Electrical Question

Can my mcb detect a short on an isolated circuit?


I changed a pendant light fitting over last night. Nothing complicated - it's just a twin and earth cable to an existing 1-gang 2-way switch.

I isolated the appropriate lighting circuit on the MCB panel and checked the circuit was dead prior to working on it, also the light fitting was turned off at the wall switch and neithter of the fittings had bulbs. Whilst working on it, the Live and Neitral cables touched the new metal light fixing and shorted, throwing the main ciruit breaker for the whole house on the MCB.

I thought that having isolated the circuit this would not be able to happen or does the MCB circuit breaker still detect a short even on an isolated circuit.

Is this a sign it's time to get the whole wiring system checked?

Thanks in advance

3 Answers

Best Answer

It'll be the RCD that tripped - not the MCB.

RCD's are very sensitive devices that detect very small leakage currents. They're designed to break if a small current passes through (for example) a human. A MCB would not trip under those circumstances.

So why does an RCD trip on a circuit that's isolated at the minature circuit breaker? Even on a circuit that's isolated, due to proximity of other live cables alongside the isolated one, a small charge can be induced in the isolated circuit. Whilst this is generally completely harmless to people, if the charge is discharged through a short circuit, the momentary discharge can be enough to trip the RCD.

By the way, it doesn't have to be line - neutral short circuit: A neutral - earth short circuit can do the same.

So - no: It doesn't necessarily mean there's a problem with your circuits. If your RCD(s) have never otherwise tripped, you can be reasonably confident that all is ok. If you're still worried, get a competent electrician registered with NAPIT to do an inspection.

BTW, don't forget to test your RCD(s) periodically - press the test button every 3 months ago to make sure they trip. If they don't trip - then get someone to look at them.

Answered 9th Apr 2013

A Brighter Spark

Member since 22 Dec 2011

From what you describe the "Main Switch" for the consumer unit is either a BSEN61008 or BS4293 Residual Current Device (RCD). As an RCD works on the imbalance od current and can detect very small amounts of current flow, it will operate i.e. trip when a neutral and earth conductor are touched together as this is a "neutral to earth" short circuit.

Although you performed safe isolation and checked that the light was dead, the BSEN60898 MCB for the circuit you were working only was only single pole, so you in effect only isolated the live cable and not the neutral.

That's why it tripped when you were working on the bare cable ends at the light.

Hope this helps please click on "like"if it does

Answered 10th Apr 2013

Electrical Safety Services

Member since 17 Oct 2011

So the RCD tripped not the MCB!

Answered 9th Apr 2013

Edris Building & Electrical Contractor

Member since 11 Mar 2013

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