RCD nuisance tripping.

Ever since the house was built it's had te odd nuisance trip.
What happens is that every spo ofetnm some surge will take out te now 100mA whole house RCD even though its virtually empty of any kit these days.
I can RELIABLY make it trip by connecting a high startup current device - like an angle grinder - to the ring that feeds my workshop and hobby room.
The same device connected to other rings is fine.
In other cases it takes a real surge - bulb blowing short - and it trips out.
I am thinking earth neutral short on that ring.
What do you think?
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In order to find that you would need to isolate each part and test it bit by bit I guess. Brian
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The Natural Philosopher wrote:

separate the ring somewhere about the middle (if you can remember how it's routed) remove each leg from the MCB one at a time and see if your angle grinder trips more or less reliably on one leg or the other?
check sockets on the bad half for incinerated spiders?
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On 14/11/2018 11:14, Andy Burns wrote:

Maybe some leakage or a capacitor on the angle grinder that is about to fail but then heals itself when mains is first applied. My power saw can sometimes take the main RCD down I suspect for similar reasons.
If the cable has got hot somewhere then there may be a partially charred wire that isn't the greatest of insulators. Does the failure correlate at all with humidity or rain?

The one that got me once was a slug that had crawled into a junction box inconveniently hidden behind part of the fitted kitchen. It wasn't an intermittent problem though more of a very hard fault as slugs even when dead are rather a low impedance.
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On 14/11/2018 12:50, Martin Brown wrote:

No. ANY load will trip it if the current is high enough.
circular saw, vaccuum cleaner, drill, whateber.#
Its not in the device its in te circuit

No, Its definitely not that.

Mmm. I can isolate larget sections of that ring with SOME difficulty
I need a round tuit.
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Andy Burns method seems right: unscrew a socket half way, disconnect the wires thus splitting the ring, test on both halves, one should trip the other not, then unscrew a socket halfway in that leg and try again, somewhere there's some insulation leaking at high currents...
And put back the 30ma RCD !
If the workshop trips out often then give it its own RCD so the rest of the house doesnt go dark every time.
I have a 100ma 10ms RCD for the whole house, and under it lots of 30ma RCDs, usually (though not always) the 100ma doesnt trip so i have some lights left
george
On Wednesday, November 14, 2018 at 11:14:11 AM UTC, Andy Burns wrote:

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On Wednesday, 14 November 2018 10:47:22 UTC, The Natural Philosopher wrote:

RCD failing to filter out hf properly. A L-N snubber near the rcd might help. Meggering all appliances too, the rcd might be overly pre-loaded.
N-E problems tend to cause trips at low loads.
NT
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On 14/11/2018 10:47, The Natural Philosopher wrote:

Is the a time delayed rcd?
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On 14/11/2018 18:12, ARW wrote:

Can't remember. Dont think so.
I had a 30mA whole house but it was always going so upped it to 100mA.
Oh well. No short route to checking it all out.
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On 14/11/2018 20:14, The Natural Philosopher wrote:

Forget the RCD? (not recommended) All RCBO CU? At least you would only loose one circuit unless there are multiple faults. Full EICR? And the first thing this would flag up is no 30mA RCD protection on the sockets etc It might however find the fault.
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I have a 100 amp 100ma 100ms RCD for the whole house, (after an isolator switch) and under it lots of 30ma RCDs, usually (though not always) the 100ma 100ms doesnt trip so i have some lights left
george
ps accidentally wrote 10ms not 100ms in my first reply
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