Immersion heater tripping probs!!

[diverdan74]

Hi all,
new to this site, so any help will be very much appreciated.
I have just replaced an immersion heater thermostat, (backermatic) but the MCB keeps tripping after a period of 1-2 hours. The old one was wired into the 32amp ring main circuit. I have since put it on it's own 16amp MCB and replaced the stat but it still trips!!!!
Continuity is sound on the wiring and the resistance on the element is 32oms.
Is there any chance the new element could have blown, or can it be i have wired the stat the wrong way round as there are no markings on the new stat...
confused, Dan

 

[ericmark]

Should be wired in series 32 ohm would be 7A something seems wrong there.

 

[1john]

Was the mcb brand new, if not you may have a tired one.

 

[diverdan74]

The element is rated at 3KW so i assumed a 16amp MCB was right? It is a new Hager MCM and a new Backermatic stat plus a new double pole 20amp switch.
Any Ideas?

 

[londonboy]

Id check the immersions wired correctly first, because 1-2 hours is roughly the time it would take for the stat to open

 

[diverdan74]

I've had a multimeter on the wiring and that read sound, earth to the cylinder, neutral to correct peg, live in one side of the stat and out of the stat to other peg. Will the direction of power through the stat make a difference?

 

[martinxxxxxx]

Have you don an insulation resistance check on the element and / or the wiring?

Martin

 

[flameport]

A thermostat is a switch. Therefore is cannot possibly matter which way round it is connected.
If the thermostat was connected to the wrong terminals/wires, either it would trip instantly, or not work at all.

If it is tripping after a certain time, this suggests that the element has an internal fault which only occurs after it has been heated for an extended period.
OR - there is an internal fault to earth, the circuit has no RCD and the actual current drawn by the element is significantly greater than the intended 13A. (e.g. an L-E short halfway along the element could result in the current being 2x what it should be.).
OR - this isn't a 3kW element at all, but some other higher rating.

 

[diverdan74]

I have done resistance test on all points of the circuit and they where ok, on the element though the reading just kept slowly dropping could that be an indication that the element has blown? I read it should be around 17-20ohms i used a 2Mohms test...that's correct isn't it?
Can i bypass the stat and wire the live direct on the element to see if it trips the mcb straight away? Don't really want to take the element out as it's been sealed in with some hard white sealant!!! Trying to find out what is from the bloke that installed the new element but no response yet!!
Thanks for all the advice.

Dan

 

[seco services]

i would of checked the element ages ago with a resistance reading of 32 ohms.

 

[diverdan74]

Did try resistance test on element but didn't stabilize. So get the bloke in that put it in as with all that gunk round the nut i don't want touch it!!

 

[seco services]

Did try resistance test on element but didn't stabilize.

thats what i mean take it out.
how old is it ?

 

[diverdan74]

brand new, apparently, been in 2 months and been on for about 5 hours on and off. More off than on!!

 

[ColJack]

we need pictures of the wiring of the stat.. sounds like it's wired wrong and causing a dead short when the tank gets up to temp and it opens..

why did you change it? what went wrong with the old one?

 

[flameport]

Can i bypass the stat and wire the live direct on the element to see if it trips the mcb straight away?

No, as this will remove the only safety device preventing the tank from boiling up.

To test the thermostat switch off, rotate the temperature adjuster to minimum, switch on.
If the MCB trips, the stat is somehow causing a short to earth and is presumably defective internally.

To test the element, remove the wiring and thermostat.
The resistance between the L&N terminals should be approx 20 ohms.
There should be no measurable resistance between L&E and N&E. (Ideally this should be done at 500v with an insulation resistance tester, and readings of 10s of megohms at least would be expected).

If the resistance reading changes all the time, either the meter being used is broken, the batteries are feeble or something else is connected to the circuit.