Mysterious RCD tripping & mysterious readings for a boil

[Polly1]

I've got a mysterious nuisance trip problem on an RCD & wonder if anyone can help?

The light circuit isn't on the RCD, but about twice a week, the RCD is tripping when lights are switched. There is one main hallway light switch where it happens the most, but tonight it tripped when a bedroom light was switched on.

As far as I understand RCD's, a problem on this light circuit shouldn't activate the RCD. Or is my understanding faulty? Can a fault in a non RCD circuit cause an imbalance that affects the RCD anyway?

I was also testing the ground floor ring & got a 0.26 meg-ohms reading on a 250 volt insulation resistance test. After checking all the outlets for faults, I came to the boiler which is spurred off the ring main. When the boiler was isolated, I got a 2.2 meg-ohms reading. But when the boiler was switched on, back to the 0.26 reading. So I suspected there was something up with the boiler, but when I tested the boiler wires independantly of the spur, I got a 99 meg ohms reading. So I assumed I had a faulty spur & replaced it. But I was back to square one though. Testing the boiler shows no fault, testing the circuit with the boiler switched off gives me a very good reading, but testing the ciruit with the boiler on gives me a bad reading.

Do you have any tips and do you think the 0.26 reading is an indication that the circuit could trip at anytime and that therefore even though the light is off the RCD, that light switching could alter the balance enough on the boiler circuit to cause the RCD to trip?

I'm trying to determine whether I should investigate the boiler more, or investigate the light or other circuits more. It's a 5 bed house, so lots of places to check.

Thanks

 

[Jim2287]

what are your insulation resistance test taken between ? Phase to neutral or Phase/Neutral to Earth ?

 

[Polly1]

Neutral/earth is the test I'm speaking of. Phase/earth was showing no fault. Phase/neutral was showing no resistance as expected.

 

[Lectrician]

You must disconnect the neutral of the circuit at the CU, and remove the fuse to test a single circuit - you are getting funny readings due to all the other circuit neutrals, and the fact that neutral end earth are connected at some point in the supply to your property, even if this is via an earth rod and mother earth.

I would also strongly advise not using an insulation tester on a boilers circuitry!

 

[Polly1]

The Phase/neutral/earth were all disconnected from the CU during testing.

I also only tested phase/earth and neutral/earth on the boiler voltage resistance test at 250v.

 

[Polly1]

I would have thought somebody could shed light on why the RCD trips from light switch activity even though the lights aren't on the RCD......

 

[cozycats]

I'd disconnect every circuit P,N & E from the cu and then test . there should be no continuity between any of the cables at all apart from p-p, n-n e-e on the ring. you will have to make sure every socket is switched off or unplugged. It is possible that poorly placed cables (abrasion) or rodent damage is causing an intermittent link between 2 circuits which should be electrically seperate. Does this happen when someone stands on a particular floorboard etc is the kind of q you need to think about

 

[Adam_151]

I would have thought somebody could shed light on why the RCD trips from light switch activity even though the lights aren't on the RCD......

Its slightly obscure but I have heard of such a thing occuring before, usually it involves a fleeting connection (you'd be wise to replace the light switch in question), which causes frequencies other than 50hz to be super imposed on the wiring, if the RCD mananged to see this (which I doubt, because you'd need quite a big light to cause much of a voltage fluctuation back at the CU) then it could get 'confused'. The more likely way this is occuring is that at the higher frequencies, capacitive coupling between adjacent conductors increases, if the switch cable ran by the ring main for example the leakage through the capaciative coupling between the circuits combined with the background leakage might sometimes be enough to trip the RCD.

 

[baldelectrician]

have had something similar before, turned out to be a faulty supply neutral.

Check for loose connections in the consumer unit area (if you are competent) , ask REC to check meter and cutout connections, if that doesn't help.

