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!

 

[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.

 

[westward10]

Excuse my non harmonized pens but this is a simplistic drawing. So you have one line supply split at the two switches and it can be seen that just isolating SW2, the emergency lighting test facility you have not completely isolated the circuit as the general switch, SW1 is still in circuit. So to fully isolate it must be done prior to these two switches. You can see the emergency light has the additional charge supply at L2 which the general light doesn't have so the neutral still carries current to the general light if only the test facility, SW2 is off.

 

[littlespark]

I did exactly the same as an apprentice.
I was told to fix or replace the lights on the stairwell at work. Wasn’t really supervised as the tradesmen were all doing other stuff and I was just spare on that day.

switched the lights off at just the switch and proceeded to remove the bulkhead.

old stranded wiring, the 2 neutral twisted together. Came out the terminal ok, but when I untwisted them…. Lots of crackling, and a shout from upstairs, the storeroom lights were flickering.


The line and neutral carry the same current. You know this, as that’s how an RCD works… looking for the imbalance.
Whatever the load is, is the same.
Seperating the neutral anywhere downstream from the main board means any load downstream doesn’t have a return path to the supply neutral.

In my case, it was half a dozen fluorescent lights finding a return path through my pliers.

 

[Electrics]

Excuse my non harmonized pens but this is a simplistic drawing. So you have one line supply split at the two switches and it can be seen that just isolating SW2, the emergency lighting test facility you have not completely isolated the circuit as the general switch, SW1 is still in circuit. So to fully isolate it must be done prior to these two switches. You can see the emergency light has the additional charge supply at L2 which the general light doesn't have so the neutral still carries current to the general light if only the test facility, SW2 is off.View attachment 89927

Legend.

This isn't exactly as it was wired since it had a sensor but i'm guessing the sensor (SW1 here) was 'closed', the SW2 was isolated at the test key and so the N back to the board was carrying the load since SW1 was still live.

There were actually 4 wires in the back, brown blue and earth and a black live.

I'll draw a pic of the set up and upload it, since i'm still not quite getting it fully. I can install basic lighting fine but figuring out someone else's always confuses me for some reason.

 

[westward10]

Legend.

This isn't exactly as it was wired since it had a sensor but i'm guessing the sensor (SW1 here) was 'closed', the SW2 was isolated at the test key and so the N back to the board was carrying the load since SW1 was still live.

There were actually 4 wires in the back, brown blue and earth and a black live.

I'll draw a pic of the set up and upload it, since i'm still not quite getting it fully. I can install basic lighting fine but figuring out someone else's always confuses me for some reason.

Yes I forgot the sensor so as you say replace SW1 with it, switching of SW2 the test facility will not stop it switching assuming it is wired like my diagram. Your four wires are normally brown (normal switched line from my SW1), blue neutral and black the permanent live for the emergency side from my SW2. It is not uncommon to carry the permanent line (black) through all fittings even the non emergencies, it is just linked through at a connector with no connection to the light.

 

[ipf]

A good example regarding 'shared returns' as we called them. Used to get it with rows of lights, say 2 rows on the same circuit, with single pole switches. They're sharing a neutral. If you work on (disconnect) one switched off leg, with the other switched on, the neutral's acting as return for the 'on' row.
The switches should really be double pole with individual neutrals to each row......although, another good reason to fully isolate the circuit before working on it.

Seen it with 3 phases feeding 3 rows with one neutral. You pulled a fuse and found a 'live' return (neutral). Avoiding stroboscopic effect comes to mind.

 

[Electrics]

Yes I forgot the sensor so as you say replace SW1 with it, switching of SW2 the test facility will not stop it switching assuming it is wired like my diagram. Your four wires are normally brown (normal switched line from my SW1), blue neutral and black the permanent live for the emergency side from my SW2. It is not uncommon to carry the permanent line (black) through all fittings even the non emergencies, it is just linked through at a connector with no connection to the light.

Thanks for the explanation.

I get the wiring diagram you did and that makes sense, but i cannot fathom out how it actually physically looks in real life and where the cables are all going.

I can't picture a 4 core going from the light to the switches and a N also going to the lights in the next room, i can't picture how it would actually be done in reality with the cables.

2 plate, 3 plate, 1 and 2 way switching i'm fine with, i can diagram them out and also picture how to do it but this has me stumped for some reason.

 

[westward10]

The wiring of lighting circuits can sometimes be bewildering even to experienced people where at times you think why the hell have you done that. In your example it depends on the wiring method, singles in conduit/trunking is generally straight forward where the lines go through the switches and the lights on luminaire couplers like Klix plugs which produce sockets for emergency lighting points. T&E is not quite so straight forward and you can three plate it with a link to the test facility and a three core and cpc between the first light and switches then you carry the same type of cable through lights and only utilising the permanent line where necessary. This is a rough drawing for three plate and T&E.

 

[Electrics]

The wiring of lighting circuits can sometimes be bewildering even to experienced people where at times you think why the hell have you done that. In your example it depends on the wiring method, singles in conduit/trunking is generally straight forward where the lines go through the switches and the lights on luminaire couplers like Klix plugs which produce sockets for emergency lighting points. T&E is not quite so straight forward and you can three plate it with a link to the test facility and a three core and cpc between the first light and switches then you carry the same type of cable through lights and only utilising the permanent line where necessary. This is a rough drawing for three plate and T&E.View attachment 89934

I really appreciate you taking the time to do this!

