extra wire on ceiling rose

[mattdh]

Hi,

just moved into a new house and have been doing some painting. Took the hall ceiling rose down to paint behind it. Everything is wired as expected (loop, switched live etc.) but it also has a single 'extra' black wire which exists in its own outer grey casing with only an earth. (all the other wires exist as black, red and earth in grey casing as expected. Does anyone know what this additional black neutral is there for? Everthing works as it should.

Thanks for your help,

Matt.

 

[mattdh]

...just realised, smoke detectors are on separate circuit anyway, so nothing to do with them!

 

[Space cat]

Everything is wired as expected --

In which case you have three red wires in the permanent live terminal, a single black wire feeding one side of the light and two black wires connected to the other side - but there's another black wire. Correct?

Question one: Does the extra wire go to the neutral terminal?

Question two: Is there another light nearby that's two-way switched?

Old two-way switch wiring typically had the live feed going to COM in one switch with switched live coming from COM in the other one. Two core cable was then used to link L1 to L1 and L2 to L2. This circuit does work but, if you think about it, the neutral return from the light is a long way from the live supply.

A common trick was to connect it to the nearest available neutral, most likely on a different circuit. This is a BAD IDEA. You will find out why if you turn off the second circuit to do some work and then disconnect a critical neutral wire. If the offending two-way light is switched on, it will go off and you will jump!

One way to find out what that wire does is to disconnect it and isolate it in choc-block. Does anything not work anymore?

 

[mattdh]

...it is connected to a switch that controls an outside lamp next to the front door. The lamp itself is next to the consumer unit and the switch is closer to the ceiling rose in question. The lamp is on the same circuit as the dowstairs lights so everything is fine (the neutral has not been "borrowed". Still not sure why this shortcut was taken though! All I can think of is that the switch is in the same case as the switch that controls the ceiling rose light so it is easier to route the cables in this way.

 

[Space cat]

The neutral is connected to the switch! That's wrong; you always put the switch in the live side. It sounds like somebody has taken the easiest route: the light is closest to the CU so they're gone like this ---

Live (in CU) -> Light -> Switch -> Neutral (on ceiling rose)

Bad plan. When the outside light is off, both sides are live.

 

[mattdh]

my mistake - it's not going to the switch - I just assumed it was because of the switch location. I was more worried about it being connected to another circuit. I have just done a continuity test and the 'extra' neutral runs to the lamp. The switch is therefore on the live side of the circuit. All the other roses in the house are wired as they should be. When I saw this extra black yesterday I was just suprised as I had not seen things done in this way before and the bloke we bought the house from said he was an electrician. I was just worried that he had had a go at some rewiring incorrecty because when we moved in I noticed he had put an extension cable into the porch by disconnecting the immersion heater radial in the CU and connected the extension directly to it. No RCD protection and its was probably installed to power a lawn mower. I can rest easy now as there don't seem to be any more abnormalities!

Thanks for your input on this,

Matt.

 

[Space cat]

You're welcome.