Neutral went bang - can someone explain this wiring to me?

[Electrics]

I was wiring up an IP65 5ft LED batten today, straight swap for a non-LED version.

The light was an emergency and worked off a sensor, no switch.

The emergency was wired through the test switch.

In the light fitting, there were 4 wires - brown switch live, black live, neutral and earth.

As i took the neutral out, i got a flash bang and a pop. Didn't get shocked and nothing tripped out anywhere. Guy i was working with said he didn't have time to explain it to me, but that it was because the neutral was coming from the lights in the adjacent room.

We then turned off the MCB for that room and i cracked on.

I'm a bit confused as to how this all works. Anyone explain it to me?

(Ps - i'm currently working for a company and wasn't just doing this DIY - i'm being directed by a qualified man at work.)

 

[Lister1987]

Shared neutral with the lights on in an adjacent room I'll bet.

 

[Electrics]

Shared neutral with the lights on in an adjacent room I'll bet.

I think that's what he hinted at but we were running round like blue arsed flies trying to finish so he didn't have time to explain properly.

Isn't shared neutrals bad?

 

[Lucien Nunes]

Breaking any flow of current can result in a spark; the greater the current / voltage / inductance of the circuit generally the larger the spark. Assuming nothing was short-circuited as you moved the neutral conductor, what you saw was the breaking of a circuit carrying current from one or more fittings.

If the neutral was looped through the terminal in your fitting to further points, then you would have broken the total current returning from them. If not, then it was just the current from your own fitting that was being interrupted. This could have included the charging current and suppressor capacitor current even if the LED was not alight.

Once a neutral wire is disconnected from neutral, any load trying to feed into it will raise its potential to line potential, making is as great a shock risk as a live line conductor. Hence the need to properly isolate a circuit before working on parts of it that seem dead. I have no idea what you were doing leaving the MCB switched on to begin with.

 

[timhoward]

If there were only 4 wires in the fitting, (SL from sensor, L via EM keyswitch, N and E) and nothing was looped in and out and off to somewhere else (e..g the next emergency light) then the only explanation is that either the sensor was still powered, or the emergency light key switch wasn't isolated.
However much you are running around like blue arsed flies you should at a minimum check at the fitting with a two pole tester before you start as one of the feeds must have been live.
He shouldn't be sending you up there without isolating the circuit first IMHO.
You shouldn't be getting hands on with it without making sure the thing is safe too ;-)

 

[timhoward]

This graphically explains @Lucien Nunes post btw:

 

[Electrics]

You guys are awesome, thanks!

 

[Electrics]

I have no idea what you were doing leaving the MCB switched on to begin with.

I think he thought it was dead because he'd turned it off at the emergency switch.

I think i'm going to ask him to make sure stuff is completely dead before i touch it in future. I'm on a trial for a potential apprenticeship and don't want to put a spanner in the works because it's going well but i also quite like being not dead.

I'm going to start doing my own voltage testing at everything i work on as an absolute minimum.

I guess the sensor must have still been powered.

 

[Electrics]

If there were only 4 wires in the fitting, (SL from sensor, L via EM keyswitch, N and E) and nothing was looped in and out and off to somewhere else (e..g the next emergency light) then the only explanation is that either the sensor was still powered, or the emergency light key switch wasn't isolated.
However much you are running around like blue arsed flies you should at a minimum check at the fitting with a two pole tester before you start as one of the feeds must have been live.
He shouldn't be sending you up there without isolating the circuit first IMHO.
You shouldn't be getting hands on with it without making sure the thing is safe too ;-)

It turned out once it was wired and back up that the sensor was naffed too.

Couldn't understand why as it worked immediately prior to taking down the light. I asked if what happened could have caused something to blow or break the sensor but the answer was a no.

Can't wait until i start college so i can have it explained to me properly and ask questions when i don't understand.

 

[Lucien Nunes]

OK so if it went bang properly and the sensor failed, you probably shorted the switched line to neutral and the sensor was a weaker link than the fuse. I was going to say that 'bang' was a bit of an exaggeration for just breaking a loaded lighting circuit.

Could it be that the newly detached neutral wire contacted the switched line terminal or an exposed section of the conductor as you moved it away from the terminal?

 

[timhoward]

I'm going to start doing my own voltage testing at everything i work on as an absolute minimum.

That's exactly what I wanted to hear. It's an essential habit to get into.

We've all made early mistakes and had near misses. I once removed a fuse, tested a cable with a non-contact tester, satisfied myself it was dead and assumed I had the right circuit. After a short delay I cut into it, got a mini-jolt and blew the main fuse. It turned out to be an economy 7 storage heater feed not the sockets circuit I thought ti was and it went live at exactly the wrong moment! So many lessons in one incident there...

 

[Electrics]

OK so if it went bang properly and the sensor failed, you probably shorted the switched line to neutral and the sensor was a weaker link than the fuse. I was going to say that 'bang' was a bit of an exaggeration for just breaking a loaded lighting circuit.

Could it be that the newly detached neutral wire contacted the switched line terminal or an exposed section of the conductor as you moved it away from the terminal?

Yes definitely could have been, all the wires were cut really short so were quite close together.

It literally made a small bang noise, some blue flashing.

I still don't really understand how because the line in was off at the emergency switch. I'll watch John Ward's video since if i see a diagram i'll get it. If not expect a diagram from me soon!

 

[Electrics]

That's exactly what I wanted to hear. It's an essential habit to get into.

We've all made early mistakes and had near misses. I once removed a fuse, tested a cable with a non-contact tester, satisfied myself it was dead and assumed I had the right circuit. After a short delay I cut into it, got a mini-jolt and blew the main fuse. It turned out to be an economy 7 storage heater feed not the sockets circuit I thought ti was and it went live at exactly the wrong moment! So many lessons in one incident there...

Thanks for not roasting me! To be fair he's turned off about 10 lights for me and they all went in without a hitch so i didn't expect it, but it's a very valuable lesson to get in my first week. I think because i'm older and can already do the basics they might take less care with me so i'll have to take it into my own hands. It's a business that was open at the time so i guess it's easier to isolate at the emergency test switch than at the board and knock a load of their lights out while it's full of customers. Still, health and safety comes first. No good learning some stuff and earning money if i don't go home safely.

As a side, i'm going to write another thread about my experience and how my mindset was before this first week on the job. Think it would be helpful for anyone wanting to try a shorter route into the industry since it's a fairly brutal reality.

 

[Petej999]

I think that's what he hinted at but we were running round like blue arsed flies trying to finish so he didn't have time to explain properly.

Isn't shared neutrals bad?

Very bad at times

 

[mattg4321]

If you’re not turning of MCB’s when you’re changing lights then even if you’re not touch ‘live’ (I mean line) parts/terminals you can still get a shock from the neutral. It’s especially true when working on systems wired in singles.

Often with singles wiring you won’t have a permanent line conductor at a light fitting. Testing with your voltage indicator will show its all dead… UNTIL… you disconnect the neutral that is looped through the light fitting being worked on. You now have a floating neutral that rises to line voltage. Turn off the damn MCB!