What sequence to identify intermittent fault causing rcd to trip .

[wsoppitt]

Guys / Girls.
How do you tackle an intermittant fault that only occurs once or twice every few weeks. What is the sequence you would use to identify and locate the issue.

 

[James]

Is it shortly after heavy rain?

 

[SparkyChick]

1. Ask the client to try and keep a diary of when it trips... what the weather is like, whether they turned a particular appliance on or off, whether any appliances were on at the time and what they were doing

2. Carry out some insulation resistance testing... if you're tester does like a fully automatic 'press the button get the result' mode of operation, turn that off and run the test manually for an extended period. Sometimes pin holes in insulation can result in sudden dips from an otherwise good result... dips that may not be picked up on a short test. Test as many circuits separately as you can so you don't miss things like neutral-earth faults on say a circuit not protected by the RCD that's tripping... and don't assume because it's an RCBO board that a neutral earth fault on another device can't trip an otherwise perfectly good circuit.

3. Ask them if they have had any works carried out recently (whether this be them doing it or another trade). If they have, dig deeper... what, when, where? Did the tripping begin after those works?

4. Poke around a bit... look for damp... look for changes (not everyone will own up to doing works when questioned if they suddenly think that they could have caused the issue).

5. Check as many connections on the incoming side of things as you can without compromising your safety. It's not unheard of for bad connections to trip RCDs.

Tracking down intermittent faults can be incredibly tricky. Sometimes if the client keeps a diary, the cause can be immediately evident after a few occurrences (as James has mentioned... it trips when it rains heavy - outside lights, outside sockets, water ingress in the loft... or it trips when we use the oven). If not then I would probably be looking for minor insulation damage... possibly so small that you can barely see it.

Hope this helps.

 

[Pretty Mouth]

Great advice by @SparkyChick . To add to that:

Is the RCD working correctly? The first thing I do with these faults is the full range of tests on the RCD itself. 1st with loads disconnected, then loads connected. Ramp test can be useful.

Then thorough IR testing, at 250V, with ALL equipment, lamps etc connected, plugged in, switched on. I test each circuit, between live conductors and earth, and between live conductors of different circuits. If I get a low reading to earth, I'll disconnect the CPC from the MET, and test to each. Is the resistance lower for one of them?

I LOVE fault finding, best part of the job for me

 

[DefyG]

And if #3 & #4 fail to locate it there is always the possibility of it being external!

Had a similar problem where unable to locate cause, only to receive call following day from client to say transformer exploded on pole that night. It has been ok since (at least 10 years) as still doing work for same client.

 

[Rockingit]

1. Eliminate the RCD itself by testing (ramp and time)
2. Look out the window and see how much water is falling
3. Don't believe the client when they say that nothing has changed
4. Repeat 2 & 3 until solution appears

 

[pc1966]

Also if you have a current clamp meter that can go down to mA then you can also see if the overall load is leaking worryingly close to In/2 or so.

Though usually that needs a lot of circuits on the one RCD, or an unusual amount of electronics, PCs, etc, with moderate leakage input filters.

 

[pc1966]

Oh, and a final thought: it can sometimes be an E-N fault and high current that causes a trip if enough L-N imbalance happens, but a global IR test (L+N to E, supply isolated) would reveral that very quickly.