12v reading on all the negative connections on 240v system

[Cravenbiker]

Had a strange experience today.
customets t&p valve keep blowing on their hot water heater (sealed pressurised electric immersion heater fed from a water pump, pped in from a 5000 litre holding tank )
thought it might be the faulty valve first, after testing that and it being fine I went onto the thermostat on the heater expecting the constant heating of the water to keep blowing the valve and I got a reading of 12-16v on the negative side of the thermostat. I followed this back to the c/u and this was the same all the way back to the main fuse on that board.

i checked if there was any solar power on the house that may have been wrongly connected but that wasn't the case.

thought I'd put a post in here first to see if this is a common issue.
On the positive side of the thermostat supply it gives a reading of 210v can anyone give me any examples of why a positive reading of 12v would occur on any sort of 240v mains fed system?

im no electrician but have a common knowledge of standards household wiring but I've never seen this before! That voltmeter was one of the best investments I've made.

thanks
tristan

 

[Sintra]

Can I ask if this problem is in your own property or for a paying customer.

Edit I see it's for a customer and as you're not an electrician I feel it would be better if you got someone who is suitably skilled to assist you. This thread is now closed.