RCD tripping overnight

[Richard C]

Hi all
We had a new consumer unit, earth upgrade (new earth rod) main & suplementary bonding, some circuit additions & the whle system tested & certified around a 15 months ago. All has been fine until 5 nights ago when, for no apparent reason the main, 100mA RCD started tripping overnight. This seems to happen somewhere between 02:30 & 03:30 - I don’t sit awake to check! Resetting the unit during the first night caused an immediate re-trip & I’ve not tried that since but re-set the thing in the morning & everything will happily run all day until the following night! At first I thought it must be timer related but through a process of elimination & overnight circuit isolation, I’ve narrowed it down to the downstairs ring main & with only 3 fridges on line, I assume it must be one of these. We had an ancient freezer that packed up last year but it tripped the circuit breaker & not the RCD so I assume that was caused by a dead short in the motor.

What I don’t understand is why it should only trip at more or less the same time overnight. The fridges & most of the other circuits are active during the day but I get no trips at all; for the sake of my sanity can someone possibly tell me:

a. is it possible the RCD is faulty & is there any way of checking?
b. is there some technical reason associated with the main incoming supply that would make the RCD more susceptible to tripping at night?
c. if it is one of the 3 fridge motors causing the problem (2 are only a year old) can this be rectified as they all still appear to work OK; or am I just awaiting impending failure of one of them?

 

[bernardgreen]

It would appear from reference to an earth rod that your supply is a TT system. ( earth is from the earth rod and not from the incoming supply cable )

Is the RCD the older voltage operated type or was a new one fitted when the new consumer unit was fitted ?

If it is the older type you may be at the mercy of a neighour's "leaky" electrical installation where a heavy load is being switched on by a time clock to take advantage of economy or off peak tariffs.

 

[reggie perrin]

Immersion heater?

 

[Richard C]

It would appear from reference to an earth rod that your supply is a TT system. ( earth is from the earth rod and not from the incoming supply cable )

Is the RCD the older voltage operated type or was a new one fitted when the new consumer unit was fitted ?

If it is the older type you may be at the mercy of a neighour's "leaky" electrical installation where a heavy load is being switched on by a time clock to take advantage of economy or off peak tariffs.

Thanks for the response.
Yes it is a TT system; main RCD is 100A /100mA & was new & fitted with the new consumer unit; mine & my neighbours supply is via overhead cables from a telegraph pole in an adjacent field (were rural). We have separate supply cables but obviously terminated on the same overhead supply.

 

[Richard C]

Immersion heater?

Thanks for the response.

Immersion heater has been eliminated by isolating that circuit overnight.

 

[industryspark]

If you suspect one of the fridges then why not unplug them overnight one at a time thus pinpointing the faulty fridge (if it is one of them). of course you'd have to empty the fridge overnight.

WARNING..... do not contemplate doing this in the current warm weather if it risks making beers warm......that just would not be good!

 

[Steve]

Electrically leaky motors doesn't mean they are going to pack up anytime soon. Domestic fridge compressors can last for decades.

 

[JohnD]

might be mice chewing a cable under the floor.

Might be water leak in the boiler or pump. During the day the boiler is hot so the water evaporates, at night maybe it can collect enough to trip.

Earth leakages are most often on watery appliances, so unplug washing machine, dishwasher, kettle and turn off shower, boiler using DP switch. Also cooker as oven elements often have leakage on them.

 

[bernardgreen]

My money would be on a "external" influence together with a quirk on that ring circuit perhaps in one of the fridges.

Are there any factories or industrial activity that start up around that time. Their heating systems may come on an hour before workers arrive.

Does one of the fridges have a water supply to a chilled water / ice maker plumbed in copper pipe ? This could create a sneak earth circuit with enough stray leakage though fridge wiring and mains filters to trip the RCD.

 

[Richard C]

Thanks for all the suggestions, it is appreciated. A process of elimination appears to have already ruled out many of the suggested culprits & the fridge/freezer doesn't have a water chiller fitted. Last night I plugged the downstairs freezer into the upstairs ring main, disconnected every other appliance on that circuit & isolated all circuits except the upstairs ring; it still tripped!

My money would be on a "external" influence together with a quirk on that ring circuit perhaps in one of the fridges.

Are there any factories or industrial activity that start up around that time. Their heating systems may come on an hour before workers arrive.

Does one of the fridges have a water supply to a chilled water / ice maker plumbed in copper pipe ? This could create a sneak earth circuit with enough stray leakage though fridge wiring and mains filters to trip the RCD.

Its beginning to look like the freezer but this external influence thing & the fact it seems time related has still got me thinking; we also have a farm next door & so just to ensure I’m not chasing a blue kipper, tonight I am going to leave all the circuits on, disconnect every single appliance in the house & plug in just one electric timer so that the circuit is drawing a small current & I will also know exactly what time it trips. Will the timer draw enough current to make it trip if it is being externally triggered? If it does trip with just the timer in circuit, can I safely assume the problem is down to an external influence & if so, what do I do about it! Is it down to the power authority to sort out problems like this or do I have to go knocking on doors myself? I get on well with the farmer (not so well with the neighbour) but he is hardly technically minded so I can just imagine trying to explain something he has only recently started doing is interfering with my power supply as even I would have been sceptical if someone came knocking on my door.

If it does turn out to be the freezer, is there anything I can do with it to prevent the nightly problem or is it a skip job?

 

[suffolklad]

if you plugged a plug in rcd and then fridge in that wont it just trip the plug in rcd and not the house one? just an idea if it is the fridge

 

[ricicle]

if you plugged a plug in rcd and then fridge in that wont it just trip the plug in rcd and not the house one? just an idea if it is the fridge

Only if the plugtop RCD were to operate first it would indicate the fridge, but this might not operate first.(I didn't see that the 100mA was time-delayed)

 

[Steve]

if you plugged a plug in rcd and then fridge in that wont it just trip the plug in rcd and not the house one? just an idea if it is the fridge

No they could both trip if same rating.

 

[Taylortwocities]

Ricicle has suggested something we didnt yet ask.

How many RCDs do you have in your consumer unit?
you should have two. The main incomer one should be 100mA and there should be a second (30mA) RCD protecting the socket circuits.

The 100mA should be a time-delayed type (S). Can you check to see if it is marked as time delayed. if you not sure, post the make and model number of the RCDs.

TTC

 

[Richard C]

Ricicle has suggested something we didnt yet ask.

How many RCDs do you have in your consumer unit?
you should have two. The main incomer one should be 100mA and there should be a second (30mA) RCD protecting the socket circuits.

The 100mA should be a time-delayed type (S). Can you check to see if it is marked as time delayed. if you not sure, post the make and model number of the RCDs.

TTC

It’s a Wylex C/U; the main RCD is Wylex 100A 100mA WRMT 100/2 & is marked ~S so presumably it is time delayed; the second RCD is Wylex 80A 30mA WRS 80/2 & is marked ~

The suspect freezer looks like it is a blue kipper as the main RCD tripped again last night (@ 02:20) with just a plug in timer & a night light on the upstairs ring, all the circuits were on but no lights were on & every other appliance in the house was switched off. Not sure where this leaves me as by a process of elimination, it doesn’t seem to be related to any circuit or any appliance; any advice would again be greatly appreciated as although I have a good basic knowledge I’m not experienced & running out of ideas!