Tripping RCD

[fudger]

Guys, i have added a 32A MCB to my main consumer unit so I can add outside lights and posibly a hot tub at some point. I have wired it to an MK IP56 double socket with 30A integrated RCD outside the house.

In the short term and to get some lighting in my shed, I thought I could simply take the mains for the lights from one the two sockets in the MK unit. Simple I thought except that I am tripping the main RCD in the consumer unit everytime I switch on the light in the shed. The 32A MCB is not tripping but the main 60A RCD is.

Is it something basic I am doing wrong? It's driving me mad at the moment.

Many thanks

Jonathan

 

[Lectrician]

What size cable did you run to the socket?? (that isnt causing it to trip - just interested).

Why did you use a really expensive socket with integral RCD if you have wired it to the RCD in the CU (assuming the cu has a 30mA RCD?)

I suspect you have wired the lighting part of your circuit with a live to earth or neutral to earth fault (damaged cable or squashed connection etc) - take appart all covers and take a look.

Have you fused down the lights with a FCU?? I hope so, remember, lights are fused at 6amp and below in normal domestic installs (1.0 or 1.5mm cable). What size and type cable have you run from the socket, and where too?? (socket to switch and switch to light?????)

 

[fudger]

Thanks for the quick reply Lectrician

What size cable did you run to the socket?? (that isnt causing it to trip - just interested).

From the CU to the socket is 38A cable (10mm I seem to remember).

Why did you use a really expensive socket with integral RCD if you have wired it to the RCD in the CU (assuming the cu has a 30mA RCD?)

Well, I did not intend to wire it straight into the CU but when I got round to doing the work it seemed the best place to wire it into. I thought that putting a hot tub onto a spur off of a ring main was not a good idea also.

I suspect you have wired the lighting part of your circuit with a live to earth or neutral to earth fault (damaged cable or squashed connection etc) - take appart all covers and take a look.

I did check this and it all seemed to be OK but I'll re-check again!

Have you fused down the lights with a FCU?? I hope so, remember, lights are fused at 6amp and below in normal domestic installs (1.0 or 1.5mm cable). What size and type cable have you run from the socket, and where too?? (socket to switch and switch to light?????)

FCU?

The mains for the lights are straight from a 13A plug to a lighting unit and then back to the switch which is not fused but I can put one in if it's better practice/required.

Cheers

Jonathan

 

[breezer]

as you have a socket what happems if you plug a light in, if the rcd doesnt trip, its your lighting wiring, if it does its the socket wiring

how does this 10mm cable get to the shed?

It should be a steel wire armoured cable, for more advice try a search for:

power to garage

power to shed

power to outbuilding etc

 

[Damocles]

Having two rcd is a bad idea because you can not be sure which one will trip first.

If there is a small leakage to earth on the light you have plugged in, this might be enough together with any other small leakage which already exists on your main system, to trip the main rcd. The lamp on its own might not be leaking enough to trip the rcd in the outside socket.

Or, you have mixed up earth and neutral. Then the outside RCD might be quite happy that it has current going out and back on the proper wires, but the house RCD sees a lovely big flow to earth, and trips.

Or something else which i have not thought of.

 

[fudger]

as you have a socket what happems if you plug a light in, if the rcd doesnt trip, its your lighting wiring, if it does its the socket wiring

how does this 10mm cable get to the shed?

The problem occurs when I plug the light into the IP56 and switch on. The 10mm cable is from the CU to the IP56 only. From the IP56 to the lighting unit it's standard twin and earth lighting flex.

When I bypass the IP56 and wire the lights to the 10mm cable direct (with a connector block) it still trips the RCD in the CU. I also then attached the light fitting direct to the 10mm cable (avoiding the light switch) and it still tripped the RCD in the CU. At no point has the MCB tripped.

Assumption at this point is that there must be another problem or the way I have added the 32A MCB in the CU as I had to extend the bus bar. However, extending the bus bar and adding the MCB is very straight forward so I can't think how I have got it wrong if indeed I have

 

[fudger]

Having two rcd is a bad idea because you can not be sure which one will trip first.

Agree totally but as I said above i still trip the CU RCD even when i take the IP56 out of the equation.

I just have to have done something wrong

 

[Lectrician]

Does it trip when you plug anything else into the socket??

Possibly got your N in the wrong N bar in the CU indoors.

 

[fudger]

Does it trip when you plug anything else into the socket??

Possibly got your N in the wrong N bar in the CU indoors.

Lectrician, you were spot on. It makes you realise how easy it is to miss read where the wires are going

Many thanks to all who have replied.