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

[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.