Nuisance RCD trips

[Lobster]

Currently being driven nuts by the above... I have a split load CU. When the RCD trips, it's only that which goes; ie none of the MCBs trips at the same time which means there's not much for me to go on.

Question 1: if, for diagnostic purposes, I switch off each circuit in turn by manually flipping its MCB, does that actually isolate the circuits for the purpose of elimination? Or would I need to open the CU and physically disconnect all the cables from each circuit in turn, completely?

Question 2: what does the fact that no MCBs are tripping; and the trips occur at apparently random intervals (4-5 times in the last day, with interval of up to ~10 hours so far, and including an overnight trip; and no problem with reinstating the RCD immediately) tell me about the nature of the fault, if anything?

Thanks for any pointers!

[Martin Brown]

MCB should do it but the circuit will no longer be live so you need to put the freezer on full bore for a while first (which might actually demonstrate conclusively that it is the root cause).

It is fastest if you drop out half at a time:

11110000 11001100 10101010

It could be ambiguous if no trip occurs. You may have to repeat with the other handed pattern so that you have three patterns that all cause a failure. Then depending on which pattern(s) trip out try just the appropriate failing circuit on its own.

My money would probably be on cooker, fridge, freezer or similar. Transient earth leakage fault as a motor starts or something like that.

[charles]

The MCB will only trip on current overload. The RCD trip indicates a leak to earth somewhere. This is frequently on things that have heating elements, but they'd probably need to be switched on for the trip to occur.

But it could be water getting into a junction box somewhere. Do you have any outdoor supplies?

[Dave Liquorice]

Sort of. If the trips are being caused by a live > earth leak then switching off the MCB will give an indication that that circuit has a problem. If the trips are from a neutral > earth leak then switcging off the MCB won't make any difference.

It's intermittent. B-) Others have pointed out likely appliances but it could be down to rodent nibbled wiring...

[John Rumm]

It may prevent the trip, but you can't be certain, since switching off the circuit leaves the neutral connected. If the cause of the problem was say for example a neutral to earth short, then the trip could still manifest.

That would ensure the circuit is isolated.

It tells you there is no associated overcurrent trip. It probably indicates that you have high leakage that is "sensitising" the RCD (i.e. using most of its trip current budget, and leaving it close to the point of tripping), and hence it does not take much to put it over the edge. Even transients on the mains that could originate outside of the house may trigger the trip.

Try here:

[Andrew Gabriel]

My parents had a couple of trips recently, but knew it was a desk fan. Fan is double insulated and all plastic case, so it can't generate earth leaks. Eventually I opened the plug, and found a dead spider nesting bwteen the live and (unused) earth connection.

[Caecilius]

I has similar issues for years, which turned out to be a combination of things:

1. An oven element, which had a leakage that was proportional to its temperature, going up to around 10ma when it was red-hot. Measured by breaking the earth wire out and measuring the current with a multimeter on mA AC range. Cured by replacing the element.
2. An outside armoured cable with a low live/earth resistance when wet. Discovered with a megger, cured by disconnecting the circuit.
3. A large number of switch-mode power supplies: household PCs, router, NAS Etc, which all added up. Cured by replacing all the 32 amp MCBs with RCBOs to give each ring it's own 30ma leakage budget.

[SteveW]

Could be anything, but my first thought would be to check anything with a heating element.

Over the years we've had it with the cooker, washing machine, dishwasher and iron, but nothing else.

If your fridge/freezer is a frost free one, it'll have a heater too.

SteveW

[Bob Eager]

And an immersion heater if the OP has one.

[Reentrant]

We started getting occasional RCD trips - every one being at at 10 o'clock at night. This was precisely when the time-switch for our garden pond pump switched off (it's a 240v pump connected by 10m or so SWA cable).

Now I leave it running 24/7 (better for the external filter, apparently) and no more problems. So it was either the time-switch or some weird induction effect between earth and the armour wires when the voltage was switched off.

[Lobster]

Thanks for all the replies. It hasn't 'popped' since 5 am today (it's now 9:30 pm so hopefully whatever it was has gone away... I'm suspecting it might have been cat pee actually; no evidence but we certainly have a problem here that way :(

[Stephen H]

Seconded. my old CU was a split load job, with 4 MCBs on a 30mA RCD feeding a cooker, two ring mains and an immersion. The other half of the CU was two MCBs feeding two lighting circuits.

Whenever I started two PCs or more at the same time, that would trip the RCD. The trouble was that 30mA leakage current budget was spread across four circuits.

I then got the CU changed for one that had RCBOs only and i split several circuits up into smaller circuits and added new circuits. This I went from 6 to 13 circuits, all on RCBO's

I also added high integrity as well, so the smoke dets, CO dets and intruder alarm are on their own RCBO, the outside sockets are on their own RCBO and the outside lights are on their own RCBO. That way anything happening outside does not affect the inside.

The boiler is also on its own RCBO so reducing the risk of the house freezing over if a different circuit develops a fault.

I've not had a single trip since.

although RCBOs are more expensive, its worth every penny from the hassle/inconvenience/ease of fault finding point of view.

[F Murtz]

Kettle or some other appliance kept near the sink.

[David]

Mine tripped yesterday due to a loose raisin from a teacake becoming stuck between the toaster element and the case. Happened twice (different raisins).

[Bob Eager]

At least it wasn't an excessively large currant.

[John Rumm]

[Dave Liquorice]

becoming

Or a large fly and excessively currant.