Radiator element tripping the RCD.

[Greenie9657]

Good morning all,

I've recently installed a towel rad on the CH system but added a 600w element bought from Screwfix.
Last night I went to switch it on and the RCD tripped on the CU. the rad is connected through a timer switch and FCU but the 3 amp fuse doesn't pop in the FCU. I've disconnected the rad and hung in a light and everything works fine so I have no doubt its the element. It's just going to be a pain as I now need to remove the rad!

Is there any way of testing the element either in situ or on a bench when I remove it from the rad? I know you aren't meant to turn them on but I quick switch on to make sure it warms up should be ok, shouldn't it? Or a con check through the element while it's still fitted (but obviously disconnected)

Thanks for you help.

 

[PrenticeBoyofDerry]

There is very little doubt that the rad element installation has contributed to the RCD trip.
But it could well be that the leakage from the element has been a factor of accumulated leakage of the whole system.
Try isolating every over circuit on the RCD and all other loads on the circuit that the element is on, then operate the element and see if the RCD trips. If it does double check all terminations you have made when installing and check no cable are trapped or exposed.
A ramp test would also be wise to perform on the RCD, to establish that it does reach a normal trip value.
You could do a leakage test on the element tails using a mA clamp meter or an Insulation Resistance Test between the live conductors (L&N) to the earth terminal or casing/body of the element.

 

[Greenie9657]

Thanks Derry

I already did the CU test and it still trips the RCD when only the one circuit is live. I don't have a Megger so I haven't got the ability to insul res check. I just have a standard multi-meter. Is there anything a con check could tell us? Also is it test able on the bench so then I can at least take it out and have a proper look.

 

[ericmark]

I will guess that some how the radiator has been empty when the element was switched on and been damaged allowing water into the mineral insulation. The problem with the mineral insulation used with heaters is if it over heats it can damage the ends where the seal is and draw in moisture. Often you need the voltage to test for this, so a multi-meter will likely show over 1MΩ but an insulation tester which uses 500 volts not 9 will show just a few KΩ. So without the special meter it is hard to be sure.

The electric elements in a wet system can work, but one has to be sure the central heating is not running and all pipes are cold, otherwise they can with therm-syphon try heating the whole house. I tried years ago when the hot water and central heating was on same boiler fitting a switch to the pump, so in summer the boiler with therm-syphon would heat domestic water only, it worked as long as system was cold to start with, but if radiators already hot then it would continue to heat radiators. Same can happen with towel rail, whole system needs to be cold before it is turned on.

 

[JohnW2]

I don't have a Megger so I haven't got the ability to insul res check. I just have a standard multi-meter.

You could at least use your multimeter to check the resistance between the L&N terminals and the earth terminal of the element (with the terminals not connected to anything), as suggested by PBoD. In both cases, the resistance should be 'too high to measure', however your meter displays that. The resistance which would have to exist to trip an RCD would be well low enough to show on your multimeter BUT, the problem is that some such faults only show themselves at high voltage (as per a proper IR tester/Megger), and not at the low voltages used by multimeters for resistance measurement.

However, it would be a start, so would be worth doing. If you found a measurable resistance with your multimeter, then that would mean that the element was definitely faulty. However, if you could not measure a resistance, then that would not give complete reassurance that the element was OK.

Kind Regards, John

 

[Greenie9657]

Hi John,

I measured the resistance across the L and N and it read 35.1ohms. Luck as it my wife was near Screwfix and picked up another element. All fitted back together now and working fine.

Guess it was definitely the element!! But it is a great addition to a towel warmer when you don't want the entire house heated up.

Thanks again for all your posts.

 

[jackthom]

Did you measure the resistance of the old element between L and Earth. That might be more interesting.

 

[JohnW2]

I measured the resistance across the L and N and it read 35.1ohms.

That's not unreasonable, bnut it was the reesistrance between L and E and N and E which really mattered (in terms of LCD trips). Anyway ....

Luck as it my wife was near Screwfix and picked up another element. All fitted back together now and working fine.

Glad to hear you've solved the problem.

Kind Regards, John

 

[Greenie9657]

Hi John,

It was open circuit across L and E. I didn't check it against the working new one as I had a million and one things to do so I just wired it in and it worked.

 

[Greenie9657]

I've still got the old one here if you want me to check anything out

 

[JohnW2]

I've still got the old one here if you want me to check anything out

Well, given the small resistance between L and N, if L-E was measuring as open-circuit, then so would have N-E, so there's not really anything more you could test with a multimeter. It sounds as if the multimeter results are all 'satisfactory' but, as I said, it could be that the L-E and N-E resistances would be different (i.e. not open circuit) if tested at much higher voltage, as with a Megger etc.

Kind Regards, John