How can I investigate a very rare RCD trip?

Question

I live in Europe, my house is serviced with a 230 V, 4.5 kW single phase line.

In my main panel I have a combined RCD + overcurrent breaker, 30 mA/25 A, and then several overcurrent breakers for various circuits, a few 16 A for outlets and a few 10 A for lights.

The combined breaker trips very rarely, two or three times a year. I have not been able to find a pattern, except it normally happens in the early morning, 5 am or so. It does not seem to be weather related, or anything related, but I have very limited data points.

I have contacted the electrician who made the wiring in my house, as this person lived there for some time, and he suggested it might be a humidity problem, and given me the option to upgrade to a class A RCD - I currently have a class AC - or to install a delayed RCD.

Both options seems very poor to me, as I'd like to root cause the problem rather than put a band aid on it.

Is there any possibility to investigate this topic further, given especially that it occurs so seldom? I am okay searching for the issue myself, but I do not have a current clamp that is good enough to investigate this issue and before investing in it I'd like to have any idea where to look.

I did measure the neutral-to-ground voltage, and it is basically zero, so it does not seem I have any high current flowing there.

Answer 1

Your desire to explore the root cause is respectable, but most likely not viable with a trip rate that low. Should you experience regular, predictable RCD trips, a root cause investigation is surely bound to succeed.

It happens that RCDs get unstable and electrical noise makes it trip randomly. (Ask me about the RCD trip during our summer vacation, and the smell of rotten food in our home. Wife wasn't pleased).

I see the following options for you (ordered by effort, cheapest first):

• Inspect all the usual suspects: outdoor outlets and lights for water ingress. Everything that has exposure to water (an eventual basement pump?)
• Upgrade to a class A RCD and hope for positive results. (Personally, I had success with that in a similar situation. My old RCD got unstable over time. Perhaps due to our exposure to lightning strikes). You'd have the benefit of improved protection.
• Upgrade the panel to a pure overcurrent main breaker, and combined RCD+overcurrent for each branch. This allows you to identify a wiring branch with a persistent issue. While this won't guarantee solving your issue, it will improve your wiring and narrows down the problem. Even better: Instead of the main breaker, install an pure (non-current-limiting) switch ("sezionatore"). Ask your electrician first if he signs it off. Just make sure that the branches are secured against ground faults and overcurrents.
• Perhaps ask for an insulation check of any water boiler, the washing machine and other fixed appliances?
• Organize an in-situ differential current monitoring until the RCD trips. Most likely too much effort for your electrician. And you'd still need to do work (find and repair the defect or the RCD).

About your comment regarding the needed class:

I've never heard of someone using the higher classes (F, B, B+) voluntarily. They were always used when somebody had tripping RCDs due to frequency converters. Class B RCDs are often required when installing an electrical vehicle charging station, unless already integrated.

If you have specialized equipment with frequency converters, you might want to think about it, else not. I'd prefer to invest the money in individual RCBOs (combined RCD + overcurrent protection) on each branch, as it will help you narrowing down on the problem and your house won't go dark if there is a fault in the outlets. Perhaps even add a surge protector?

Answer 2

I think @Martin may have a point that efforts to diagnose a transient problem that occurs once a year are probably doomed. But FWIW here is what I would think through and attempt in your situation:

[ This answer assumes you're looking for an earth leak but see my comment to the question ... are you? ]

If the problem usually occurs at 5am, or perhaps while you are sleeping and you notice it at 5am? it may have to do with

• Anything with a timer on it. But that's too obvious
• The actual act of your waking up. Your alarm clock? Electric blanket? CPAP machine? IDK. This is a stretch.
• Pets? What is their behavior at the time(s) this happens?

Ok and now to get a little more serious:

• It's probably outside, something to do with temperature changes, condensation (dew), animals, neighbors ....

If you can isolate all your outside circuits (lights, outlets etc), that might help you point to the outside. You have to disconnect hot and neutral wires to all circuits before they exit the house, not simply turn off the power. If you can leave it that way for a year, and the problem never happens, that's a good data point. If you can sometimes reconnect it at 5am and see if that pops the RCD, that would be helpful too.