Earth problem from boiler tripping RCD

24 Mar 2023
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United Kingdom
Hi everyone,
I am after some advice. We had a new consumer unit fitted with an RCD and when we switch the boiler/heating on it trips the
The boiler is wired to the consumer unit via the ring main with a 32a fuse.

We have an old boiler (ideal Mexico floor standing boiler with a separate hot water tank.)
We have checked the fused spur and it is working correctly. (When in the off position sockets work fine)

The electrician checked this.

We have found a bad earth between the controller unit (Honeywell st6400c) and the boiler (Honeywell type v 460 E).
The blue wire!

Has anyone had this problem?
The electrician stopped at this point.

Would changing the Honeywell Type 4600e be the next step or can any more fault finding be done?
(Obviously for a plumber/boiler engineer)

Many thanks.
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A neutral (Blue or Black in old money) connected to earth (Green/Yellow- old or new) or vice versa would explain an RCD tripping. Check there are no connections between N and E anywhere in the heating system.

Once wired an oven and the clock worked fine, as soon as heat was required it tripped the RCD. I had N and E transposed and the clock current wasn't enough to trip the RCD - load current would.
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Hi, thank you for your reply.... update. Had a boiler breakdown engineer who thinks it may be the wiring center upstairs next to the tank. He wasn’t 100% but recommended changing it.
it wont be the wiring centre, it is simply terminal strip in a box, it cant go faulty, post pics of the "Blue wire " at both ends
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It is one of the blue wires on the left. The one with the 3 going into the same terminal.
One blue goes from the fuse spur, the other to the wiring center and the other to the boiler below. (This is the one that ‘goes to earth’ when tested with a multimeter on continuity.


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Show us a picture of what you mean by "wiring center". More pictures are better than fewer. Be careful using terms like "below" - it doesn't matter to us what the orientation of things in your house is/where your boiler components area screwed to the wall relative to each other, but when you write a post, words and pictures are above/below each other, which can make for confusing reading (neither of the pictures "below" your words appear to be a boiler, so "boiler below" doesn't make sense)
when you turn the fused spur on does it trip straight away or not
and does it then trip when you select heating or hot water
I thought N to E should read 0 ohms and only read "infinity" if the consumer is isolated from the supply with say a DP isolator.
Thank you everyone for your feedback. It will try to be less confusing with my explanations.

When you turn on the fused spur it trips the rcd (straight away)

We found that 1 of the 3 blue wires in the connector on the left was measuring to earth (using continuity). The wire was connecting Honeywell ST6400c to the boiler Honeywell Type v 4600e.
I have drawn a diagram and have taken more photos


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Tripping straight away First thing you should be checking is that no water has got into the motorised valves possibly dripping from pump above them .
The blue (appears to be neutral) wire from programmer seems to be connected to the earth connection at the Honeywell wiring center? Could also be a switch live causing a short circuit.
Thanks, will check this out. But visually it looks dry but will investigate further
Your wiring is wrong but shouldnt cause the fault you have, the permanent L,N&E from the fused spur should go to the bottom right hand terminal on the wiring centre, yours goes to the programmer back plate this is wrong the wiring centre needs a permanent live, yours is running on switched lives
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