RCD both tripping and refusing to trip!?

[Greg:]

I have been away from a refurb job while other (inc. the owner) have been in and on my return I found that the RCD (for a cooker circuit and three power circuits) is tripping. It triped even when a fairly basic piece of equipment such as my makita drill charger of the owners kettle was plugged in. Thankfully my kettle was fine. It tripped when items were put onto each of the 4 circuits with the other circuits switched off. I put in a new RCD and the same happened.

Then, at least for me, it gets stranger. I did an RCD auto test on the new RCD in connection with the 4 circuits mentioned and it refused to trip. I consistently got a reading of >30mA and I was perplexed and began to question my tester. I went out of the premises, knocked on some neighbours doors and presented a request which roughly said "this may sound strange but it would really help me if I could test your electrical equipment". With good grace someone let me and my tester tripped the RCD in an expected fashion.

I took the new RCD out and wired it in as an external item to the board. sequence of equipment was Main switch> breaker> RCD> socket> tester and the results that I got for my auto test were 34ms, 34ms, 24ms and 10.9ms.

I did some brief IR tests (I was expect elsewhere and missed NE on three of the circuits) and got results typically in the 300 Mohm range. Ze measured 0.25 ohms.

Nothing was plugged in while tests were conducted and the isolator for the boiler was switched off.

This whole thing doesnt make any sense to me and advice would be appreciated.

 

[Sintra]

Did you check for a borrowed or shared neutral?

 

[Specialist]

Anything been altered while you were away ?

 

[telectrix]

had similar a few times. has always been a neutral fault. not necessarily on the same RCD.

 

[SPARTYKUS]

Di you ramp test ? Just out of interest.

as pointed out above check for separation of circuits.

 

[Octopus]

Did you do, or try to do, a Zs on one of the circuits??

 

[i=p/u]

Neutral crossed is the affects of applying a load and trip. Belie me I've been there...

 

[Greg:]

Thanks all for replies

Did you check for a borrowed or shared neutral?

The cooker circuit runs straight to a choc block and I have worked on two of the three circuits. I got the same results on all circuits when three of the four breakers were off. The kitchen circuit needs a lot of investigation (some sockets not working but they may just have been disconnected). all the power circuits are rings and gave good readings at initial testing and at subsequent tests. In my initial testing (done months ago) the RCD tripped like a well trained puppy.

had similar a few times. has always been a neutral fault. not necessarily on the same RCD.

I dont think anyone would have altered the circuitry unless to try to fix a bodge. At one stage I found of the ring cables had been mostly cut through so that only the line conductor held it together. This was must have happened during the demolition of some stud partitioning in which the related sockets had been positioned.

Di you ramp test ? Just out of interest.

as pointed out above check for separation of circuits.

Yes and there was no response. I feel frustrated replying. I know the RCD trips but it doesnt behave when in tests.


Other potentially relevant info.
When I first returned to the job I tinkered with a lighting circuit and then put radio on and then, within the duration of half a song, the radio adaptor exploded. The casing literally flew apart. Its a while since I used the radio not on batteries but that was the first sign of trouble. This was on Saturday before I got the new RCD and I speculated that if might have been this that caused the RCD failure. I checked the weather on getting home and there were no report of lightning in the area.

The gas is not bonded and, curiously, there are two bonding cables connected to the water.

We have previously had three workers in all with battery chargers and other equipment in use with no problems.

 

[SPARTYKUS]

Seems alot going on there. I think Tel answered it, its tripping under load, i.e, when the neutral is employed.

 

[MDJ]

Neutral crossed is the affects of applying a load and trip. Belie me I've been there...

Hey now come on stop drinking, the match was over yesterday.

 

[Corpy76]

I had one today like this (Sunday call out). It turned out to be the little plastic clips which hold the earth/neutral bars to the board. It was a wylex 16 edition. The one between the RCD neutral and non RCD neutral had melted and was causing the rcd to trip. They didn't look to be any phisical connection but the plastic was all melted. Removed the offending clip spread out the bars, went to pub!

 

[trev]

Sounds like a neutral to earth fault. Have you conducted IR tests?

 

[Greg:]

Seems alot going on there. I think Tel answered it, its tripping under load, i.e, when the neutral is employed.

Yes and it seemed to be going well until today.

I just cant see how all four circuits on the one RCD (cooker and three power) can be affected when the only other circuits are lighting!? I've not seen evidence of a circuit crossover.

I also don't know how it is possible for an RCD not to trip when tested in one circumstance but to trip as expected in another. This is a burning question for me.

I will also fabricate a "circuit" from the RCD in situ test it in another way.

 

[Octopus]

Did you do, or try to do, a Zs on one of the circuits??

??

You also note issues with the kitchen circuit - so best you start there!

 

[jaypp]

When I first returned to the job I tinkered with a lighting circuit and then put radio on and then, within the duration of half a song, the radio adaptor exploded. The casing literally flew apart.

what song/station was on? :smiley2:
joking aside, if initial tests good, re-test and compare results. sockets not working, walls removed and cables damaged = someone's repaired a 'boo boo'. employ detective mode pointing out time=money.