Electrics question

[puffernutter]

This is a 1890s terrace with a TT supply. The incoming mains is protected by a time delay RCD, all other circuits either have RCD protection or are RCBOs.

There are two consumer units:

1. Split unit. Non RCD protection, but circuits protected by RCBOs; and RCD protection with circuits protected by MCBs

1. Non split unit RCD protection with circuits protected by MCBs (this is where the garage circuit is fed from)

The feed to the garage is through an RCD then a 32A MCB.

The garage has a separate consumer unit with 4 circuits, each of which is protected by an RCBO.

After heavy rain this afternoon, the RCD associated with the garage circuit in the house would trip when the outside lights were energised. However the RCBO in the garage feeding the outside lights didn't trip! Note, I have narrowed the problem down to the outside lights and yesterday (and all night) they worked without any problems.

So is the problem that a combination of leakage around the lights in insufficient to trip the garage RCBO, but in conjunction with potential leakage in the garage - house cable it is enough to trip the RCD associated with the garage circuit?

If so, I have space in a split consumer unit with the RCBOs on a non RCD protected circuit, can I move the garage MCB to the "non RCD" side and still meet the IET Wiring Regulations requirement for an RCD on a TT supply, even though it is a delay RCD?

Regards

Peter

[ss]

I need to extend the electric mains cable on the household freezer by about 5 feet is there any reason (safety) why I shouldnt cut off the existing moulded plug and add on the cable and replug. All cable will be above door height or behind freezer. Would the freezer be 3 or 2 core cable?

[NT]

Its fine if you use a proper enclosed connector with cord grips. A much neater option is remove the existing flex and replace with one longer piece. 3 core, minimum current flex.

NT

[puffernutter]

Having had my subject line hijacked!

This is a 1890s terrace with a TT supply. The incoming mains is protected by a time delay whole house RCD, all other circuits either have RCD protection or are RCBOs.

There are two consumer units:

1. Split unit. Non RCD protection, but circuits protected by RCBOs; and RCD protection with circuits protected by MCBs

1. Non split unit RCD protection with circuits protected by MCBs (this is where the garage circuit is fed from)

The feed to the garage is through an RCD then a 32A MCB.

The garage has a separate consumer unit with 4 circuits, each of which is protected by an RCBO.

After heavy rain this afternoon, the RCD associated with the garage circuit in the house would trip when the outside lights were energised. However the RCBO in the garage feeding the outside lights didn't trip! Note, I have narrowed the problem down to the outside lights and yesterday (and all night) they worked without any problems.

So is the problem that a combination of leakage around the lights in insufficient to trip the garage RCBO, but in conjunction with potential leakage in the garage - house cable it is enough to trip the RCD associated with the garage circuit?

If so, I have space in a split consumer unit with the RCBOs on a non RCD protected circuit, can I move the garage MCB to the "non RCD" side and still meet the IET Wiring Regulations requirement for an RCD on a TT supply, even though it is a delay RCD?

Regards

Peter

[Brian Gaff]

I think I'd want to know the exact cause of the leak myself in case its a potentially serious issue that would need some recabling. Brian

[A.Lee]

No, all RCD's trip at different times. Depends on make/model/age/condition. An old RCD could trip in

[Dave Liquorice]

Yes, forgot to say in my previous post that needs to be investigated and sorted out. Then worry about the cascaded 30mA RCDs.

[puffernutter]

Dave,

Thanks for the reply. I think you have it correct. The route from the incoming supply to the outside lights is as follows:

[In House:Time Delay RCD - 30mA RCD - 32A MCB] - cable to garage - [In Garage: 6A RCBO - outside lights - 1.5mm FTE all run in conduit]

As has been suggested the additional dampness is probably enough to tip the balance as they were working fine the previous night!

My suggestion was, as I have a space in a split CU is:

[In house: Time delay RCD - 32A MCB] - cable to garage - [In Garage: 6A RCBO - outside lights - 1.5mm FTE all run in conduit] .

I should add that the reason we went for a time delay RCD was that we have had a number of issues with the house wiring and we would regularly lose everything. By putting the socket rings on RCBOs meant that we only lost the circuit that was causing the problem and it could be tracked easier. In fact, since we made the change, we have had no nuisance trips at all, so it looks like separating the circuits out has reduced the leakage at each RCBO to below the threshold.

Cheers

Peter

[Andy Wade]

That's OK if the cable run to the garage is surface wired, or is SWA (or one of the other prescribed cable types), or is in metal conduit etc. It would not be OK for T&E buried in a wall at less than 50 mm depth since this needs fast 30 mA protection for 'additional protection'. Also of course any 13 A sockets in the garage will need to be fed via a

30 mA RCD or RCBO.

[puffernutter]

Andy,

Thanks again for that. To answer the last question first, all sockets in the outbuildings (3) are fed through separate RCBOs.

The cable to the garage was installed by the previous owner (a builder

- that explains the state of the electrics!) and cable to the garage is 2.5mm FTE which runs in 20mm blue polypipe buried in the garden.

Not strictly to the rules, but pretty well protected!

Cheers

Peter