Night Storage Heater/Economy 7 tripping out

Please help. I have a basic understanding of electirics, but 5 Electricians are stumped and I don't know where to go next. My mother in law heats her house by 4 Night Storage Heaters, all of which are wired to a Economy 7 supply/fuse board, with no RCD just MCB. The largest of the Night Storage Heaters began to fail and trip the MCD. To cut a long story short, she ended up having a new one fitted. Unfortunately this new one also now trips the MCB. The electricians have tried swapping the various supplies to the various heaters around in the E7 fuse board and the same heater is tripping the different MCB. It would be an obvious thing to consider that there is a fault on the circuit between the board and the night storage heater. However, they have now wired it into the day time board and it is not tripping. Has anyone got any ideas as we need to get it wired back into the E7 circuit??
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On Tuesday, 19 December 2017 17:44:05 UTC, Steve Poulton wrote:

Whoever those 5 individuals were, they weren't electricians. An electrician would test the heater, circuit, and MCB using a calibrated test meter. Swappin things around and hoping they don't go bang is not a suitable test methodology [1]
I'm going to guess a phase-earth fault on the heater circuit and a reversed phase-neutral incomer on the daytime board (or vice versa) so the fault presents as a neutral-earth on the daytime board, which does not trip the MCB (but would trip an RCD).
Simple polarity testing at the various locations would rule that out.
Owain
[1] For electricians, it works fine in computer repair :-)
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The question is though, if this has been in place on the old heater all that time how come its suddenly reversed? I'd imagine this is the mindset which has caused the confusion. It might have been wrong for years but only showed up due to a fault and some components in the new heater are causing it to still show up, It can be very dangerous to assume that just cos it used to be fine, that the wiring was always correct.
Brian
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On Tuesday, 19 December 2017 17:44:05 UTC, Steve Poulton wrote:

There's even more context at
https://www.electriciansforums.co.uk/threads/totally-stumped-night-storage-heater-tripping-out-mcb-but-ok-on-daytime-circuit.124737/
So far possibilities seem to be (a) a polarity error as I suggested - possibly introduced due to a meter change - or old 15A MCBs overheating and thermal tripping.
Owain
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On 20/12/2017 10:32, snipped-for-privacy@gowanhill.com wrote:

The polarity error is possible, but it seems hard to believe that none of the visiting sparks have done a continuity test between L+N and E on the circuit and the heater (even if they did not do a full insulation resistance test).
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John.
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On 20/12/2017 12:06, John Rumm wrote:

I don't think it is a polarity error, it has to be thermal operation of the E7 board oldish MCB(s). Try replacing the MCB for the heater circuit that is tripping.
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On 21/12/2017 15:20, Ash Burton wrote:

That would only make sense if the new heater is more powerful than any of the others, since the trip moves with the heater to other MCBs.
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On 22/12/2017 03:39, John Rumm wrote:

The OP said "The largest of the Night Storage Heaters began to fail and trip the MCD." So, I assume the replacement is also large.
These heaters are expensive items, so replacing one that was possibly working okay is a shame.
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On Friday, 22 December 2017 09:55:43 UTC, GB wrote:

Especially when the fault turned out to be arcing within the off-peak CU main switch
Owain
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On 22/12/2017 03:39, John Rumm wrote:

But does not trip when connected to an mcb on the 24 hr supply board IIRC.
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