Electrical query - can anyone explain?

[Maroochydore]

The set-up in my free-standing garage is that power comes from the house through an armoured cable and into a fused switched spur.

Two cables lead out of the spur, one to 13A sockets and one to a lighting circuit.

The switch and the fuse only operate in relation to the lighting circuit.

This was all installed by the house builder some 40 years ago.

Today I decided to install a new light switch into the lighting circuit. Switched off at the spur, no lights. For additional safety I then removed the fuse.

Cut the wire where I wanted to install a junction box and the whole circuit tripped at the distribution board. The circuit includes part of the lounge and kitchen in the house as well as the garage.

Reset the MCB and no further issues.

I am now completely bewildered as to why cutting a 'dead' wire should/could trip the circuit.

Any thoughts?

[BullDog]

Lighting circuits in our house have to be switched off with two of the MCB's at the distribution unit, or they are still live. I have no idea if that's normal. But it's how ours is. The distribution unit was fitted new 3 years ago in a 28 year old house.

[DrFfybes]

The switch and fuse isolate the live.

At a guess the trip is because your cutters have connected neutral to Earth, the the one that trip was a Residual Current Device rather than a plain MCB.

Paul

[Lanark]

My guess is this, when you turned off power at the spur there was still some kind of capacitive load on the line which meant the live wire was sitting at say 5v or so.
When you cut the cable, that 5v made contact with the earth cable, the power draining out was enough to trigger an earth leakage detection and trip the circuit breaker.

[Itsallaguess]

The switch and fuse isolate the live.

At a guess the trip is because your cutters have connected neutral to Earth, [and] the one that tripped was a Residual Current Device rather than a plain MCB

I agree that this is the most likely explanation.

Cheers,

Itsallaguess

[Maroochydore]

Thanks for all the responses. One less thing to puzzle about. Now, does anyone know the meaning of life?

[Dicky99]

Probably a good opportunity while you're making changes to run the live into a little consumer unit?
£40 from Screwfix complete with 2 x mcb's and a spare way for future use.

[DrFfybes]

Probably a good opportunity while you're making changes to run the live into a little consumer unit?
£40 from Screwfix complete with 2 x mcb's and a spare way for future use.

Could be - that way if you do have a switched and fused spur you don't test the switch and find it isolates the lights, then remove the 13A fuse and assume this protects the sockets and attack the socket cable with the wire cutters. Those ones with the red and yellow handles are great, apart from a slight flash burn here the jaws exploded I was fine.

Paul

[Lanark]

The switch and fuse isolate the live.

At a guess the trip is because your cutters have connected neutral to Earth, the the one that trip was a Residual Current Device rather than a plain MCB.

Paul

I hadn't thought about neutral, for it to be tripping in this situation I would think about checking how good the connection is. Normal voltage difference between neutral and ground is < 2v which I wouldn't expect to be enough to trip an RCD.

[88V8]

The switch and fuse isolate the live.
At a guess the trip is because your cutters have connected neutral to Earth, the the one that trip was a Residual Current Device rather than a plain MCB.

I hadn't thought about neutral, for it to be tripping in this situation I would think about checking how good the connection is. Normal voltage difference between neutral and ground is < 2v which I wouldn't expect to be enough to trip an RCD.

When I'm working on a supposedly dead circuit with these RCBO/RDC thingummys, I try to avoid connecting anything to anything, live, neutral, earth.

Was wiring some wall lights last week, had 67V of 'phantom voltage' but managed to connect them OK.

Better when we had porcelain fuse holders in a hardwood case, fuse wire you could 'upgrade' if it kept blowing for no apparent reason. Those were the days.

V8

[Urbandreamer]

Normal voltage difference between neutral and ground is < 2v which I wouldn't expect to be enough to trip an RCD.

Err... I assume v stands for volts and RCD stands for Residual Current Device/disconnect?

Do I have to point out that voltage does not require a flow of current, while the point of a RCD is to disconnect when a current flow exceeds a given amount.

Dare I point out that wires are usually low resistance and that 2v though a 1 ohme resistor is 2 amps!

I would bloody hope it trips at that!

While 2v won't produce that current flow through a person, 2 amps through a person will kill.

Oh, there is a old adage that it's volts not amps that kill. That's because you need a lot of volts to push any serious amps through a human body, which has quite a high resistance. Nothing to do with RCD's.

[ReformedCharacter]

Better when we had porcelain fuse holders in a hardwood case, fuse wire you could 'upgrade' if it kept blowing for no apparent reason. Those were the days.

V8

Quite, and wooden 2 pin round pin plugs. And those useful adaptors that enabled an appliance to be plugged into a light socket, like an iron. I remember my father attempting to fix a blown plug fuse by drilling a small hole in the cap at either end and trying to solder a new length of fuse wire into the fuse. Unfortunately my father couldn't solder to save his life

RC

[Mike4]

Normal voltage difference between neutral and ground is < 2v which I wouldn't expect to be enough to trip an RCD.

Err... I assume v stands for volts and RCD stands for Residual Current Device/disconnect?

Do I have to point out that voltage does not require a flow of current, while the point of a RCD is to disconnect when a current flow exceeds a given amount.

Dare I point out that wires are usually low resistance and that 2v though a 1 ohme resistor is 2 amps!

I would bloody hope it trips at that!

While 2v won't produce that current flow through a person, 2 amps through a person will kill.

Oh, there is a old adage that it's volts not amps that kill. That's because you need a lot of volts to push any serious amps through a human body, which has quite a high resistance. Nothing to do with RCD's.

