RCD Randomly Tripping

[AndrewCollis]

Hi Everyone
I have a puzzler.
I have a split rcd cu. The left hand side is fine which powers everything other than lighting and power (including the kitchen ring) The right hand side has 6 mcb's, basement lighting, basement power, downstairs lighting, downstairs power, upstairs lighting, upstairs power. The right side rcd trips randomly. tonight I have been sitting in one room upstairs on the computer and the rcd has tripped 8 times. I havent moved or done anything to make it trip. The only thing plugged in is the laptop charger. Everything else is plugged in from the kitchen ring which has no problems. Each time, I switch all mcbs off, switch rcd on then 1 at a time switch mcbs back on. the first time, when I switched the downstairs power mcb on, it tripped the rcd again so I removed the neutral wires, and left the mcb off but switched on all the others and it was fine. less than an hour later and it trips again. This time I check and the basement lighting mcb trips the rcd when I switch it on so I remove the neutrals and leave off the mcb and try again and all works fine. I continued this until there was only one circuit connected. When it tripped again, I took out the last 2 neutrals, tested continuity on all neutrals and all was fine so I put them all back in the right places. I went through the whole cu checking all the screws were tight and switched it all back on. All the circuits worked fine for about 90 minutes and then it tripped again. This has been happening for the last few days. Today, it was fine for about 4 hours then tripped twice in 30 minutes then fine for a few hours. There doesnt seem to be a pattern.
Can anyone point me at a place to start?
Any advice would be very much appreciated

 

[JohnW2]

Do these trips still happen if you switch off the other (left-hand) RCD (i.e. 'disconnect' all the 'left-hand circuits' loads)??

Kind Regards, John

 

[AndrewCollis]

Hi John
Thanks for your reply.
To be honest, I haven't tried. I didn't touch the left hand side because it was working fine.
I will disconnect all the left hand side first thing in the morning and see what happens
All the best
Andrew

 

[ericmark]

You seem to understand the problems with neutral being connected. But you could try testing the neutral has no connection to earth. One problem is load on the system will increase the difference between earth and neutral so will increase the leakage current. So a small leak on one RCD protected circuit may not trip the RCD being under 15 mA but when a load is put on the other RCD circuit the voltage between neutral and earth gets larger and the leakage reaches some where between 15 and 30 mA which trips the RCD.

I had it in daughters house, my son did not have his mega, so could not find the fault, so I went up with my mega, and slowly split the ring until I found the bad socket, I was lucky the neutral was always touching the fixing screw so I could test and isolate, had it been an intermittent fault it would have been a nightmare.

 

[JohnW2]

Hi John, Thanks for your reply. To be honest, I haven't tried. I didn't touch the left hand side because it was working fine. I will disconnect all the left hand side first thing in the morning and see what happens

It's only a long-shot, and a 'partial' one at that (it would not localise a fault). However, given the rather odd nature of what you're experiencing, it's just possible that there is a fault on the 'left-hand side' which is resulting in fault currents going partially through a right-hand-side circuit (hence MCB) if that circuit is connected. That could, for example, explain why you have experienced trips when switching on different (RH) MCBs.

That (fairly esoteric) possibility aside, we obviously do not yet have information that points us to any specific cause, but I think I might personally have a fairly high degree of suspicion in relation to the (RH) RCD itself.

Kind Regards, John

 

[ericmark]

You seem to understand the problems with neutral being connected. But you could try testing the neutral has no connection to earth. One problem is load on the system will increase the difference between earth and neutral so will increase the leakage current. So a small leak on one RCD protected circuit may not trip the RCD being under 15 mA but when a load is put on the other RCD circuit the voltage between neutral and earth gets larger and the leakage reaches some where between 15 and 30 mA which trips the RCD.

I had it in daughters house, my son did not have his mega, so could not find the fault, so I went up with my mega, and slowly split the ring until I found the bad socket, I was lucky the neutral was always touching the fixing screw so I could test and isolate, had it been an intermittent fault it would have been a nightmare.

 

[Iggifer]

How dry is the basement? I haven't been in all that many that don't have some level of dampness - how much you have may be an indication of a problem.

On a side note to have the following on one RCD is not great design

basement lighting
basement power
downstairs lighting
downstairs power
upstairs lighting
upstairs power

You would normally have it split so upstairs sockets / downstairs lights were one one, and downstairs sockets / upstairs lights were on another RCD

 

[JohnW2]

How dry is the basement? I haven't been in all that many that don't have some level of dampness - how much you have may be an indication of a problem.

Indeed so - but, as I understand it, the OP still got trips when he isolated (switched off MCB and disconnected neutral) the basement circuits.

Kind Regards, John

 

[deadshort]

What brand is the rcd ? I have had issues with Crabtree RCCB's in the past.

DS

 

[Iggifer]

I'm sure more OP has arrived since I replied! Must be the heat!

