Consumer unit - If i need to change a light pendant Will turning off the one switch be enough?

[reoreborn85]

Hi.

This may be a really silly question - I need to change a 4 pendant lights UPSTAIRS. I have changed them many times but when I do I normally turn the MAIN switch off.

For years I've been told "Just turn the one switch off, in this case the upstairs lights (circled in red) When i turn it off the lights turn off upstairs etc.
I was just wondering if there's ANY benefit to turning them all off (turning off the main switch) to just the one (Lights first floor).


THe reason I ask if we live with my mother in Law who uses an electric bed with an air controlled mattress etc so turning off the main switch for 20-30mins will be very inconvenient.

Thanks for any advice <3

 

[CountryFan]

I would say work safely and dead test, otherwise it’s how you could end up, dead. Not all consumer units are labelled up correctly, so whilst isolating one circuit to work on is ok, it does need to be proven dead.

 

[Harry Bloomfield]

For years I've been told "Just turn the one switch off, in this case the upstairs lights (circled in red) When i turn it off the lights turn off upstairs etc.
I was just wondering if there's ANY benefit to turning them all off (turning off the main switch) to just the one (Lights first floor).

The safest way, is always total isolation of the main supply, which isolates everything, including both L & N. A potential risk, when working on a circuit where just the L is isolated by a single MCB, is that there might be what is known as 'borrowed neutral'. This is where a neutral is used, which belongs to another circuit. This happens often, on a stair lighting circuit, when an L is used from the downstairs lighting circuit, but the N from the upstairs circuit. Disconnect the borrowed neutral, and it immediately becomes L, if that particular light is turned on.

 

[reoreborn85]

The safest way, is always total isolation of the main supply, which isolates everything, including both L & N. A potential risk, when working on a circuit where just the L is isolated by a single MCB, is that there might be what is known as 'borrowed neutral'. This is where a neutral is used, which belongs to another circuit. This happens often, on a stair lighting circuit, when an L is used from the downstairs lighting circuit, but the N from the upstairs circuit. Disconnect the borrowed neutral, and it immediately becomes L, if that particular light is turned on.

Thank you for the reply.

Ah, i see what you mean. Our landing light has upstairs and downstairs switch which makes sense.
The three lights are in the bedrooms and only have one on and off switch. Does that make any difference?

I can turn the main switch off but i'd have to turn it on every 10-15mins to allow for her to move her bed/use her bed Etc ,3

 

[Murdochcat]

Turn the lights on you wish to change. Turn off the circuit breaker you think supplies the lights. Check that they are off

If they are still on try another circuit breaker

 

[pete01]

Agree with Harry, borrowed neutrals can catch anyone out, switch the lot off.

 

[Murdochcat]

Agree with Harry, borrowed neutrals can catch anyone out, switch the lot off.

With that CU a borrowed neutral would be tripping at least one of those RCDs

 

[pete01]

With that CU a borrowed neutral would be tripping at least one of those RCDs

Yes the lighting circuits are on different rcds i didn't look to be honest

Would still switch the lot off though.

 

[Harry Bloomfield]

Ah, i see what you mean. Our landing light has upstairs and downstairs switch which makes sense.
The three lights are in the bedrooms and only have one on and off switch. Does that make any difference?

A borrowed neutral, can occur absolutely anywhere, it's just that they tend to be more common, where more that one circuit, are in close proximity. In the home, where the lighting circuits are split in two sections, upstairs and down, they come together on the stair lighting. On the stairs, some electricians would take the easy way out, and borrow a neutral for the landing light, from the upstairs circuit, but with the L fed from the downstairs circuit.

 

[ericmark]

The RCD isolates, the MCB's only turn it off, so theory should turn off the isolator. But I will turn off the MCB, but also test for dead. We should ensure sockets and lights are not on the same RCD for any one room, so if the RCD trips due to a fault with something plugged in, your not also plunged into darkness, but this is not always done.

 

[ericmark]

A borrowed neutral, can occur absolutely anywhere,

Not across RCD's as it would trip the RCD. So turning off RCD one should be safe.

 

[Harry Bloomfield]

Not across RCD's as it would trip the RCD. So turning off RCD one should be safe.

I agree, I hadn't spotted the two lighting circuits were split across the two RCD's. Nonetheless, just switching the MCB off, is never foolproof, against a borrowed neutral, which was the point.

 

[morqthana]

Also turning off just the MCB doesn't disconnect the neutral, only the live, so if you short neutral to earth when working the RCD will trip.

Happened to me once & I couldn't understand why the thing tripped until I looked into it. Still happens sometimes, but at least I'm no longer mystified.

Can you run an extension lead from the kitchen to power the bed for the duration of the job?

 

[Jurassicspark]

Also turning off just the MCB doesn't disconnect the neutral, only the live, so if you short neutral to earth when working the RCD might trip.

 

[TAMD8389]

Hi I don’t want to create a thread due to a lot of arguments and opinions so I’m looking for a quick answer here.
If a circuit is Fused spur protected then would turning off the fused spur be okay to work on it?

Ta