Immersion tripping RCD

[Pete123a]

Hi, sorry but another immersion heater tripping RCD, question! I’ve read loads but no seems to have the same issues I’m having so would really appreciate some help.

Two weeks ago the RCD tripped. Narrowed it down to the immersion heater. As by coincidence I was planning to change to the hot hot water tank, I just assumed it was faulty and turned it off.

New tank, new immersion heater same problem. As soon as the immersion switches on, RCD (80amp 30milli amp MK unit) trips. Changed the lead, changed the isolation switch, no difference. Tried powering the heater from a different circuit, same thing.

So my thoughts are that either I’m unlucky and brought a faulty heater which seems unlikely or the RCD is faulty. But this seems unlikely as it hasn’t tripped for anything else.

Can any one help?

Many thanks Pete

 

[EFLImpudence]

Was the different circuit covered by the same RCD(RCCB)?

 

[HERTS P&D]

Have you wired it up correctly?

Andy

 

[Pete123a]

Hi, thanks for the quick replies.

Yes, tried it from a nearby ring main covered by the same RCD

Yes, both the old and new wired correctly. The old one worked fine for at least 20 years

Pete

 

[sparkwright]

Unless you have somehow wired the lead into the immersion incorrectly, it does seem like another faulty element.

 

[ericmark]

A RCD measures power in and power out, if the same it assumes nothing leaking. Selecting wrong neutral would cause it to trip in same way as leakage to earth.

We have three tools to help us.
1) Insulation tester I got one for around £35 and it puts 250, 500, or 1000 volt across the test probes to see if any leakage.
2) The clamp on ammeter with 0.001 amp scale, shows the leakage, but if trips then clearly will not help.
3) The RCD tester, it tests if 15 mA trips the device, then if 30 mA trips the device, and also times how fast 150 mA trips the device. The limit is 40 mS.

Without these it is guess work, the problem is if there is a small leak neutral to earth any load will increase the leak, so the example I use is a bit of damp toast in a toaster switched off, and using the kettle causes it to trip.

With my house with all RCBO (MCB and RCD combined) an immersion heater causing a trip it is reasonable to assume the immersion at fault, but where one RCD supplies many MCB's not so easy, silly idea to combine circuits like that, but common.

So step one is unplug all you can, switching off is no good, as switches often only switch the line, not the neutral, so a neutral earth fault still exists, if the fault remains then it needs test equipment, and even £35 is likely more than an electrician will charge, so likely best option is to get an electrician to test.

 

[Pete123a]

Hi, yeah, checked and double checked. To be honest they are pretty straightforward, even for me! I’ll leave it for tonight and check again tomorrow, as been a long day!

Pete

 

[Pete123a]

Thanks for the reply Ericmark, all makes sense. I’ll have another check tomorrow and then may have to get in help!

Thanks again, Pete

 

[bernardgreen]

The immersion heater may be OK and it is some other item from the same RCD that has a fault. Even though that other faulty device is switched OFF when the immersion heater is switched ON the RCD can trip. This makes it appear as if the fault is in the immersion heater. Damaged or water logged outside lights are often the real cause of the RCD tripping when something else on that RCD is switched ON
V

 

[Pete123a]

Hi Bernard green, thanks for your reply. So if I try the heater with other circuits switched off, that may help identify a problem on the disconnected circuit.

Just one thought, if that was the problem, wouldn't turning on another high load device like the oven, cause it to trip?

Thanks Pete

 

[bernardgreen]

So if I try the heater with other circuits switched off,

Switching OFF a circuit / appliance by switching only the Live ( Neutral still connected ) does not isolate the circuit.

wouldn't turning on another high load device like the oven, cause it to trip

If the other high load device was on the same RCD as the faulty device then yes it would in theory trip that RCD

,

 

[ericmark]

wouldn't turning on another high load device like the oven, cause it to trip?

The length of neutral cable with a load on it will determine the volt drop and that will alter the current flow, so two 3 kW loads on the same RCD one may cause it to trip and another fail to trip it due to being on a different circuit or less cable so less volt drop.

 

[Pete123a]

Hi, really appreciate the replies and explanations, cheers.

So next plan is to get another heater and try that, if that doesn't sort it I'll disconnect the neutral and live from each circuit in turn and see what happens. I'll let you know, but will be a while as other jobs (and work!) are pulling up.

Thanks again, Pete

 

[OvertonPlumber]

Change the cable probably has a break. In the neutral

 

[ericmark]

Since talking about an immersion heater there are problems with mineral insulated devices, the mineral is hygroscopic so if seal is damaged they can fail, also have to ensure they are in water, not unknown for the cistern not to be full of water when first switched on.

New tank, new immersion heater same problem.

This is why looking for other reasons for it to trip. I have a 500 volt insulation tester so first thing I would do is test the immersion heater, I also have all RCBO consumer unit so unlikely for a trip to be anything but the immersion heater, no other items on same circuit.

This is the problem with cheap consumer units with one RCD for many circuits. Just before I moved we lost two freezers full of food when the RCD tripped when away from the home, so when this house had RCD's fitted went for all RCBO's which is RCD and MCB combined, however when they trip there is nothing to show if due to overload or earth leakage. Old house RCBO's as used today were just not available.