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StevieV 08-22-2008 01:00 PM

Linking the 5amp lighting live to the 13amp neutral causes tripping of the rcb. Why?
As an after thought in my kithen I decided to add undercupboard lighting, and wanting to swich this from my main lighting switch, I have used the common live in the switch but due to location issues I cannot link to the lighting circuit neutral.

However I can link to the 13 amp socket circuit neutral which I have done. When I switch the swith the lighting rcb trips instantly, any ideas why as I thought the neutral wires were all linked back to the same location in the consumer unit.


SD515 08-22-2008 06:04 PM

Hey Stevie

My first thought is that you may be located in the UK or similar possibly?? If you are, FYI, many of the responders here tend to deal with US or Canadian electrical systems, so you may not get a quick responce. I personally am not very knowledgeable with UK/European systems, so I'd rather not field this one.

Stubbie 08-23-2008 12:08 AM

Hello Stevie

Well mate the fact of the matter is you can't use the neutral of the socket circuit because it does not come back to the rcb neutral terminal bar in your consumer unit. Instead it lands on the main switch non rcb terminal strip in your consumer unit. If you open up your consumer unit, you will see your main switch usually on the far right it feeds the rcb's its power and then the rcb feeds several circuit breakers next to it. Those circuit breakers neutrals come back to the rcb neutral bar and their current then through the oversized neutral connected to the rcb. The rcb keeps track of the current flow for all the circuit breakers protected by it. So when you turn on your lights the rcb sees the current flow on the live but it doesn't see the same current on the neutral sees 0 current... and trips out.

You have 3 terminal bars in the consumer unit, your main switch neutral bar for non protected rcb circuits , your rcb neutral bar for protected circuits, and your ground bar. If you connect to a neutral that is not on rcb protection the rcb cannot see if the neutral current exists for that circuit and trips out. Lets say three circuits protected by the rcb are each using 15 amps so the rcb is seeing 45 amps on the live and 45 on the neutral. Now you connect to the live of one of those circuits for the cupboard lights but you connect the cupboard light neutral to a neutral from another circuit that is on a non rcb circuit. Lets say the cupboard lights carry 1 amp. You turn on the light and rcb now sees 46 amps on the live but only 45 on the neutral and trips out due to the imbalance between live and neutral.

Hope that all makes sense

StevieV 08-23-2008 03:12 AM

Thanks - great answer, that makes sense.

joed 08-23-2008 08:17 PM

Just so some of our members will understand and RCB is the same a GFCI.

Stubbie 08-24-2008 12:04 AM

That's true Joe, only difference is in the UK they have 250 volts to neutral or ground and we have 120 volts ( I know that you know this but bear with me...:)) They use a single rcb in the electrical panel that clips to a rail with other normal circuit breakers who's circuits are to be protected the rcb (residual current breaker). The branch circuits protected by the rcb have their own neutral bar and a common connection is made to that neutral bar from the rcb. After the neutral current flows through the rcb it then connects to the neutral bar that bonds with the service neutral along with all the neutrals from the non protected circuits. So a little different than what we do here with individual gfci breakers for each branch circuit
We shouldn't be to worried about giving advice to our UK friends as it really is the same thing we have here just a higher potential to ground. Once you learn a few of the quirks they have.. like the rcb and they do not use wire nuts but compression set screw terminals built into the fixtures and devices. They also have fuse blocks that protect the switch loops which takes some getting use to as that is quite differrent from us here in the USA and Canada. They also have a whole different wire color scheme than here and Canada. And they recently in the last few years changed all the old colors to the European standard colors. Should cross your eyeballs...:) OHH.... and they call a load center a consumer unit...

Here is a pretty good link for anybody interested in familiarizing themselves with the folks in the uk and their electrical system....

Pudge565 08-24-2008 11:35 AM

They should rename GFCI RCB here in the states. GFCI confuses people makes em think that it needs a ground to operate.

SD515 08-24-2008 07:48 PM

Thanks Stubbie !!:thumbsup:

Stubbie 08-24-2008 09:42 PM

You are welcome SD515. I hope you find the information on that link helpful. It is one that I used often when I was first trying to help our friends in the UK.

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