Not only all that but there the obvious puzzle remains. How does ANY current flow when the circuit is isolated by a switched fused spur (FCU in today's parlance) which is a double pole switch, i.e. one which disconnects both live and neutral conductors?

I reckon the FCU must be faulty and only disconnecting one conductor.

[quelquod]

Oh, there is an old adage that it's volts not amps that kill. That's because you need a lot of volts to push any serious amps through a human body, which has quite a high resistance. Nothing to do with RCD's.

The old adage I used to tell my guys is “it’s volts that jolts but it’s mills that kills”. There’s a reason for the common 30mA breaker limit. How many volts it takes to achieve that is dependent on conditions at the time (eg how sweaty your hands might be) but even the standard industrial 110V centre-tapped to earth isn’t guaranteed safe.

[jfgw]

Lighting circuits in our house have to be switched off with two of the MCB's at the distribution unit, or they are still live. I have no idea if that's normal. But it's how ours is. The distribution unit was fitted new 3 years ago in a 28 year old house.

That is definitely not normal!!! It is not as dangerous as it might seem (for a lighting circuit) but I, personally, would want to rectify the fault ASAP.

Regarding the OP, I agree with the comments regarding an rcd (or rcbo) tripping. If it was an mcb, post back! (an mcb does not have a test button.)

Not only all that but there the obvious puzzle remains. How does ANY current flow when the circuit is isolated by a switched fused spur (FCU in today's parlance) which is a double pole switch, i.e. one which disconnects both live and neutral conductors?

I understand that single-pole FCUs exist, not that either of us would buy such low quality products.


Julian F. G. W.

[88V8]

Lighting circuits in our house have to be switched off with two of the MCB's at the distribution unit, or they are still live. I have no idea if that's normal. But it's how ours is. The distribution unit was fitted new 3 years ago in a 28 year old house.

I wonder if there is a two-way switch with a borrowed neutral.

V8

[BullDog]

Lighting circuits in our house have to be switched off with two of the MCB's at the distribution unit, or they are still live. I have no idea if that's normal. But it's how ours is. The distribution unit was fitted new 3 years ago in a 28 year old house.

I wonder if there is a two-way switch with a borrowed neutral.

V8

There's several of them upstairs and downstairs lighting circuits. Definitely have to switch two MCBs off to make the lights go out. That's how I know this. I changed a few light switches myself and actuated the MCBs until I knew I'd selected the correct lighting circuits.

I'm definitely not an electrician (a retired CEng) so I'm super cautious with electric electrickery. The consumer unit was replaced 3 years ago by a local electrician we have known for years and he issued a testing certificate to us to keep. I don't really think we have a problem.

[Itsallaguess]

I wonder if there is a two-way switch with a borrowed neutral.

There's several of them upstairs and downstairs lighting circuits. Definitely have to switch two MCBs off to make the lights go out. That's how I know this. I changed a few light switches myself and actuated the MCBs until I knew I'd selected the correct lighting circuits.

I'm definitely not an electrician (a retired CEng) so I'm super cautious with electric electrickery. The consumer unit was replaced 3 years ago by a local electrician we have known for years and he issued a testing certificate to us to keep. I don't really think we have a problem.

Having separate lighting circuits for each floor of the house is a common situation, but I suspect that the way you've introduced that concept in the linked post below might have been mis-understood to read that any particular light in your house can only be electrically isolated when TWO different MCB's are turned off at the same time, which of course would be a dangerous and incorrectly wired situation -

https://www.lemonfool.co.uk/viewtopic.php?f=40&t=39534#p595957

When I read your earlier post, I understood you to be simply talking about two separate electrical circuits, so it might be worth helping to clear up the above linked confusion if that is what you meant?

Cheers,

Itsallaguess

[BullDog]

Having separate lighting circuits for each floor of the house is a common situation, but I suspect that the way you've introduced that concept in the linked post below might have been mis-understood to read that any particular light in your house can only be electrically isolated when TWO different MCB's are turned off at the same time, which of course would be a dangerous and incorrectly wired situation -

https://www.lemonfool.co.uk/viewtopic.php?f=40&t=39534#p595957

When I read your earlier post, I understood you to be simply talking about two separate electrical circuits, so it might be worth helping to clear up the above linked confusion if that is what you meant?

Cheers,

Itsallaguess

Thanks. For example, for the downstairs lights to go out, I need to switch two MCBs. They are both marked lighting circuit by the electrician who installed the consumer unit.

[Mike4]

Having separate lighting circuits for each floor of the house is a common situation, but I suspect that the way you've introduced that concept in the linked post below might have been mis-understood to read that any particular light in your house can only be electrically isolated when TWO different MCB's are turned off at the same time, which of course would be a dangerous and incorrectly wired situation -

https://www.lemonfool.co.uk/viewtopic.php?f=40&t=39534#p595957

When I read your earlier post, I understood you to be simply talking about two separate electrical circuits, so it might be worth helping to clear up the above linked confusion if that is what you meant?

Cheers,

Itsallaguess

Thanks. For example, for the downstairs lights to go out, I need to switch two MCBs. They are both marked lighting circuit by the electrician who installed the consumer unit.

This sounds very wrong to me. So to turn off any one single downstairs light, any light, you have to turn off two MCBs at the same time? Or do you mean to extinguish ALL of the downstairs lights (as opposed to just some of them), two MCBs must be turned off?

Assuming the former, having turned off these two MCBs, turning either of them back on will make the light come back ON again?

If so, then you must have two MCBs in parallel. Either a wiring error, or a monumental bodge to stop tripping when lots of lights are on at the same time, I'd have thought.