Testing RCD without power

[bsr]

Hello

My shed is supplied by a 40 amp dedicated circuit, through around 25m of 6mm T&E and SWA, to a garage consumer unit. It's a TN-S supply and the earth is exported. The garage CU is a BG unit. It has a 40A RCD and two MCBs, 32A ring and 6A lights. Rather overkill but there you go!

I've lost all supply to the shed. I won't have a chance to get the multimeter out until the weekend, but I noticed this morning that when I pressed the RCD test button it didn't trip. My questions to help me troubleshoot are:

1. Should the RCD trip on the test button if there is no supply? Or does that indicate a faulty RCD?
2. How does one test if the RCD has failed open? Can I just test for continuity across the input and output for each pole or would the multimeter current trip it?
3. Similarly for the MCB in the CU, would you test continuity across the MCB while it's still in the (de-energised CU)?
4. If the RCD has failed, would you replace with another RCD or a DP switch? I have an 18th ed split load board, so the circuit is already 30mA RCD protected.

Thanks in advance

bsr

 

[EFLImpudence]

1. No. It needs power to work.
2. Yes, continuity across the terminals. It is just a switch.
3. Yes.
4. Are you certain the garage cable is supplied through the house RCD?
If yes, then a 'Main Switch' will do.
If not, then another RCD - 30mA is the important number, the 40A can be that or anything higher.

 

[bsr]

Thanks for your response - the MCB at the CU end and both poles of the RCD at the shed end both have infinite resistance when open, and around 1 ohm resistance when closed, so they seem fine.

I've found the junction box where the T&E from the consumer unit is jointed into the SWA - with the MCB energised there is no voltage at the junction box so I guess that indicates a cable break / fault. I have not found any sign of damage to the cable, but I guess it could be in a cavity or where the cable runs through a wall.

Would you do any more testing? I was thinking I could join the phase and neutral wires at the junction box and test for continuity at the CU end to confirm a break. I was considering testing the MCB live i.e. testing voltage with the CU energised, but I am rather nervous of poking round in an energised CU.

The T&E cable run is about 10m, mostly in trunking. Would you rip and replace, or try to find and repair the break with a crimp? I'm guessing rip & replace. Can I use singles in surface mounted PVC trunking? They would be easier to run and perhaps less likely to get damaged while going through the wall. This page suggests I might only need to use 2.5mm2 rather than 6mm2 for the CPC - is that right?

Thanks


bsr

 

[SFK]

Good idea to be very wary of CU.

If you are competent I would
1a). At CU end, with CU powered off, remove Phase cable from MCB and Neutral from CU and join together.
Then test for Continuance (resistance) at JB.

or

1b) get another single length of loose wire that is as long as your 6mm T&E and test each wire speratly using that loose wire, ie connect one end to Phase, drape wire to your multimember at CU, connect to one side of multimeter, use otherside of multiuser to test for resistance along Phase wire. Repeat for neutral and earth.

2) To test MCB with CU closed, wire a new short cable (say 1m) to MCB and Neutral in CU.
Put connector block on other end of wire to keep wires separate.
Close CU and then test this wire for mains voltage using multimeter.

Again only if you are competent, otherwise get in an Electrician. They are a small cost compared to your likelihood, family and life.

SFK

 

[bsr]

SFK, thanks, doing (1) as you said showed the cable was OK. I took out the MCB and tested it, and it was fine. Put it all back together and it works OK.

No idea what happened! Maybe it was stuck open and taking it out and shaking it helped? Dunno, waste of time, but at least it's working now and I don't have to rewire.

 

[Taylortwocities]

At a number of sites, I have seen problems like this where the busbar is not properly clamped into the MCB.
If care isn’t taken, with some maker’s products it’s possible for the busbar prong to sit behind the MCB clamp. The circuit works ok for a while, then one day, a large load is applied and the dodgy contact breaks down.

 

[bsr]

Ah, you know, that's definitely a possibility here. That's what I did the first time I put it back in, I only realised because I saw that the busbar screw was much further in than the busbar screw on all of the other MCBs.