Fault finding an intermittent RCD trip.

[Myron]

Hi all,

I’ve had a headache of a fault and can’t think what to do next.

The house has an RCD main switch, and I’ve verified the circuit which has the fault to be the kitchen ring final. The RCD has tripped once without interference, and several times when a switched fuse spur is turned on. It doesn’t always trip, sometimes it can take 5+ switches on and off to trip.

The layout is as follows: twin socket above worktop on ring, spur out feeding FCU. Load from FCU goes down below worktop to feed a connection plate with a dishwasher and a washing machine hardwired in. I will be adding a separate FCU so they’re controlled independently.

I’ve done an IR test on the whole circuit, and found nothing of note. I’ve changed the FCU, I’ve bypassed the connection plate. It still trips intermittently. If both loads and completely removed the RCD will not trip. However, if I connect the appliances to another circuit on the same RCD it will not trip. What am I missing here?

Thanks for the help.

 

[Wilko]

Hi - a puzzle
Have you got an earth leakage clamp meter or RCD ramp tester? Reading your note, I suspect the FCU, DW and WM combination. As you say, with them gone RCD does not trip.

 

[Midwest]

I'd go back to the beginning again. Testing wise.

 

[littlespark]

Fault finding an intermittent problem on RCD just calls for an "optimistic"

Be prepared for a long day/night trying to find it.
Could the RCD itself be at fault?
Just rereading the OP... both appliances on the same FCU? not ideal.
Switched cable down to the connection plate ok? Cables not squashed in the box or anything silly like that?

When you add another FCU, you'll be completing the ring, right? not a spur off a spur?

 

[westward10]

Connect something else to the connection plate and see what happens.

 

[marconi]

Have you PAT tested the two machines?

 

[Deleted member 105166]

I'd measure the leakage of the installation with neither appliance connected, then isolate every other circuit and measure the leakage of each appliance individually. Hopefully this will provide some clues.

 

[Lucien Nunes]

The RCD has tripped once without interference, and several times when a switched fuse spur is turned on.

So it trips mainly when the switch is operated, not while it is left switched on and the machines used? Some combinations of load and DP switch are known to cause nuisance tripping when operated. Since there are two machines on one switch, even when the machines' own switches are off, their two sets of suppression capacitors are switched onto the circuit together. If the circuit is already using some some of its leakage budget, I would hazard a guess that the switch transient applied to the two machines at once might be pushing it over the threshold. The alternative switch that you tried on the same circuit might be SP, or switches its two poles in a less disruptive sequence, and hence not causing a trip.

If this is true, splitting the machines onto 2 RCDs will cure it, but the leakage should be checked as it may be excessive anyway.

 

[DPG]

This is where a leakage clamp meter would be useful. Now who was asking about them the other week?

 

[Lucien Nunes]

In my post above (#8) for '2 RCDs' read '2 FCUs', i.e. what you are doing anyway.

 

[Myron]

Thanks for the replies. My clamp meter doesn’t go low enough for leakage current, will be on the lookout for a bargain on one that does. I retested fixed wiring with everything screwed back, no red flags. Pat test on appliances comes in okay.
I’m going back on Monday to add another FCU anyway as it shouldn’t have them both on one. The RCD trips when only one appliance is connected so not to do with two appliances on one switch anyway.
When I go back I’ll connect a different appliance to the FCU that’s tripping the RCD.

Thanks for helping, will let you know (should be back Monday).

 

[marconi]

At this stage there would appear to me to be five strands to this problem:

1. troublesome wiring ie: one spur off the FRC to one fcu feeding two connection units for a DW circa 2kWpk and WM circa 2kWpk;

2. a question 0n why the RCD trip problem has only now become a problem or has it always been there and for some reason is only now manifest? Accumulated standing earth leakage? Transient earth leakage?

3. the electrical state of the DW and WM.

4. Some as yet undetected FRC wiring problem.

5. Another appliance plugged in to the same FRC which has an electrical defect.