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 Post subject: rcd randomly tripping
PostPosted: Thu Oct 19, 2017 3:30 pm 
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Hi chaps.
A few weeks ago i had a new consumer unit installed along with a new kitchen ring circuit and a new circuit for the range cooker. The reason for this is that I've swapped the lounge with the kitchen.
The range is all electric and is 6 years old but has been in storage for 4.
The rcd for the circuits that include the cooker (there are 2 rcds on the board) keeps tripping- coincidentaly when the cooker is used. When its running and the right side oven is switched on it trips immediately.
The electrician came back and tested the circuits which are ok. When he tested the cooker circuit his megger showed zero resistance when the right side oven was switched on. This would point to an element fault if i am not mistaken.
However, the cooker wall switch has now been disconnected with one blue wire and the mcb is off- but the rcd tripped only moments after they left.
Could the faulty element short to earth this still tripping the rcd?
I'm not an electrician but would like advice on this if possible.
Thanks.


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PostPosted: Thu Oct 19, 2017 4:11 pm 
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Its possible.

The "problem*" with a RCD is you must isolate Live AND Neutral to say some thing is isolated. If the neutral touches earth it can cause an RCD to trip

*Its actually a good thing most of the time.

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PostPosted: Thu Oct 19, 2017 4:26 pm 
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Update.
I have completely removed the cooker switch and it still tripped. I have now turned off my 1980's hifi and it hasn't tripped yet.. Let's see..


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PostPosted: Thu Oct 19, 2017 4:40 pm 
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You can get different leaks on circuits adding up until you get a "cumulative" trip. An existing slight earth leak on something can be an amount that doesn't quite trip the RCD, then another leaky item will cause a trip. You need the two on together for the RCD to trip.
A fridge and a cooker for example.


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PostPosted: Thu Oct 19, 2017 5:23 pm 
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Thankyou for the replies. So far-after a few hours with the cooker switch disconnected and the hifi off everything seems ok. I will turn the hifi on soon as I've had a San Miguel so fancy a dance.. If it trips then, well it's goodbye retro.
I think a new element is definitely required for one of the ovens anyway.


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PostPosted: Thu Oct 19, 2017 7:21 pm 
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Remember that most circuits and equipment will have some inherent leakage. Again it can add up.
If any cooker elements or rings are looking rough that'd be my next move.


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PostPosted: Thu Oct 19, 2017 9:28 pm 
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When you say looking rough what do you mean? I've just ordered a new element for the oven that showed the fault but that particular oven has actually only been used once in 6 years. It would seem that the insulation has broken down over time sat around in the garage.
Well, that's the theory anyway.
So far, with the hifi off and cooker disconnected the rcd has not tripped all evening. In the morning I shall ramp up the radio and see what happens.


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PostPosted: Fri Oct 20, 2017 7:27 pm 
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Your electrician is making hard work of this.

Yes an element in your cooker has failed[1]. However to isolate the cooker and exclude it as a possible cause of the second RCD trip he only needed to switch it off at the double pole switch - there was no need to remove cables from the switch.

Now you have a dual RCD CU.

What happens if the hifi is plugged into a socket fed from the other RCD? Does the other RCD trip?

[1] Depending where stored (ie damp places) the elements insulator which is made from magnesium oxide will absorb water and become magnesium hydroxide



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PostPosted: Fri Oct 20, 2017 8:47 pm 
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The hifi does not trip the rcd. Today I removed the back of the cooker and to my suprise found that there are two elements (one above and one below) that heat the oven. I had the element that was visible inside delivered today so I thought I would do some investigating..
The resistance on the top element and the newly delivered one were identical on my meter where as the reading was lower on the bottom element. I disconnected them one at a time and each time turned the oven on. Every time the rcd tripped- even with no elements connected.
The good news is that the rcd hasn't tripped all afternoon and I'm not going to turn the oven on as I know it will. I've got an appliance repair chap coming tomorrow to sort it as I've got other things to do.



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PostPosted: Sat Oct 21, 2017 8:56 am 
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Did your electrician issue a certificate if so he would done an insulation resistance test on all circuits although cooker elements are common failures so is faulty wiring, you cannot measure element correctly with a multimeter, I have replaced oven elements in the past, whenever I replace consumer units I ensure all is working before leaving also did he do ramp test on rcd ?

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PostPosted: Sat Oct 21, 2017 3:43 pm 
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Yes I got all the certs.
It seems to be fixed now anyway- both elements on one of the ovens were toast. Leaking to earth apparently. The top and me has been replaced and the lower is on order and will be installed in a few days.
So far nothing has happened since the chap was here..



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