Cooker MCB/RCD

[Grumps]

Hi Father-in-Law had a new NEFF oven installed by John Lewis. Their installers said that the MCB needed changing to a 32A part. The electrician called and checked the wiring and swapped the MCB. He said that the method the JL installers had used to connect wasn't great; ideally the copper wires should be wrapped around the screw terminals.

The problem is, that when the oven is switched off, sometimes the RCD trips. FIL just says "the fuse blows", but other circuits are also affected - hence I'm guessing he means the RCD. The NEFF engineer turned up and took a look at the consumer unit and said "You need a class A fuse. You've got a class B". They're my FIL's words.

So, can anyone decipher this and say what needs changing and why? Can you even get a class/type A MCB? Why would changing the RCD type from B to A make a difference?

Thanks, and a Merry Christmas!

[tabbypurr]

can be caused by either oven element leakage or a faulty RCD. The former is much more common.

there's no such thing as either. And no reason to change from MCB to fuse.

90%+ likely the oven is faulty.

NT

[ARW]

This must be a RCD thing. There is and has never been a type A MCB. There are however type A and type B RCDs.

I would be surprised if your FiL has a type B RCD unless he has an electric car charging point..

Any chance of a photo?

[John Rumm]

Apologies for the thread hijack, but that reminds me. I added a bit to the RCD article to cover the different sensitivity / detection type classes of RCD:

It could probably do with some more practical guidance on selection.

[Brian Gaff]

Delay time? Brian

[John Rumm]

Normally when a RCD trips on a switch off event, it indicates that the RCD is "sensitised" i.e. there is already lots of leakage being seen by it, and it is close to its tripping point. The step change in current demand then only needs cause a little extra leakage in the system to cause a trip. Hence the fault may be elsewhere, or for that matter may just indicate too many circuits protected by a single RCD.

Hard to say what that might have been about. Perhaps the Neff engineer just wanted something to blame and get out quick.

There are different types of MCB; Type B, C, or D. They differ in how tolerant they are to switch on surges. Typically you use type B devices on most domestic circuits. Can't see that being an issue here - especially on switch off.

There are also different types of RCD. The basic one we know and love is a type AC. Other types include a type A, F, and B. Each of those adds abilities to detect additional leakage scenarios that the lower level device may not respond to. Again it seems unlikely that there is an issue here.

[Grumps]

Thanks. Hope this works:

[Grumps]

Thanks. Hope this works:

[tabbypurr]

So if power to everything was lost, it has to be the RCD that tripped. That leaves 2 possibilities: Oven element leaky near the neutral end RCD faulty. The 1st is more likely. Hipot or PAT test the oven & you'll know which it is. One thing's for sure: a class A fuse has nothing to do with it.

NT

[John Rumm]

ok so that tells us the RCD is a bog standard double pole device, with a

30mA trip threshold, a maximum switching capacity of 80A, and no time delay.

The circuit breakers are all normal type B devices...

Nothing there that should have any compatibility problem with an oven.

Is that photo of the whole CU, or are there more circuit breakers to the right of the big red switch?

If there is another bit we can't see, does that have another RCD?

Either way, it has quite a number of circuits protected by the same RCD...

[Grumps]

That is half of the CU. The other half has a similar number of circuits and another RCD. I'll get another photo later.

Is having lots of circuits on one RCD bad?

[Grumps]

If there are too many circuits on this RCD, is a simple solution to replace the oven MCB with an RCBO?

[ARW]

When you say the oven is been switched off when it trips is that at the oven or at the switch on the wall?

[ARW]

And an odd layout. That looks like a new CU and MK main switches are on the left.

[Grumps]

On the oven itself.

[Tim Watts]

I'm impressed the electrician printed a nice custom label (I'm going out on a limb and say it looks like Brother P-Touch tape) instead of scribbling on the manufacturer supplied paper strip :)

[Tim Watts]

Not at is - the busbar would still be supplied from the RCD that would be equally likely to trip as the RCBO.

Unless the other side (assuming a split load board) is non RCD - that would work, if the wires had enough slack to be moved over.

[ARW]

Then I would be wanting a polarity test on the oven terminals.

[ARW]

see the picture on the 3rd page down, would be the more traditional internal set up for these CUs.

Looks to me like the sparks has modified the setup to match the incoming cables.

[John Rumm]

ok that makes sense (the labels seemed to suggest there were circuits missing). The layout is a bit strange though... Other than that, it looks like a fairly standard 17th edition style "split" CU with a pair of RCDs and the circuits distributed between them.

Its not "bad" as such, but if you assume that each circuit may have a small amount of natural leakage[1], then when you add up lots of them you can get close to the tripping point of the RCD. At that point they are also more sensitive to being tripped by switching loads.

Do you still get a trip if the oven is turned off at the cooker point / isolator switch?

[1] For example, socket circuits will often have electronic devices connected with modern switch mode PSUs, and those will tend to have a mains input filters that have a small capacitor across the supply, which will leak a tiny amount at normal mains frequency.