Mysterious Fault Connecting a Garage Consumer Unit

[EasyGoingPat]

Hi,

(I have a HNC in Electrical Engineering, but I am more than a little rusty because I rarely do any electrics as part of my day-to-day job.)

I have just added a garage consumer unit in a new shed in the garden. The cable used is 2.5mm armoured and is contained inside an outer plastic tube. The cable is connected into the main household consumer unit using its own dedicated 20A MCB. In the shed, I am using an MK garage consumer unit (40A incoming RCD, two outgoing MCBs, one for lights one for sockets).

Using an electrical test meter, I get 240V at the input terminals to the garage consumer unit. If I click the incoming RCD ON, this 240V is passed through to both ougoing MCBs.

If I now press the test button, the garage incoming RCD does not trip out.

If I wire a brand new electrical socket into the outgoing socket-MCB in the shed consumer unit, I get power at the socket okay. If I now plug a device (any device, even something as low-current as a radio) into the socket and switch the socket on, the incoming RCD now trips out.

There are a couple of tests I haven't done yet (mainly because the weather is so bad), but I wondered if anyone could suggest what the most likely fault(s) might be here.

It all seems very odd to me. After all, basically all I am doing is connecting a single 3-core wire.

Any help or advice would be very gratefully received.

Kind wishes ~ Patrick

 

[davelx]

Check that the incoming neutral is not connected to the neutral block of the garage consumer unit! The incoming Line and Neutral should connect ONLY to the input side of the RCD. The outgoing Neutral of the RCD then connects to the neutral terminal block, to which the neutrals of the local circuits are connected.

 

[Taylortwocities]

A very very common problem.

As Davelx says, You have wired the garage consumer unit up wrong.
The incoming and outgoing neutrals must be connected in some way, probably you failed to remove the busbar link.

Your HNC in Electrical Engineering must be very rusty indeed.

This does remind me of the time when Building Regulations Part P was introduced. There was an outcry from some time-served members of the (then) IEE that they would no longer be able to (legally) do electrical work in their own houses. You have demonstrated why….

 

[winston1]

You don't even need a CU in the shed. The sockets can be connected directly and the lights via a switched fused connection unit. You need a RCD if there is not already one in the main CU.

 

[JohnW2]

You don't even need a CU in the shed. The sockets can be connected directly and the lights via a switched fused connection unit. You need a RCD if there is not already one in the main CU.

True, but that horse has already clearly bolted and, in any event, the CU is as probably as good an enclosure as any in which to house an RCD (assuming that one is required), and it also keeps the 'box count' down by not requiring a separate FCU for the lights.

Kind Regards, John

 

[333rocky333]

You need a RCD if there is not already one in the main CU.

Why

 

[JohnW2]

You need a RCD if there is not already one in the main CU.

Why

Because, even if there are no other reasons, the new sockets will require RCD protection?

Kind Regards, John

 

[RF Lighting]

You could use RCD sockets.

 

[AndyPRK]

Be rather more expensive than a 2 way cu.

 

[JohnW2]

Be rather more expensive than a 2 way cu.

Indeed - and, by all accounts, they can also be a bit iffy (although I have little personal experience).

Kind Regards, John

 

[winston1]

and it also keeps the 'box count' down by not requiring a separate FCU for the lights.

Kind Regards, John

Not really. You need a light switch. That can be a switched fcu.

 

[333rocky333]

You need a RCD if there is not already one in the main CU.

Why

Because, even if there are no other reasons, the new sockets will require RCD protection?

Kind Regards, John

Hasnt the OP just fitted an rcd controlled 2 way board for the whole garage, im baffled why winston says he needs an RCD in the main cable to protect the SWA

 

[JohnW2]

You need a RCD if there is not already one in the main CU.

Why

Because, even if there are no other reasons, the new sockets will require RCD protection?

Hasnt the OP just fitted an rcd controlled 2 way board for the whole garage, im baffled why winston says he needs an RCD in the main cable to protect the SWA

I think you're misunderstanding. Yes, the OP has installed a CU with an RCD. However, AIUI, winston was saying that he needn't have done that - and if he hadn't he could have direct wired the sockets and run the lights via an FCU - but then added that he would also then have to have added an RCD if the circuit was not already protected at the house end. That was my reading of it, anyway!

Kind Regards, John

 

[winston1]

You need a RCD if there is not already one in the main CU.

Why

Because, even if there are no other reasons, the new sockets will require RCD protection?

Kind Regards, John

Hasnt the OP just fitted an rcd controlled 2 way board for the whole garage, im baffled why winston says he needs an RCD in the main cable to protect the SWA

Someone else who does not read posts properly. I said a CU was not necessary in the garage but there would need to be an RCD if one was not already protecting the system.

 

[JohnW2]

Someone else who does not read posts properly. I said a CU was not necessary in the garage but there would need to be an RCD if one was not already protecting the system.

To be fair to Rocky, what you wrote was, IMO, very ambiguous. The bit about needing an RCD was a separate sentence from the one talking about what the OP could have done without a CU.

Kind Regards, John