Both RCD's trip randomly

[dawsirn]

Scratching my head a bit on a job today. Been to clients house to investigate fault with installation.

For the past few months client has reported that both RCD's on his dual RCD board have tripped at the same time on odd occasions thus losing all power to house.

House was rewired by another contractor few years back. TN-C-S earth.

It happens at all different times of the day and night and the RCD's reset with no problems afterwards too. Only happens maybe once a month or even less than that.

Went today to have a look and carried out Insulation Resistance tests. First a global test between N-E bars then for each individual circuit N-E & L-E.
-Circuits protected by left RCD were all fine. Lowest IR reading obtained was about 50Mohm which quickly rised to >99.9 during test
-Circuits protected by right RCD had low IR readings on two of the socket circuits. cct2 0.63Mohms and cct3 2.63Mohms
Both ccts were fine once all items were unplugged. IR readings went up to >99.9. Likely culprit(s) seems to be the numerous surge protection extension leads and IT equipment in home office in cct2.

Both RCD's tested and trip times were fine and ramp tests showed they tripped at 25mA

Carried out IR test between both Neutral bars and interestingly I did get a reading of about 60Mohms quickly rising to >99.9Mohms. Narrowed this down between cct's 2&3 on right side with cct7 (sockets) on left side.

All end to end readings on socket circuits were fine and as mentioned above all IR readings with wiring seem fine once everything was unplugged.

Bit of a loss to what would cause both RCD's to trip at same time. Opinion is that all the IT equipment is enough to occasionally trip the one RCD but can't think why both would trip.

Only time I've seen both RCD's trip is when borrowed neutral is present or wrong connections in consumer unit after a change, so the random nature of them both tripping is confusing me a bit.

I'll stress that I haven't actually witnessed this symptom happening but him and his wife are adamant that both trip at same time.

Is there anything external to the installation that can cause both RCD's to trip at same time or any else I could look out for. I've asked them again to take notes if it happens again and recommend to remove surge extension leads where not really necessary.

Thanks for reading and any help would be much appreciated.

 

[Octopus]

Observations/ comments

Do you have a clamp meter?

Did you try to move the incoming tails in the meter and /or head?

And yes a fault on one side of a CU can trip the other RCD ........ but both together is unlikely.

What is the Ze / Zl and is the incoming voltage L-N stable?

 

[dawsirn]

Don't have clamp meter with low enough range to detect mA. Might be tempted to get one now.
Didn't think to check tails and now I feel daft as I didn't check Ze or voltage readings. Got a bit carried away with other dead tests and had other jobs to get too. If issue persists I'll check these.

Thanks.

 

[Octopus]

^^ none of us have all the answers but if you do domestic an earth leakage clamp meter is a must .....

 

[Octopus]

^^ none of us have all the answers but if you do domestic an earth leakage clamp meter is a must .....

 

[Wilko]

Another thought from left field is the cutout connection to met and bonding. Perhaps check all is in order?

 

[Adam W]

My first impression is that both RCDs tripping suggests a borrowed neutral.

 

[marconi]

I think you should:

a. buy a clamp meter and take some earth leakage current readings.
b. check the tightness of all connections in CU, intake, and earthing and EP. Check the continuity of EP bonds and state of clamps.
bb. Open up cct2, 3 and 7 and forensically inspect the wiring, its insulation and connections.
c. PAT test any item with an earth.
d. PAT test all white appliances, electric cookers and hobs and extractor hoods, and electric shower.
e. ask about any water leaks or floods.
f. have any holes been drilled or screws/nails been driven in floors, ceilings and walls.
g. look outside the house at any wiring which may be damp or suffering water ingress.
h. ask if any work done on CH and HW system.
i. ask about when problem started and explore with client what changed about that time.
j. do the lights ever flicker/dim?
k. measure the pair of Lin-Lout and Nin-Nout voltages on Main Switch and RCDs - each pair should be nearly the same value (but there will be differences between pairs).
l. measure N-E voltages around the wiring installation.
m. study the post rewire test results and compare with what you have measured.
n. PAT test the in-use surge suppressor sockets (some of which do degrade over time).

Probably enough to do for your next visit. It would be good to hear how you got on.

 

[Octopus]

^^ I love the use of the word "forensically" - does he need to be wearing white overalls at the same time?

 

[marconi]

^^^ I echoed what you drum home every time there is an RCD problem - you gotta be able to measure amps as well as volts and ohms to explore what is going on in electrical wiring.

Yes, forensically is a bit poetic and legalistic for this context - a good hard slow look with a torch is what I meant to say.

It's London Fashion week so maybe ask them what is a la mode at the moment for the style conscious electrician to be wearing.

 

[dawsirn]

Many thanks for the input. We're going to wait and see if and when it happens again and I'll be happy to see what further tests show and I'll update this thread if I find anything interesting or conclusive.