Boiler tripping

Hi

im a sparky myself but can’t seem to work this out. I’m doing a rewire of a house.before it was a rewirable fuse box. Now  Installed duel rcd fuse box. Everything works apart from boiler. Boiler has a flex as usual. With the boiler breaker turned off but all connected at fuse box and fuse spur, when ever I touch the earth from the boiler flex to fuse spur earth it trips straight away. But live and neutral of flex not connected to fuse spur. Also the boiler is around 12 years old (valiant). Water and gas not bonded at the moment. Another thing is that the boiler side rcd is not tripping it’s the other rcd that’s tripping. Checked for reverse neutral in the fuse box but all correct. Any idea? Thanks in advance. 

Sounds to me like you have a short somewhere in the house between a neutral (from a circuit on the side that trips), and the central heating pipes.  Have you checked the immersion heater if there is one?

If you make the bonding right you will most likely have a permanent fault.

I would agree with the above but, just in case it is a fault from line to central heating pipes, have you checked the voltage between the water/central heating pipes and earth as if the water pipes are not currently connected to earth they could become live in this case.

If it is a neutral to earth fault then the pd should be minimal.

A good one for this sort of fault is a damaged cable in a wet environment by the pipes.

I am unclear how have you energised other circuits with no bonding in place to Gas/Water. Or how you have done satisfactory insulation resistance tests on these other circuits also without bonding connected. What are your IR test readings for this boiler circuit?

Doc H.

Concur with the doc. 

What testing have you done prior to or after the cu change? 

Jmatt said:

Hi

im a sparky myself but can’t seem to work this out. I’m doing a rewire of a house.before it was a rewirable fuse box. Now  Installed duel rcd fuse box. Everything works apart from boiler. Boiler has a flex as usual. With the boiler breaker turned off but all connected at fuse box and fuse spur, when ever I touch the earth from the boiler flex to fuse spur earth it trips straight away. But live and neutral of flex not connected to fuse spur. Also the boiler is around 12 years old (valiant). Water and gas not bonded at the moment. Another thing is that the boiler side rcd is not tripping it’s the other rcd that’s tripping. Checked for reverse neutral in the fuse box but all correct. Any idea? Thanks in advance. 

Click to expand...

Rewireable box..??   How many circuits?...  4,  6   8... ?

I guess a fully equipped RCBO board would have been much better than a Dual RCD?

Howeverl....   

based on the fact that RCDs for basic domestic requirements must trip at 30ma BUT NOT at 15ma..

That would suggest a resistance somewhere between   15333.33ohms  and 7666.66ohms between earth and Live OR Neutral...

and or maybe lower...….

Shouldn't be that had to find for a Spark Yourself.