 

[westward10]

To quote @DPG it's just wires and stuff.

 

[Electrics]

To quote @DPG it's just wires and stuff.

I think I get it, is this the basic principle?

If this is correct, then I still don't understand how my light was wired since the light itself had a black and brown switch live, not a blue switch live. Unless they've simply put them the other way round. Let me know if this drawing is pish.

I think from looking at your diagram what has happened is, the live to the 'switch' is still live since the MCB isn't off and because of the movement in the room, it's always been ready to switch on because it's a sensor and not a switch.

That's the best I can make of it.

Much more needed from me in terms of lighting. Everything else seems fairly straight forward but some lighting proper mashes my head when i try to fathom out how it would be done in cabling. Back to the books and i really think getting some actual cabling and physically doing it would be a great help to me.

In singles it seems fairly straight forward but i always get confused as to how to do it properly in cabling.

Thanks for all your help mate, and everyone else!

EDIT: maybe it's because your drawing is 3 plate and i think the method used on site was 2 plate since it was only push connectors at the fitting not a rose.

 

[westward10]

In your drawing I would use the brown as the live to the switches and the black as one of the returns plus in a three core and cpc you have no blue it is grey. Three plate doesn't have to have a rose the connections can be within the light fitting. The term three plate is used because years ago there were three terminals on a batten holder back plate hence three plate. Prior to this with only two terminals at the base you had two plate.
There are multiple ways lighting circuits can be wired some of them quite baffling and over complicated.

 

[Electrics]

In your drawing I would use the brown as the live to the switches and the black as one of the returns plus in a three core and cpc you have no blue it is grey. Three plate doesn't have to have a rose the connections can be within the light fitting. The term three plate is used because years ago there were three terminals on a batten holder back plate hence three plate. Prior to this with only two terminals at the base you had two plate.
There are multiple ways lighting circuits can be wired some of them quite baffling and over complicated.

Well this has definitely baffled me!

There is zero chance of me figuring this out without taking the sensor out and the key switch and looking at the cabling.

Since the light literally only had a 3 core and earth in the fitting, it couldn't have been wired like my drawing above even if the principle is the same.

This is going to really wind me up.

I think from memory the sensor had red black and earth in the back of it but i only briefly glanced at it on my way past, someone else took it out and was looking at it.

We need to change the sensor since it's naffed (they linked the line and switch line together so the light is now permanently on (it's in a store cupboard) until the sensor is changed) so if i get sent back there next week i'll take some pics of the wiring at both switch and sensor.

Thanks for taking the time to try to explain it to me, definitely owe some beers!

 

[westward10]

Imagine the sensor as the switch which it essentially is so you were correct previously where you said it replaces SW1 in my drawing the only slight difference being it also requires a neutral to function.

 

[Electrics]

Imagine the sensor as the switch which it essentially is so you were correct previously where you said it replaces SW1 in my drawing the only slight difference being it also requires a neutral to function.

I get how it works in your drawing, like if i had to wire it in singles i could, quite easily. I just can't get my head around exactly how it was wired in the light i was replacing! Hopefully i get to go back and can have a proper look.

 

[westward10]

It will all become clear don't try and cram too much in, once you get the basic principles you will eventually see how it all works.

 

[Vortigern]

A sensor would have a L N and a L out to the light so you should have seen three wires, not just black and red and earth. Shame there is no pics. The light you were working on, just to be clear, there was only four wires going in to it? Not eight wires, i.e. looping to the next fitting in line???

 

[nicebutdim]

This is going to really wind me up.

Don't let it - if you get the opportunity to figure it out, then well and good. If not learn what you can and add to that at each subsequent opportunity.

There's a mountain to climb and the only way to the top is one step after another.

 

[Electrics]

A sensor would have a L N and a L out to the light so you should have seen three wires, not just black and red and earth. Shame there is no pics. The light you were working on, just to be clear, there was only four wires going in to it? Not eight wires, i.e. looping to the next fitting in line???

The light was fed by 3 core + earth flex, one cable in, push fit connections at the fitting and black, blue, brown and earth.

I think it was the last on the circuit from the lighting in the room next door since that room was a main shop front but the light i was changing was in a store cupboard so maybe added as an afterthought. Might be why i still got a bang since the cupboard was only turned off at the keyswitch and not the MCB - when the MCB was turned off the lights in the main shop room went off.

I'll draw a pic of the set up.

 

[Electrics]

You know thinking about it, the wires were really short and when i took the neutral out it's possible some of the other wires touched.

I'm wondering if the line from the sensor has shorted which is why the sensor is now broken and doesn't work. It's not a coincidence imo that the sensor stopped working immediately after this.

If the sensor was still live then it's going to switch live as soon as it senses my movement in the room isn't it? Normally i think you switch off the light and then kill the emergency switch so the emergency light comes on the battery power but if it's a sensor and not a switch then you can't kill the power unless at the MCB, which was still on?

Still confused mind as to how the RCD never tripped.

 

[westward10]

A flex indicates a coupler in the ceiling void.