All-RCBO CU: Any circuits that shouldn't be on an RCBO?

[Dippy]

It's been a few weeks since there were some 17th ed. discussions and I asked if it was maybe a good idea to fit an all-RCBO CU.

Now I just read on another thread someone suggesting that smoke detectors should not be on an RCD circuit.

Now I have heard that cookers can be leaky and trip RCD circuits, but I intend to replace my kitchen and appliances and so I hope my new cooker wouldn't do this?

In my mind, any healthy circuit with healthy devices/appliances on it should not trip an RCD, so whilst maybe being 'over-protective', having an all-RCBO CU should not be a problem.

Hence my question - is there anything that should NOT be on an RCD circuit?

(BTW I am fitting a lighting control system which will be spread over several RCBOs so there should be no single-point trip that means someone is left in darkness).

 

[JohnD]

Fire alarm

Burglar alarm

Freezer

Fishtank if it contains valuable fish. It would be worthwhile having a extra-low-voltage heater and pump to avoid the risk of shock to yourself when you have your arm in the water.

electric cooker*

immersion heater*

The non-RCD circuits should not have sockets on them, use FCUs


*these ones are just because they are prone to nuisance tripping. if they are on their own RCBOs it is not such a prob, it will just prompt you to get then repaired or replaced when they start to leak.

If you are having an all-RCBO installation you might consider using 100mA RCBOs on the circuits I have said should not be on RCD. Will give a degree of protection, and are less prone to nuisance tripping.

If you put your freezer and fishtank on RCBOs, what will you do when they trip while you are away?

If the alarms and lights on stairs trip their RCBOs in an emergency, you had better have standby power for them

I use RCBOs, and on one occasion recently I had all three trip at the same time. It appears to have been while contractors were digging up the road outside, and I suspect they touched a supply cable.

 

[Steve]

I use RCBOs, . . . all three . . .

We know . . . just three John?

 

[JohnD]

I keep the others pinned on the wall to ogle, instead of a Playbeast calendar

 

[Steve]

 

[RF Lighting]

If you are having an all-RCBO installation you might consider using 100mA RCBOs on the circuits I have said should not be on RCD.

You'd best start saving up now then

I had reason to fit a 100mA RCBO a few weeks back. It was special order from our wholesaler, and had a handsome price tag attached too.

 

[JohnD]

you'd be surprised what I've got on my shelf

 

[Dippy]

Thanks John.

I'm not so worried about the immersion heater as we never use it unless the boiler breaks (not happened yet).

What about cookers then? Can you confirm that modern cookers will start to leak after a few years?

I have looked for the fish tank but can't find it. I guess we don't have one or else we ate the fish last time Sainsbury's didn't deliver the weekly shop.

On reflection I'm not sure about the lighting. The RBCO would only be protecting the short cable up to the 8-channel lighting controller and the controller itself anyway so maybe the need is not so great.

However I'd like to understand your reasons a bit more. You mention nuisance tripping. Are you saying that RCBOs are prone to trip when there is no current leak? If so they I'll certainly heed your warnings. But if there is a genuine circuit fault then surely isolation is necessary, even if it is an alarm or freezer?

 

[JohnD]

no, by nuisance tripping I mean that sometimes you get leakage which is not a danger. Occasionally a trip may be caused by something outside the house (like my road-diggers). nuisance tripping risk is lower with RCBOs than with a split CU or a whole-house RCD, because you do not get the accumulation of slight background leakages from all appliances and all circuits coming through a single device. Occasionally a fault on one circuit can trip an RCD on another circuit. This is not very common, and is one of the sources of much head-scratching.

If your freezer circuit trips off while you are away, you could lose your year's supply of frozen meat pies, so this is a reason for not giving it an RCD. My freezer is on a dedicated radial (no RCD) as is my cooker.

I've seen no evidence that cookers have stopped getting earth leakage. The heating element degrades slightly over time. As an electric cooker will be on a dedicated radial and is not portable and is not often used outside the house, so it does not need an RCD. The body of the cooker should be securely earthed so no great shock risk. You can put it on an RCBO is you particularly feel like it.

 

[Dippy]

Thanks John. I guess my concept of all-RCBO came from reading the discussion about the 17th Ed, that's all.

I still quite like the idea of having an RCBO per circuit instead of a split CU, but I think I'll just put the rings on RCBOs now.

I haven't looked into buying my new CU yet. If I'm going to fit RCBOs, do you have a manufacturer you particularly recommend or are they all much the same?

 

[JohnD]

ha ha ha ha ha!

MEM is a good one, and due to the way the RCBOs are assembled, you can have practically any rating you want.