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

Electrics · Sep 10, 2021

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.)

westward10 · Sep 11, 2021

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.

[ElectriciansForums.net] Neutral went bang - can someone explain this wiring to me?

littlespark · Sep 11, 2021

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 · Sep 11, 2021

westward10 said:
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 · Sep 11, 2021

CamoElectric said:
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 · Sep 11, 2021

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 · Sep 11, 2021

westward10 said:
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 · Sep 11, 2021

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 · Sep 11, 2021

westward10 said:
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 · Sep 11, 2021

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

Electrics · Sep 11, 2021

westward10 said:
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 · Sep 11, 2021

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 · Sep 11, 2021

westward10 said:
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 · Sep 11, 2021

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 · Sep 11, 2021

westward10 said:
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 · Sep 11, 2021

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.

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

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