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Spurious RCD trips

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I'm having a problem with my RCD 'Safe-T-Cut' spuriously tripping. This happens at random times, seemingly uncorrelated with any activity - morning, afternoon, evening, night, completely unpredictable. This happens every few days - the RCD trips and can be immediately reset. OK for several days before tripping again. The fault - if indeed there is a fault - is obviously transient in nature.

The usual advice is to isolate various circuits and turn off appliances to try to isolate the cause but this is obviously impractical when the trip only happens infrequently. I can hardly just disconnect the fridge/freezer for a week to see what happens - and this would not prove anything anyway!

i am wondering if the RCD unit itself is developing a fault or is just oversensitive to mains spikes or general voltage instability? It's only a minor inconvenience - unless in the middle of the night - but I would like to get it sorted out.

Anyone out there experienced any similar behaviour? Any suggestions?

The house has no ground circuit, if this is relevant.

You should probably sort out a ground for your Class-1 appliances, but that's unlikely to be your issue.

Is the Safe-T-Cut one of the adjustable ones? What is it set to?

Do you have any test gear, a multimeter, neon driver etc?

Start by popping off your outlets and switches and check for wildlife and/or dampness inside.

"I don't want to know why you can't. I want to know how you can!"

  • Author
14 minutes ago, Crossy said:

You should probably sort out a ground for your Class-1 appliances, but that's unlikely to be your issue.

Is the Safe-T-Cut one of the adjustable ones? What is it set to?

Do you have any test gear, a multimeter, neon driver etc?

Start by popping off your outlets and switches and check for wildlife and/or dampness inside.

Set to 30mA

I have a clamp meter and have tested everything that I have access to for leakage currents. Nothing untoward shows up.

No wildlife - ants or whatever - found in switches or sockets.

The real mystery is why this happens only infrequently.

I'll bet you have a Neutral - Earth (the actual earth since you're not wired for grounds) fault somewhere, they can be an absolute dog to locate even with the correct test gear, you stand no chance with a regular multimeter.

One thing, have you verified the polarity of your incoming supply (live really is live)? It wouldn't be the first time that odd problems have been traced to swapped incoming supply.

"I don't want to know why you can't. I want to know how you can!"

  • Author
5 minutes ago, Crossy said:

I'll bet you have a Neutral - Earth (the actual earth since you're not wired for grounds) fault somewhere, they can be an absolute dog to locate even with the correct test gear, you stand no chance with a regular multimeter.

One thing, have you verified the polarity of your incoming supply (live really is live)? It wouldn't be the first time that odd problems have been traced to swapped incoming supply.

Nothing would surprise me about the wiring in this house - bit of a dog's breakfast. Having said that though, it has worked OK for the last 20 years. The RCD problem only started a few weeks ago and, as I said, has been completely sporadic. I have read that UPS devices can cause RCD trips. We have two of these. One protects my modem and router and the other protects the rest of my computer equipment. The one on the modem and router did get replaced a few weeks ago - I can't remember the exact dates - so I wonder if this could be the culprit?

  • 2 months later...

We had same problem. I found out it was the back yard where the pump was. I put plastic over the pump switch/ outlet. the plastic fell off. it started raining. safety cut...

  • Author
2 hours ago, advancebooking said:

We had same problem. I found out it was the back yard where the pump was. I put plastic over the pump switch/ outlet. the plastic fell off. it started raining. safety cut...

My problem turned out to be an actual faulty RCD unit. Very odd behaviour - with sensitivity set to 25mA the unit trips sporadically but if set to 15mA the unit is stable. Electrician has confirmed that the unit is faulty and should be replaced. He also recommends replacing the whole consumer unit. Under consideration but, right now, has been stable at 15mA for a few weeks.

3 hours ago, doctormann said:

My problem turned out to be an actual faulty RCD unit. Very odd behaviour - with sensitivity set to 25mA the unit trips sporadically but if set to 15mA the unit is stable. Electrician has confirmed that the unit is faulty and should be replaced. He also recommends replacing the whole consumer unit. Under consideration but, right now, has been stable at 15mA for a few weeks.

3 hours ago, doctormann said:

My problem turned out to be an actual faulty RCD unit. Very odd behaviour - with sensitivity set to 25mA the unit trips sporadically but if set to 15mA the unit is stable. Electrician has confirmed that the unit is faulty and should be replaced. He also recommends replacing the whole consumer unit. Under consideration but, right now, has been stable at 15mA for a few weeks.

and then if you replace it and it still trips? do you have any switches or wires outside exposed to the rain.

  • Author

The only outside equipment is our water pump and that is not actually exposed to the weather.

As I said, the RCD has been checked and is definitely faulty. What I don't understand is why the spurious trips occur at 25mA sensitivity but not at 15mA. This does not seem to make sense unless the RCD is indeed faulty in some way. It certainly indicates that the problem is not with a faulty appliance or wiring or water incursion. There was never any correlation between the trips and any activity - totally random.

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