky type of strip connector from wickes or wherever.

[Dave Plowman (News)]

1.5mm might well be ok for a short run.

[dennis]

On a CU with fuses not MCBs and a double socket, I don't think so.

[John Rumm]

If they are cartridge fuses[1] then you might just about get away with it - the max rating for 1.5mm^2 T&E clipped direct is 20A - and that is also a typical design load for a double socket.

(not recommending this cable choice - just highlighting it is not as wrong as first appears)

[1] with rewireable ones the de-rating factor to be applied to the cable makes it a non starter.

[Dave Plowman (News)]

If you do the sums for 6kW which is the theoretical max load on a double socket and the cable is enclosed the size needed is 4mm...

Of course the earth impedance is likely to be the limiting factor anyway.

[Fredrick Skoog]

Hi All,

My consumer unit is doing a funny thing. One of the bulbs went in my kitchen and tripped the MCB on the consumer unit, so I dutifully changed the bulb and tried to flick the switch back to the on position. Unfortunately it wouldn't stay there. Presuming that there must be something else causing it to go (I can't see what as it only appears to control the lights to the kitchen and they were switched to the off position) I flicked the main switch off and tried again. Even this time it wouldn't stay on and the small coloured indicator also didn't budge.

Obviously there is something else wrong but I'm at the limit of my limited knowledge. Can anyone give me some advice or is it time to call an electrician?

Thanks.

[dennis]

Take the new bulb out and try the mcb again, let us know what happens.

[Lobster]

Sounds like something else is causing a short, which is still present: I would think the most likely cause something wrong in the bulb holder; failing that the ceiling rose above it. Unscrew them and have a shufti.

David

[Andrew Gabriel]

The MCB needs replacing if it won't stay on even when the main switch is off. (Are you sure the main switch you switched off does cover that MCB?)

Replacing MCB's varies in complexity depending on the make/model of Consumer Unit. Some simply plug in, others can require quite a bit of disassembly of the CU.

[John Rumm]

Depends on what other de-rating factors are in play. Two sockets at full load would be 26A, and 2.5mm^2 clipped direct is ok for 27A. However calculations often assume a little diversity, and use a load of 20A since it is relatively unlikely that you will need two 3kW loads on the same double socket. (Also in the case of a single unfused spur, you can consider the fault protection of the cable (i.e. the MCB/fuse at the head end of the circuit) separately from the overload protection - which could be down to the fuses in the plugs)

Obviously if you know that you will be placing a high longish duration load on the pair of sockets then you take this into account! Kitchens / utility rooms are the most likely locations for this to be an issue.

Depends on the circuit length and the type of earthing at the head end.

4mm^2 does not generally perform much better than 2.5mm^2 cable since it still has the same 1.5mm^2 CPC as the smaller cable.

[Allan]

I flicked the main switch off and tried again. Even this time > it wouldn't stay on and the small coloured indicator also didn't budge. >

Either your MCB is knackered, or it may be of a type that needs a reset, ie push the switch down further than the normal off position and then back up to on. If indeed it only feeds the kitchen you must have (at least) one other 6A MCB . AS a temporary measure pull the wire out of it and stick it in with another 6A feed till you get a replacment. Obviously main switch off first!!!

Allan.

[John Rumm]

If a MCB has tripped as a result of a short duration fault current (i.e. like the brief short circuit that can occur when a bulb blows) they ought to reset straight away.

Some models however can refuse to reset straight away if the cause of the fault was a sustained overload for several minutes. Here the trip mechanism is different (it relies on a bimetal strip like that used in a thermostat), and the device may not reset until its thermal trip mechanism has cooled a little. If you have a heavily loaded lighting circuit it is possible you are seeing a borderline thermal trip as well.

If after a cooling off period it still fails to reset - even with the power off at the main switch, then I would suspect that the MCB itself has failed.

What make of CU is it? If you post a photo of it somewhere for us we can advise on what options you have.

[Fredrick Skoog]

Cheers Guys I waited a while and tried again and it reset ok, seems like it may have been a thermal issue.

Thanks again though