Immersion heater(s) tripping rcbo when runs for a while

2 Oct 2010
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United Kingdom
Need some guidance on trouble shooting a new issue that has arrisen with my imersion tank.

I have a vented Heatrae Megalife cylinder with twin immersions. Both immersions are hooked up to a Horstman Electronic 7 timeclock, The lower immersion is on the timed output and the upper immersion is on the Boost output. It's a single circuit, no economy 7 wiring.

For the last two days the circuit rcbo has been tripping when the immersion is on. I might get anywhere upto an hour of run time before it trips. initially i thought it was the lower imersion failing, however the circuit still trips when using the upper boost immersion. same behaviour - runs for a while then trips.

Whats the best strategy to troubleshoot and isolate the issue ? I've got test equipment so can do IR testing, trip time testing etc

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Have you carried no out a resistance check across the terminals on the immersion? Have you done a visual of the wiring?
I had the same problem and it was one of the relays in the timer had failed so that one immersion heater had power all of the time. When the second one switched on, both were running and the breaker tripped. I replaced both the relays for less than £10

Do you have a multimeter or volt stick?

Whats the best strategy to troubleshoot and isolate the issue ? I've got test equipment so can do IR testing, trip time testing etc

You need to be aware that either leakage L to E, or leakage N to E can both cause an RCD to trip. Hence, both poles leading to the element, need to be disconnected when trying to work out which element is the cause of the trip. IR tests, should be easily able to identify which element is failing.
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It is stated that it is an RCBO that is tripping so the cause could also be overload like brig001 has posted.
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Would a faulty element or thermostat not trip it then?
Yes, but there are two elements and both (coincidence?) are said to trip the RCBO so we don't know if it is overload by both elements or a faulty switch (as per brig001) or earth leakage (as per Harry).
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If one looks at the tripping curves 1684636717511.pngwith a 16 amp RCBO it would be reasonable for it to hold for around one hour with a 6 kw load. Likely each immersion heater has a thermostat, the normal would be for the lower one to be set higher that the upper as in the main two immersions are used with Economy 7 and the like, I know not using that here, but still likely set that way, with a modern cistern there is really no point having two immersion heaters unless using off peak, as the tanks are well insulated. But unless a lot of DHW is used, likely the top immersion will rarely switch on, and if it does, not for very long.

It does ask the question why fit a Horstman Electronic 7 time clock? My tank is heated with the oil central heating three times a week for 20 minutes each time, the water cools that slowly it will stay hot for 2 days, and not a well insulated tank, I have thought about using a electric immersion heater timed to come on just after boiler turns off as there is a thermostat in the immersion but non on the tank, so yes there can be a reason for a time clock, could be set to switch on bottom immersion before the weekly bath, but unless some thing like Economy 7 there seems to be no real reason for the timer.

However it could also be tripping out on earth leakage, the mineral insulated immersion heater will draw in moisture if the seals are damaged on the ends, and the heat tends to move the moisture, so in the early stages yes could run for an hour before tripping.

So the question is what is easy to test? I have a clamp on ammeter which would easy show it two heaters running, would also likely show leakage if I could put it around just the lives (line and neutral) and I also have a insulation tester which would show if water ingress was the problem, which I used would depend on ease of access.

Since @Licklieder has IR test equipment that would seem to be step one. Once that is done and we can rule out earth leakage, then we start on the rest.
Thanks for the comments.

As the tank has heated up the trips have stopped - it started tripping after a bath on friday night, when the tank was cold.

Unless there is a fault on the immersion controller, only one element should run at a time - which will keep the load at 3kW. Checking my power monitoring shows this to be the case.

@ericmark I do have an E7 meter. The whole CU / property is wired for E7 rather than just the heating / hot water circuits.

I'll IR test the elements / thermostats and see where i go from there
The way E7 is used has changed over the years, I still remember the white meter, this means the control devices have also changed, early timers had two independent supplies, latter this was reduced to one, so looking up a Horstman Electronic 7 time clock wiring diagram seems we are now on series 3 earlier ones may have not worked in exactly the same way.


shows the latter versions with a single supply, but earlier versions had two independent supplies from memory, I have the same problem with my central heating boiler, there are that many versions, finding the installation instructions for my version is hard.

But if a contact did weld together then you could get what you have described. I says protect with a 15 amp fuse or 16 amp MCB so clearly both should not switch on together, but seems likely they have. It also says 100 mA RCD which does show the age, we have been using 30 mA for some years now.
Is the feed to the top immersion heater still live?
You are describing exactly the scenario I had, where once the top half of the tank was hot enough, the thermostat in the top immersion opened solving the problem


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