Intermittent Fault Headache

[jibjob]

Visited a property today that has an intermittent earth fault. RCD trips sometimes 3 times a week sometimes once a month but it's been going on for some time now and owner wants it sorted.

IR'ed across N/E and L/E and all ok until the boiler is switched into circuit. Then the L/E values dip to under 0.1M ohms on all but one of the 5 circuits on the RCD.

Would you expect to get such low readings across L/E on a 2 year old boiler? I would of said not but perhaps I'm missing something. I've got to go back tomorrow with plumber so any advice or comments would be most appreciated. Install is approx same age as boiler.

 

[GT1]

Have you tested IR with the boiler connected..??
If so are you sure you havent damaged it..!

Or are you saying you have tested the circuit UPTO the boiler..?

 

[jibjob]

Tested at 250v rather than 500v but only across L/E and N/E, not L/N. Would of thought damage could only be caused if IR'ed at 500v across load.

 

[GT1]

Yep,
sorry, makes sense now you clarified that.. have you done the circuit upto the boiler anyway..? just to rule out the cable..?

 

[jibjob]

Wiring up to boiler is good.

 

[jimsky007]

Hi

Is the rcd part of a split consumer unit?? lookout for borrowed neutrals and outside lights or sockets?? I had a problem similar and it was the pir light at the back door everytime it rained

You also get leakage currents from most appliances in an installation and sometimes they all total more than 30ma and cause rcd to trip

Can you swing the suspect boiler supply onto the non rcd side of the board?? and try it for a week??

hope this helps you? one of the hardest faults to find??

 

[jibjob]

Boiler is spurred from kitchen ring (doh!). C.U. is split load (16th Ed), all lights on non RCD side as is oven/cooker. No spare ways even it was possible to re-wire boiler on own circuit (which it would appear it's not). Probably gonna have to call in boiler man and see if he can find boiler fault, if not it's back to square one.

 

[jimsky007]

Hi

Might be worth while running temp supply from socket outlet on CCU for 1 week then you can say for def??
Also check RCD I had one that was faulty (Wylex) and was tripping intermittently at strange times normally through the night when I tested at 50% 15ma it was tripping but it had been ok for first 3 weeks in use

 

[UNG]

What type of boiler is it, combi or conventional ?

 

[jibjob]

Hey, thanks for everyone's help btw. Boiler is conventional, Worcester. Tested RCD also, no obvious problems found.

 

[UNG]

Not too familar with the Worcester conventional boilers with regard to faults they may have but have you checked the motorised valve on the hot water tank feed had a few intermittant faults on them over the years when they operate
Also the circulating pump depending which type is fitted can cause problems as they have water in the main body

 

[jibjob]

Going back to job in the a.m. will let you know if I find anything relating to the valves/pumps etc that you mention

 

[UNG]

As with all intermittant faults it is trying to recreate the A+B+C operation that causes the fault

Does the fault occur at different times or similar times each time it trips out

 

[GT1]

had a similar job a while ago, and it was down to the boiler.
As jibjob has done, i proved the wiring upto the boiler was good and fault free, and had to simply hand it back to the client with a contact number for a local friendly boiler engineer i knew..
never found out what it was other than an internal fault with boiler.

 

[jibjob]

Been back with plumber, checked pump, boiler, motorised vales etc still no joy. Trouble is, the RCD may not trip for another 2/3 weeks and then were back to square 1. Gonna suggest I install seperate circuit for boiler (shouldn't be to difficult as they're in close proximity of each other) and move onto non-rcd side of board (spare ways permitting). Then we'll see what happens. Of course it could be nusiance tripping but try telling that to the person that has to live with it. Fun & games eh.