Electric power to outbuilding (rewiring)

fifthofwhisky

I had an existing electric power extension to an outbuilding using armoured cable. The house was rewired (and old fuse box replaced by a breaker) and thereafter any time I use more than a lightbulb in the outbuilding (e.g. power tools) the breaker trips. I'm guessing I might need to have it rewired somehow but just wanted to check in case there is something else that needs attention. As far as I know a twin an earth cable (2.5mm2?) runs internally from the distribution panel/breaker and then connects via an internal junction box to the armoured cable. I'll get a certified electrician in to do any wiring but would like to have some idea of what might be needed to get it working properly and safely.

TheCyclingProgrammer

Is the existing power to the outbuilding a spur off your main ring or on its own circuit?

fifthofwhisky

TheCyclingProgrammer wrote: »

Is the existing power to the outbuilding a spur off your main ring or on its own circuit?

I cannot say for sure. Before the house was rewired it was definitely direct to the distribution board. I guess I will have to try and trace the wiring back. So it should be direct to the breaker on its own circuit?

TheCyclingProgrammer

My understanding (and I'm not an electrician) is that power can be connected to the outbuilding in one of two ways under modern regs.

A spur off the main ring, with an isolation point at the house end (fused connection unit) or a separate circuit from the fuse board. In either case the power must be RCD protected.

I believe is considered better to have a separate circuit so if something trips in the house it doesn't leave you in the dark in the outbuilding and vice versa.

That's under current regs anyway. Our garage is connected to the house from the main ring and I have no idea where it's spurred from. The cable to the garage connects directly to a standalone RCD and then into a fuse box with a power and lighting circuit. This wiring predates the existing consumer unit which also has its own RCD protection.

fifthofwhisky

Thanks for the info. It definitely is on an RCD and since when it trips some power in the house goes out it must be on a spur from the main ring? There is a switch on the interior wall with a LED lamp (not sure if it is fused) that toggles power to the outbuilding. So it sounds like it could be wired OK, leaving the question why is the breaker tipping?

Wookey

Do the same power tools trip it when they are plugged in inside the house?

fifthofwhisky

Wookey wrote: »

Do the same power tools trip it when they are plugged in inside the house?

That is a good question. No they don't.

Just tried the outbuilding power supply with a coupe of tools. Firstly a 375W motor. No problem (so far). The other tool has a 600W motor and this is what I was using when the breaker last tripped. I'm guessing there is some kind of surge when it's under load and, whatever that value is, it is tripping the breaker.

Looking at the breaker box, the RCD it is a Hagar 63Amp 30mA RCD, so I doubt any problems there. It must be a problem in the circuit/wiring to the outbuilding since no internal equipment on that ring main trips the breaker. Incidentally, I did find the internal power switch is fused. I'll swap the fuse out for a new one to rule that out.

gmgmgm

What precisely is tripping? If it's the breaker, what rating is it?

fifthofwhisky

gmgmgm wrote: »

What precisely is tripping? If it's the breaker, what rating is it?

The RCD at the breaker box. It's a Hagar 63Amp 30mA RCD

Ectophile

It could be that whatever you are plugging in is faulty. That would easily be checked by plugging something else in.

Or the wiring is faulty and/or seriously damp. For instance shorts between Neutral and Earth can cause all sorts of strange tripping problems with RCDs. And these may only show up when the circuit is heavily loaded. Unfortunately, that's the sort of thing that needs an electrician with the right tools do diagnose the problem.

Mr.Generous

The 63a Hagar isn't a breaker its a RCD. It is presumably tripping because there is a leak to earth. I would suspect water ingress at the junction to the armoured cable or possibly somewhere en route to the garage, or a wiring fault. Again the starting place would be the joints, terminals, the mains board connections etc.

The other possibility is you are exceeding the total board (or side of the board) capacity of 63a, if you have a 10.8kw shower and an electric cooker on at the same time for example.