One for the sparkies

[MudSurfer]

We have two 400W metal halide lamps in the shed, wired back to a C 6A MCB. They worked fine for a while, but just lately when switched on and warmed up, they not only trip the MCB on the CU in the shed, but also the 63A RCCB on the incomer where it leaves the house.

It may be coincidental, but this only seems to have happened since the lift was installed, but, even after isolating that, on the post, the wall and the CU I still get the same problem.

Today I got up to the two lamps and replaced both bulbs to see if that was the issue - and instead of tripping the breakers after only 10 seconds, they lasted about a minute, then pop!

Is it a fault in the neutral connection somewhere that's feeding back, or earth? If it's in the CU then that's OK, as I can get to that without help, but if it's the lamps I need a cheery picker or my mate with his telescopic loader and a pallet again.

I can check the connections on the lift too, and see if there is anything on there that may be amiss, but I can't see it being there if the problem still exists when it's completely isolated, so it's (in my simple mind) either the ballast failing in one or both lamps, or a neutral or earth issue in the CU.

Any and all help greatly appreciated, as I can't do anything in the shed beyond 5pm, and that's only going to get worse.

TIA.

[MudSurfer]

I have a theory on earth leakage on the basis that the fault bypasses the main breaker on the CU in the shed, and runs right back to the house and trips the 63A there. Is it doing this because there is a good earth at the house, but poor/faulty earth on the CU?

When the lift was fitted, the guy installed two sockets too, which go back to the same CU in the shed, onto a new 16A breaker, but when this was fitted, with the new 20A one for the lift, he took the coomon neutral bar off the bottom of all the breakers and had a right job getting it back on. Is it possible that it's not on right and the lamps are causing this issue when they heat up?

Everything else works fine, everything tested ok, all sockets ok, lift ok....just these lamps tripping.

[Dobbslc]

What you're calling the common neutral bar sounds like the live busbar to me!
Don't touch that!

The breakers / mcb's in the cu operate just on overload only ie if you pull 10 amps through a 6 amp mcb it will trip.
The 63a rcd in the house picks up earth faults so if that is tripping you could have and earth fault like a faulty ballast in a light fitting, but as the lift is the newest thing to be changed it could have a problem.
Earth faults occur on both live and neutral to earth so just turning off something doesn't mean it is isolated as the neutral isn't always switched ie a normal light switch only turns off the live side.
You need to break the installation up into bits to narrow down the fault.
Try completely isolating the lift from the cu ( it should have a double pole isolator really) then try the lights.
Both lights should only draw about 3.7 amps so if they are tripping the mcb one could have a dodgy ballast.
You could also have earth fault accumulation where if you have a 30ma (milliamps) rcd in the house the various individual parts of the installation have small amounts of "normal" earth leakage, on their own this is ok but add it all together and if it equals more than 30ma it will trip.
Light ballasts and motors are good at giving out a bit of leakage especially when they get old.

I try the easiest to reach and most newly installed things first when I go fault finding on a site so I would check the sockets and lift connections, make sure the neutral and earth connections are the correct way around in the cu, lift switch, back of sockets and inside the lift controls too if you can.

Let me know what you find.
Dobbs

[MudSurfer]

Cheers Robin,

You are correct, it's the live bus bar. I've completely disconnected the lift - removed the cable between the 20A isolator on the wall next to it, and the control box fitted to the side of the lift. On each test I've done, this isolator has been switched to off, as has the control isolator on the lift itself.

Tried the lights again - they fire up, then as the bulbs warm up and the light output increases - they went off. This time, bypassed everything in the shed CU, and tripped the 63A RCD in the house. It think it's fair to assume it's not the lift.

I'm going to take the sockets out of the equation next by removing them and taping off the conductors, but it's hammering down out here, and the shed is a far walk away, so it may be later.

[Dobbslc]

If you've disconnected the lift completely and do the same for the sockets then it most likely is one of the lights, you can get spare ballasts for them but it's only worth changing them if the rest of the fitting is in good condition.
You might be able to test them with a multimeter and see which one has a low resistance to earth

[MudSurfer]

Cheers again. The units are only about 5 years old, but have hardly ever been used, one often takes much longer to start, and it's likely that is the offending article since it does seem that when that one warms up, pop!

I'll get back in there in an hour or two - I have my usual weekend list to get through before I can do my stuff.

[Dobbslc]

Those sort of lights can get damp inside after long periods of no use and that will trip an rcd.

[MudSurfer]

I wonder then if it's worth putting up with it a few times to let them dry out, or is that just not going to happen, since they don't stay on longer than a minute?

I may just take them out and fit a few fluorescent tubes instead, at a lower lefel so I don't need to be lifted up on a pallet again.

[MudSurfer]

OK, isolated the socket, terminated conductors with a 16A terminal block, tried the lamps that are in the other bay, no problem (these are on a 16A type B), then tried the ones in the shed, and the same one came on first, followed after about 30 seconds by the other one. By the time that had started to heat up, it took out the RCD on the CU in the shed, and the house 63A.

Looks like it's that lamp, probably knackered ballast. I may just replace both ballasts, as they are (looked at the bulb packaging) 250W, so that is 2 x 21,000Lu, and if I swap to strip lights, they are 2,000 each.....

[MudSurfer]

Thanks for all your help Robin, I really appreciate it.

I got up to one of the lamps in the barn this morning and removed the whole unit, then managed to get up to the one in the shed, like this:



Then took this out:



Looks like the offending article, and it's melted the back of the case of the ballast too....

Put the unit from the barn into the shed, wired up, reset the breakers and tested and all OK!!!

[Dobbslc]

Yep that would do it!