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Topic Title: Electric Hob & Split Type Dist Board
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Created On: 16 March 2017 02:33 PM
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 16 March 2017 02:33 PM
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Mandem

Posts: 3
Joined: 14 March 2017

Hi I recently replaced a Distribution board from an old Wylex fuse type to a split dual DB with Main switch & 2 RCD's protecting the circuits.
In the old fuseboard the Electric Hob which is a very old electric plate style was supplied through a 30A rewirable fuse into a cooker point & is now connected on one of the RCD sides to a 40A Type B MCB, but is tripping out the RCD when it's turned on while the MCB stays up.
It worked fine before on the old 30A rewirable.
All cabling has been checked for faults etc, Neutral has been checked that it's connected to the correct bar in the split, and all is OK.
I'm assuming the RCD is tripping because of the sensitivity of this old hob with the extra protection being given by the RCD.
What i was wondering is would this circuit be OK to be fed by MCB off the main switch only, and not have to be on an RCD protected side of the board, or can anyone else suggest anything else?

Thanks.
 16 March 2017 02:56 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

Sounds like a faulty hob to me (checked for a dead mouse across the main terminals?) - even years ago you would have expected at least half a meg (quarter at worst) from a cooker on an insulation test (i.e. 0.5 - 1mA leakage) - 15-30mA needed to trip a sound RCD is way over the top.

If you did want to tolerate such leakage you should really be looking at high integrity earthing arrangements (e.g. duplicate c.p.c.s) - otherwise you're back to a single fault (broken c.p.c.) to danger.

- Andy.
 16 March 2017 02:59 PM
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Tof99

Posts: 18
Joined: 18 March 2011

Does the hob trip the RCD whenever a hot plate is turned on, or is it a particular hot plate?
It is likely that the heating elements are leaking down to earth and the RCD is picking up on this. have you tried isolating the other circuits being protected by the same RCD. ie the kitchen ring main?
It may be that other devices on the RCD circuit, are leaking to earth and when the hob is switched on, it is taking the RCD over the 30mA level.
It is worth performing an insulation resistance test on the hob itself, turning each hotplate on in turn.

I wouldn't run any circuit without RCD protection, as you have updated the installation to 17th and all circuits should be RCD protected.
 16 March 2017 02:59 PM
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runway

Posts: 18
Joined: 15 September 2013

have you checked the IR on the hob, could be one of the elements starting to break down. if it trips there's an earth fault. so putting it on
just an MCB would not solve the problem just mask it.
 16 March 2017 03:00 PM
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geoffsd

Posts: 1704
Joined: 15 June 2010

Edit - didn't realise there were other replies.
 16 March 2017 05:50 PM
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dustydazzler

Posts: 1350
Joined: 19 January 2016

element

You don't have an rcd side , you only have rcd sides

You customer will likely end up needing to buy a new stove
 16 March 2017 08:54 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

to trip a 30mA RCD takes more than 15mA and less than 30 - but we cant be sure its all the cooker, it may be the straw that breaks the camels back.
You could put the cooker on its own RCBO, rather than its own MCB so not sharing the RCD function, and still claim 18th compliance. Actually from a nuisance trip perspective this is quite a good idea anyway, problems or not. Then as others have said, its searching for the source of leakage in the cooker - spilt soup or similar in the works across the element terminals, or ingress of Mr. Muscle kitchen cleaner or similar around the knobs is a common problem, or as others have noted, a failed element.

-------------------------
regards Mike
 19 March 2017 08:14 AM
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Mandem

Posts: 3
Joined: 14 March 2017

Thanks everyone for the advice.
The hob seems to work when 2 of the 4 plates are turned on & RCD stays up, although only 1 plate heats up suggesting the other is faulty. When the other 2 plates are switched, they trip out the RCD immediately. So at present only 1 plate out of 4 works.
I'll be checking through all the solutions advised a little later on today & hopefully get it sorted out.
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