 

[RF Lighting]

I had a similar fault on a job a couple of weeks ago. I fitted a new split load consumer unit , and on the upstairs socket circuit there was a neutral to earth fault. The RCD would hold in fine with no load at all on the installation (which is unlikely in an occupied house) but as soon as any load was run through the installation whether from an RCD protected circuit or not the RCD would trip. Also similar to baldelectrician I was called to a house which had a split load consumer unit fitted (sockets RCD protected, Lighting not protected). The customer told us of how the lighting in the house would start to flicker and become progressively worse until the RCD tripped. We eventually tracked this down to a loose connection on the incoming side of the REC cut out fuse.

 

[Polly1]

Thanks for the suggestions. I'll be working on this tomorrow & will post how I get along.

 

[daviegarden]

Disconnect the complete ring in question at the consumer unit, go round the house and make sure everything else still works, even the lights. Carry out a ring main continuity test on the ring, this will tell you if the ring IS a ring, the readings should all be below 1.0 ohm. remove each of the sockets on the ring and check them for polarity and additional cables, you should only have two of each in each socket, apart from the socket that the boiler is fed from. If you can't find a fault do an earth continuity test and a polarity test on the lighting circuit, include the neutral in the polarity test and do these tests BEFORE you reconnect the ring. If any of the readings are not typical, while both circuits are still disconnected, check for continuity between the socket circuit neutrals and the lighting circuit neutral at the consumer unit. The reading should be over-range, do the same with the feed cables, same reading should apply. Then check neutral to earth common to both circuits for continuity and earth from one with neutral from the other. These earth-neutral continuity tests should also be over range because all the cables are disconnected at the consumer unit. If you get an ohmic reading between the earths and neutrals you have either got interconnected earths/neutrals or there are cable compressed against each other somewhere in the building. In summary, it sounds like a common neutral problem, maybe combined with a slight earth-neutral fault.

 

[Polly1]

Well what a day. Thanks for all the tips. Previously I've been unable to observe the problem myself, but today the problem was very clear as everytime I used the hall light switch a few times, the RCD went. I turned off all breakers on the RCD & the problem persisted with the RCD tripping with no load at all on it. Later in the day I also discovered when doing a voltage resistance test on one of the RCD breakers that the phase to earth had a 0.26 reading at 500v & a 1.08 meg ohms at 250v. Could that indicate a dodgy breaker or dodgy RCD?

Anyway I made some good progress & isolated a phase to earth problem in one of the ring mains. I also as suggested changed the main offending switch. Immediately after I changed the switch the RCD stopped tripping. But that didn't explain why the light circuit was causing the RCD trip whilst not on the RCD protected circuits. But I felt I was getting there. So I carried on with the work to isolate the ring main problem.

However, after I'd packed all my tools into my car, I switched the light once more & the RCD blew!!!!!!

So tools out again. This time into the REC connections as suggested by a couple of posters. What I found was that the feed into the meter was tight on phase & neutral. On the the supply to the CU side, phase was tight & neutral was in need of tightening, but not by a huge amount. So I tightened it about 2 turns of the screw until it was tight.

I had to go after that & I hope it was that REC screw, but I want to ask the poster RF lighting. How loose was the connection causing the RCD tripping you mentioned?

Thanks

 

[Polly1]

For anyone who reads this I should say that the original boiler problem that I described turned out to be a dodgy link in the ring main that was adjacent the boiler, so it wasn't the boiler at all.

The leg of the main seemed to go underground through concrete & may have water penetration or damage from when it was first installed. So I removed it & replaced it with a new surface leg in trunking.

 

[RF Lighting]

It was loose enough to let the conductor move about in the fuse holder and probably only needed one turn to 'nip it tight'. when there was some load on the system my cutout was crackling away and got really rather quite hot. You really shouldn't be messing about with REC equipment though, and if this is where the fault lies then it is the responsibility of the REC to put it right. Also the problem I went to only seemed to cause problems when the system was loaded up and the failed connection heated up.

HTH

Rob