DIY Doctor

Newbie question on electric oven problem

Postby wozza955 » Wed Jan 05, 2011 5:35 pm

Evening all. Newbie here, who will happily buy a round of virtual beers for any help on this one....

I have recently (beginning of December) installed a new electric oven as a replacement for one that was a few years old, but wasn't installed by me. Apologies for the length of the post but I will try to give as much information as I can on the problem, hopefully someone will then be able to add some wisdom.

The new oven came with a flex already connected, which I then wired to the cooker connection plate under the worktop (I am reasonably competent with DIY/electrics). All has been working fine until yesterday when the oven was in use and it tripped the main circuit breaker. Now every time the oven is switched on and in use it runs ok for approx 1 minute then trips the main breaker again.

Some more detailed info:
- Oven is a Candy model FPP6071X, rating is shown as 2100W
- The cooker is on a separate circuit in the Consumer Unit, with a 32A MCB. The only appliance on the cooker circuit is the oven, the hob is gas
- Oven is wired to a connection plate under the worktop; I have checked the connections in this plate both from the oven and the switch above the worktop, all are connected correctly
- Switch above the worktop does not have a socket in it; I have checked the connections in the switch, all are connected correctly
- When the oven runs for approx 1 minute, the master breaker trips - not the MCB for the cooker circuit
- The master breaker appears to be an RCD double breaker (with test button)
- Electrical circuit and CU installation was all done at the time of the house being built, approx 17 years ago

I have tested the oven circuit by turning off all the other MCB's in the CU and just left the cooker circuit switched on. Again, when the oven runs for approx 1 minute it trips the main breaker, not the cooker circuit MCB.

I am assuming that the problem therefore isn't a circuit overload, as the MCB for the cooker circuit doesn't ever trip, it is always the main breaker, so I was wondering if this problem was due to an earth leakage??

I can't see from the back of the oven whether the earth cable from the flex is connected internally to the casing. And as the oven is only one month old I don't want to start taking it apart to confirm this as that will no doubt invalidate my warranty.

So, to my questions:
- Am I correct in thinking it is possibly an earth leakage problem?
- How can I verify this?
- Is there anything else that could be causing this?
- If it is earth leakage, can I overcome this by earthing the casing myself?
- If yes, there is a copper gas pipe behind the oven which connects the hob supply, can I earth to this pipe?

All help greatly appreciated!!!

Thanks in advance.
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Simply Build It

Postby ericmark » Thu Jan 06, 2011 10:05 am

It does seem like an earth leakage problem. Heating elements are often mineral insulated and it is impossible to get perfect seal. They do absorb moisture from atmosphere and can easy have a high earth leakage.

There are two ways to test the cooker. Either a special clip on ammeter or a special 500v ohm meter. Likely you have neither.
The installation can be checked with a special RCD tester that checks point at which it trips in milliamps and time taken in milliseconds. Again unlikely you have one.

There is a dangerous way to dry out the oven and if you call an electrician who is able to take the precautions required he will likely both test supply and oven and if oven could dry it out for you. However if so new then I would be tempered to return it as faulty.

You say connected to cooker connection point which rings alarm bells as you are running an item designed to work on around 12A on a 32A supply. So if there was a fault the 32A MCB could fail to safely disconnect. Normally we would use a Fused connection unit to reduce the amps available to 13A.

So again getting an electrician to check seems a good idea anyway. If he writes out an inspection and states how the oven does not comply and the leakage is too high then returning it may be easier to do.

Personally I would not dry out as likely some time in future the fault will return.
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