I've had a hager RCD trip when lights on the other side of the split were turned on (nothing wrong in the board) we never did get to the bottom of it - problem only happened when you turned on 8 floros,16 downlights & undercabs all at once and then was hard to recreate

 

[AndrewCollis]

Hi Everyone

Thanks very much for all your input, I really appreciate it.

This morning, I disconnected everything from the left side of the cu as suggested by John. Within 15 minutes the right hand RCD tripped.
I thought as the left side has not had a problem I would use the left side as a test side. I removed the mcb for the outside lights (on the left side) so I could move the mcb and wiring for each circuit on the right side over one by one, each time changing after the right side had tripped. when I got to the upstairs lighting (typical last one on the right side) it tripped the left side RCD straight away. I guess this is because the kitchen ring is on the left side which has everything plugged in like you said ericmark. I turned off all mcb's and switched on the RCD then one by one switched the mcb's back on. As soon as I switched the kitchen ring back on it tripped again. I tried this again and the same result. So I disconnected the upstairs lighting again (except earth) and left it off to see if anything would trip. That was about 10am and there has been no more trips today so I am fairly sure the upstairs lighting circuit holds the fault. I guess this rules out the RCD John?

Moving forward, Is there a way for me to find the fault without tracing the wiring from cu all the way through the circuit? I am only armed with a multi-meter which is a pretty good one but I dont have proper electricians tools yet (still training) My only other thought was to un-wire a ceiling rose completely to split the ring in half and only power one side and then the other if the first trips to narrow down the fault?

The basement is pretty good Igifer, it is a 3 story house, the basement has a bedroom, 2 reception rooms and a cloakroom. It is more damp than the other 2 floors though. I took the light switches off the walls down there to check one wasnt wet but they all seemed fine. The layout is poor in the cu, wasnt any clever thinking that went into the design. After this I may look at moving things around (wire length permitting)

Thanks again everyone

All the best
Andrew

 

[JohnW2]

.... when I got to the upstairs lighting (typical last one on the right side) it tripped the left side RCD straight away. ... So I disconnected the upstairs lighting again (except earth) and left it off to see if anything would trip. That was about 10am and there has been no more trips today so I am fairly sure the upstairs lighting circuit holds the fault. I guess this rules out the RCD John?

It sounds as if it does and also that you seem to have probably identified the culprit circuit.

Moving forward, Is there a way for me to find the fault without tracing the wiring from cu all the way through the circuit? I am only armed with a multi-meter which is a pretty good one but I dont have proper electricians tools yet (still training)

To be honest, you've probably reached the stage at which an electrician, with the appropriate kit, would probably be the most sensible way forward - but all the work you've done so far should enable them to concentrate their efforts on the most likely problem area.

My only other thought was to un-wire a ceiling rose completely to split the ring in half and only power one side and then the other if the first trips to narrow down the fault?

Unless it's extremely unusual, the lighting circuit will not be a ring - so that idea is probably a non-starter.

Kind Regards, John

 

[AndrewCollis]

Hi John

All 3 lighting circuits are rings. I bought this house after a building company owned it and had done a lot of work (mostly bad) so Ive had all floors up and ceilings down, replaced lots of old rotten beams and joists. The kitchen nearly ended up in the basement at one point as only one side was supported. The wiring was mostly done so I went round, traced and checked everything (corrected what I needed to) and replaced all the roses, switches and sockets. I ran a wire back to the cu from the last light on each floor to make them rings so I could check continuity on the circuits. I had to run a new earth from twin rods outside the basement front door too as there wasnt any earth connected when I bought the place.

I have worked on wiring a lot over the years (only my own) and enjoyed it very much. I always had an electrician come and check what I had done and make sure it was correct and safe. (touching wood here) each time I have, everything has been fine and the electrician has complemented me on the quality of my untrained work. I left my full time job in the motor trade because I wanted to change my career and become qualified as an electrician. I find it very interesting and the more I learn makes me realise how very much more I still have to learn. With that in mind, if there are things I can do to try and find the fault myself, it will be another learning experience for me.

Thanks again for all your help

All the best
Andrew

 

[EFLImpudence]

As you are so keen and interested buy an Insulation Resistance/Continuity tester.
http://www.ebay.co.uk/sch/i.html?_f...nkw=insulation+resistance+tester&_sacat=92074

and then an RCD tester and Earth Fault Loop tester

or a Multi-function tester which does all three.


At the moment, as you have wired the lights in a ring, disconnect one side and see what happens.

 

[JohnW2]

Hi John ... All 3 lighting circuits are rings.

Fair enough - although, as I said, that very unusual - nothing 'wrong with it', electrically, but of no useful purpose unless the circuit is incredibly long and one wanted to reduce the voltage drop and loop impedance. Lighting circuits are so 'over-engineered' in the first place (in terms of cable CSA), that there's no reason to have a ring from that point of view. However, it means that you can (and probably should) try splitting the ring and just feeding one end at a time - in that way, as you've implied, you ought to be able to narrow down the fault even further.

Kind Regards, John