Protecting freezer from thawing if RCD's trip when away.


Postby Jon121212 » Wed Apr 21, 2010 1:36 pm

This is my first time here, and I am impressed with replies, advice I have just read.

So please help me with a small problem.

I have wired in a new CU 17th edition with a split board and two RCD's.

I want to protect the freezer contents, especially when on holiday.

The problem is if the freezer radial circuit RCD, which has other circuits protected by it is tripped by some other fault, then the freezer will thaw.

So firstly, will a power cut trip an RCD?

So even if a power cut does not trip an RCD, I still think it sensible to connect the freezer radial wire to its own fuse ie MCB or RCBO and take the power ( feed ) from the main switch.

I can't make up my mind which would be better, but consider the following;

An MCB with feed taken directly from the main switch, may not work if the RCD is tripped, as the return would go through the DPDT RCD , effectively cutting off the return.
But maybe I could bypass the return bar, and take the freezer return direct to the main switch.

An RCBO with both the feed and return via the main switch would remain independant from the CU RCD's and providing power cuts don't trip the RCD element of it, then would maintain power to the freezer.

This is a bit of a puzzle to me, brought on by a builder I was chatting to, who insisted that with new CU with all circuits protected with two RCD's , it created this potential freezer problem.

I look to your learned selves for advice as the few people I have asked to date seem to develop a sort of glazed expression when asked before developing a slight twitch.
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Postby moggy1968 » Wed Apr 21, 2010 10:06 pm

it does create a problem!! one way is, as you have suggested, to put the freezer on it's own RCBO, which will reduce the likelyhood of nuisance tripping.
The other way depends on the rest of your circuit. you could have it not protected by an RCD at all, provided it is just that socket and it is clearly labelled that it is just for the freezer. a lot of other conditions have to be met as well, cable not burried in wall, not a TT supply etc, etc. You probably need a sparks advice to ensure everything is in order. if your consumer unit is in the garage though there is a fair chance you could put the freezer on a non RCD circuit, but best to check.
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Postby jimmy_one_ball » Thu Apr 22, 2010 12:31 am

An RCBO has an RCD built in and will operate in exactly the same way. An RCBO is an MCB and an RCD all in one.

Just leave it as it is, an RCD will only operate under earth fault conditions which are unlikely to happen unless you have loose wires floating about. An RCD won't operate in overcurrent conditions, be that overload or short circuit so don't worry about your freezer, leave it RCD protected.
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Postby ericmark » Thu Apr 22, 2010 7:29 pm

So firstly, will a power cut trip an RCD?

Not normally. There are two types of RCD passive and active the latter will trip with power cut but not normally used in a consumer unit. However often when there is a power cut there are also spikes and these can trip an RCD.

So even if a power cut does not trip an RCD, I still think it sensible to connect the freezer radial wire to its own fuse ie MCB or RCBO and take the power ( feed ) from the main switch.

I can't make up my mind which would be better, but consider the following;

An MCB with feed taken directly from the main switch, may not work if the RCD is tripped, as the return would go through the DPDT RCD , effectively cutting off the return.
But maybe I could bypass the return bar, and take the freezer return direct to the main switch.

An RCBO with both the feed and return via the main switch would remain independant from the CU RCD's and providing power cuts don't trip the RCD element of it, then would maintain power to the freezer.

This is a bit of a puzzle to me, brought on by a builder I was chatting to, who insisted that with new CU with all circuits protected with two RCD's , it created this potential freezer problem.

I look to your learned selves for advice as the few people I have asked to date seem to develop a sort of glazed expression when asked before developing a slight twitch.

As to cure yes using a dedicated RCD will reduce the problem but since the problem is spikes it will not remove it. Using ali-tube cable and a MCB is permitted but as with the dedicated RCD it needs a dedicated cable which is not the easiest way.

Lucky insurance now has to cover and they can’t blame you for fitting and RCD and my house with twin RCD since 1992 has hot had a problem while we were on holiday mainly because nothing else in the house is being used while we are on holiday.

I have considered for my disabled mother fitting an auto reset RCD but at £350 for RCD alone I would say insurance is best option.
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