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.)

 

[Electrics]

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!

Could you please explain what looping is? I've looked it up and I'm a bit lost.

 

[Dave OCD]

It's just when a conductor is connected in the same terminal from fitting to fitting , simple as that really.

 

[Vortigern]

A shared neutral is perfectly normal and required within a circuit. A borrowed neutral is a no no as it is from another circuit. When you took the neutral out did you notice lights going off elsewhere?

 

[nicebutdim]

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.

Fingers crossed it works out for you.

I'm loving every minute of it and wishing I'd made the move years before.

 

[Electrics]

Fingers crossed it works out for you.

I'm loving every minute of it and wishing I'd made the move years before.

Yeah it's been graft, i started short notice on Saturday and did 65 hours this week, with a couple of late night finishes. I'm glad to be getting stuck in again after essentially being a stay at home dad for the past 5 years.

Here's hoping we smash it pal.

 

[Electrics]

It's just when a conductor is connected in the same terminal from fitting to fitting , simple as that really.

So if i'm understanding right, say the N is connected from rose A to rose B, and rose A is off at the switch (which is essentially what happened here) and rose B is on at the switch, because the neutrals are connected i can get a shock from the line conductor of rose B?

 

[Electrics]

A shared neutral is perfectly normal and required within a circuit. A borrowed neutral is a no no as it is from another circuit. When you took the neutral out did you notice lights going off elsewhere?

Nope. I'm starting to think it was the sensor

 

[Dave OCD]

So if i'm understanding right, say the N is connected from rose A to rose B, and rose A is off at the switch (which is essentially what happened here) and rose B is on at the switch, because the neutrals are connected i can get a shock from the line conductor of rose B?

If the neutral ran from A to B and A was off but B was on at the switch and you then separated the neutrals while replacing A the neutral going to B would then rise to mains potential as it's 'open' with nowhere to return to - apart from through you to earth....

 

[westward10]

The error was made when your colleague only switched off the test facility, this was a serious error on their part because if wired correctly it only isolated the permanent live which is essentially the part of the circuit which charges the battery at the fitting. A maintained emergency light also has a switched line which turns it on as per a normal light and the permanent line as previously mentioned. When I isolate a circuit and a colleague asks if it is dead I say from my end yes but double check, never rely on another person's confirmation no matter who they are.

 

[nicebutdim]

The error was made when your colleague only switched off the test facility, this was a serious error on their part because if wired correctly it only isolated the permanent live which is essentially the part of the circuit which charges the battery at the fitting. A maintained emergency light also has a switched line which turns it on as per a normal light and the permanent line as previously mentioned. When I isolate a circuit and a colleague asks if it is dead I say from my end yes but double check, never rely on another person's confirmation no matter who they are.

Am I right in stating that the light has two completely separate feeds looped through it, with neutrals commoned between them?

 

[westward10]

Am I right in stating that the light has two completely separate feeds looped through it, with neutrals commoned between them?

Not separate feeds as such they should be from the same circuit. A maintained emergency light which is switch for illumination at material times, in other words it should be on when the general lighting is switched on has two line supplies one to switch as a normal light, the other for the battery charge circuit which is controlled through the emergency lighting test facility. The neutral isn't really "commoned" it is the same circuit so if you isolate the test facility you have not isolated the general line conductor for the circuit, the test facility will only kill the permanent/charge supply.

 

[Electrics]

If the neutral ran from A to B and A was off but B was on at the switch and you then separated the neutrals while replacing A the neutral going to B would then rise to mains potential as it's 'open' with nowhere to return to - apart from through you to earth....

So basically i could have gotten a proper belt off this? I'm a bit concerned that nothing actually tripped - it is an RCD protected 3 phase board. Shouldn't it have gone off when that happened? So much for me to learn..I still don't get how the N could feed back through the next door, i need a diagram lol.

 

[westward10]

Later this morning I will do a drawing of the emergency lighting circuit I suggested.

 

[Electrics]

Later this morning I will do a drawing of the emergency lighting circuit I suggested.

I owe you a beer.

 

[DPG]

I owe you a beer.

Best post this week.