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Topic Title: Bizarre Problem
Topic Summary: Please can someone help me understand whats happening
Created On: 07 September 2013 11:34 AM
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 07 September 2013 11:34 AM
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esspeegee

Posts: 127
Joined: 06 January 2008

Hi all,
Sorry to trouble you all again.
Customer has a rangemaster oven.
Recently the main RCD started to trip and that seemed to be when the oven was in use. I tested cooker supply IR and RCD operation and all was in good order so left cooker isolated and they got Rangemaster in. In the meantime they had no problems and the RCD didnt trip once.

Rangemaster changed lots of components in the oven but although the clock illuminated, any attempt to turn on a hob and the RCD tripped. So they said it must be a faulty RCD and that it should be changed. I doubted that, but took the cooker circuit off the RCD and put it on its own RCBO. However, although the clock still illuminates, if I try and use any hob the new RCBO also trips.
In my limited experience, that sounds like the fault must be with the oven?
OK - so here's the mind boggling bit (for me)

I was blaming the cooker while the customer went to make a cup of tea. When she turned on the kettle, which is on the kitchen ring and so on the original RCD, the new RCBO tripped. That is the RCBO which I had just installed and supplied just the cooker. The kettle continued to boil unaffected. I repeated this three times and every time, the RCBO tripped.

Please, can any one advise me what is going on and recommend a course of action.

Thanks
SImon
 07 September 2013 11:42 AM
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michaelbrett

Posts: 930
Joined: 28 December 2005

Simon

I suppose that the new RCBO and its circuit is totally isolated from the RCD in question?

The fact this is happening with more than one appliance seems to indicate a wiring problem.

Have you looked at the neutral conductor? Could be an issue of it breaking down or poor connection somewhere.

Regards

Mike
 07 September 2013 11:43 AM
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JZN

Posts: 556
Joined: 16 November 2006

My guess is a N-E fault on one of the circuits (not necessarily the cooker or kitchen ring.). When you turn on a reasonable load the RCD sees an imbalance and trips.

You might also find that the RCD trip times are screwed up.

Start doing IR tests on the other circuits.

Jon
 07 September 2013 11:52 AM
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esspeegee

Posts: 127
Joined: 06 January 2008

RCD and RCBO are in the same CU, but yes totally isolated from each other. In fact there are three other RCBOs making the new one the 4th. None of them were affected.
I disconnected the cooker circuit at both ends and with the cooker isolator closed did a 500V IR test and and got >500m Ohms suggesting the cooker circuit is fine.
I havnt done any tests on the kitchen ring because I couldn't, without a lot of disruption, remove all loads and anyway, the signs to me still point at the cooker.
 07 September 2013 11:56 AM
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esspeegee

Posts: 127
Joined: 06 January 2008

Why would the RCBO supplying just the cooker circuit trip when the kitchen ring is used. Surely if the fault was N/E on the kitchen ring then kitchen ring RCD would trip, not the cooker RCBO.
I tested RCD and RCBO times and ramp and all fine
 07 September 2013 11:57 AM
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stateit

Posts: 2157
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I've had this problem... The customer begins to think you are incompetent.
As above, IR test ALL circuits individually.
Then if that's clear test the equipment plugged in etc. to those circuits.

And yes, it is a lot of disruption....

-------------------------
S George
http://www.sg-electrical.com
 07 September 2013 01:34 PM
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esspeegee

Posts: 127
Joined: 06 January 2008

Thanks folks, for your help.
UPDATE
I went back and disconnected all other circuits from the board and the cooker hob still tripped the RCBO. Reconnected everything and then took a load to other power circuits elsewhere in the house. Every time a load was energised the cooker RCBO tripped but the circuit I put the load on was unaffected.
Sweating now, I disconnected the cooker and put a socket on it. I then connected a 3kw load (fat fryer) and no problems anywhere. Proving, I am certain, that the fault must be N/E inside the cooker.

I still dont quite understand how but the sweat has dried

Thanks again.
 07 September 2013 01:34 PM
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esspeegee

Posts: 127
Joined: 06 January 2008

Thanks folks, for your help.
UPDATE
I went back and disconnected all other circuits from the board and the cooker hob still tripped the RCBO. Reconnected everything and then took a load to other power circuits elsewhere in the house. Every time a load was energised the cooker RCBO tripped but the circuit I put the load on was unaffected.
Sweating now, I disconnected the cooker and put a socket on it. I then connected a 3kw load (fat fryer) and no problems anywhere. Proving, I am certain, that the fault must be N/E inside the cooker.

I still dont quite understand how but the sweat has dried

Thanks again.
 07 September 2013 02:01 PM
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alancapon

Posts: 5760
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I agree. The neutral to earth fault will be inside the cooker, as this is the rcbo that is seeing the imbalance. Applying a largeish load to the installation (such as a kettle), you are raising the potential of your neutral conductor within the installation, providing a voltage difference to drive current through your neutral - earth fault.

Regards,

Alan.
 07 September 2013 02:14 PM
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Simannjo

Posts: 10
Joined: 09 January 2008

Had a VERY similar fault with a brand new dual fuel rangemaster...
Turned out to be the neutral unshielded terminal of the element or element thermostat (can't remember which) touching the casing.
this was visible behind the panel that is removed to connect supply.
There were a lot of unshielded terminals...
 09 September 2013 08:21 AM
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davezawadi

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I assume that the rangemaster does not have electronic controls?
So insulation test L&N together with everything switched on and hot. Is it still OK?
This has to be a N - E fault somewhere. My next test would be to run the range via an isolation transformer to definitely isolate the fault to the range. Once proved then its back to them.... Another question, does the kettle trip the circuit with the range isolated? Another pointer to the fault being in the range.
If it does have electronics then it is much more difficult because the fault may be isolated from the mains with no power applied. Why did rangemasters man change things if he couldn't find a fault? Very strange, I expect he had no test equipment!

-------------------------
David
CEng etc, don't ask, its a result not a question!
 09 September 2013 07:22 PM
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iomtt

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Also not read whole replies but is it an induction hob and have you ruled out pulling more amps than mcb?
Induction jobs in my experience have a natural,leakage current
 09 September 2013 07:22 PM
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iomtt

Posts: 20
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Also not read whole replies but is it an induction hob and have you ruled out pulling more amps than mcb?
Induction jobs in my experience have a natural,leakage current
 09 September 2013 07:50 PM
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weirdbeard

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Originally posted by: iomtt


Induction jobs in my experience have a natural,leakage current


Hi iomtt, Not done many of these myself, what readings of leakage current are normally found?
 10 September 2013 07:25 AM
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davezawadi

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Any results from my suggestions yet?

-------------------------
David
CEng etc, don't ask, its a result not a question!
 10 September 2013 08:00 AM
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primo

Posts: 431
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Originally posted by: davezawadi

My next test would be to run the range via an isolation transformer to definitely isolate the fault to the range.


You are the only electrician I know of who has a 240v isolating transformer capable of running an oven / hob element in their test kit!
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