Unusual two switch setup

[conroyster]

I was recently changing the switch plates in my brother's house, when I discovered a strange setup with the landing light and I was wondering if anyone could shed some light on how it works.

The light on the top of the stairs in controlled by either a switch at the top of the stairs or the switch at the bottom of the stairs. The switch at the top of the stairs includes 3 wires (2 red, 1 black), were 1 of the red wires seems to be in it's own sleeve and the other 2 wires (1 red and 1 black) are sleeved together.

The switch at the bottom of the stairs has 2 wires (1 red, 1 black) in the same sleeve. However the red wire is connected into 2 terminals (i.e. the end of the red wire is connected into a terminal and the same wire has a section of sleeve remove to allow it to connect to another terminal). This setup, with the switch at the bottom of the stairs, is something that I have not seen and discovered through browsing the internet.

As you may have gathered I am not an electrician, but I am interested in how this particular setup works. Apologies if this is not enough information, but I cannot remember the actual wire-to-terminal setup.

 

[ericmark]

There are two basic wiring methods

method two is more common as it does not require and junction blocks in switch. There are a few variations including borrowing the live wire from another light circuit and I have explained further here however I am sure there are more if you really look.

 

[1john]

thats the conventional method, check for shared neutrals with this setup.

 

[RF Lighting]

Yep definate risk of borrowed neutrals there.

Please be VERY careful when working on these lighting circuits. Even if they appear and test as dead, splitting a connection at the lights can unsuspectingly liven some of the wires back up.

If you are in any doubt, make sure you switch off the MAIN SWITCH to your installation, rather than the individual fuse or MCB.

 

[martinxxxxxx]

I second that!



Yes follow the isolation procedure. Today it saved my life potentially. I was asked to fit a pendant onto some wires hanging out of a new ceiling, I switch off both the upstair and down lighting MCBs. But testing for dead showed that actually the light had been wired to a fused spur from the down ring.

Least it gave me an excuse to play with my Locate220. My latest investment / wasted money.

Martin

 

[conroyster]

Thanks for the responses.

As I mentioned previously I am not an electrician, but I am intrigued by the setup. I realise that there could be more than one way to do this, but I thought the following setup would apply:

Although I would have thought that the wiring would be swapped in the 2 switches:SwitchA:L1 to SwitchB:L2, SwitchA:L2->SwitchB:L1. This would ensure that the light is on only when the one and only one switch is on.

What throw me when I seen the setup was the single red wire used in 2 terminals, I should also mention that the switch at the bottom of the stairs is a double, which the second switch independently controlling a light at the bottom of the stairs.

 

[1john]

the permanent live as pictured in your left hand switch is shared over two switches to provide a feed to both. You may find installations with this type of two way switching are wired in mostly in singles, neutrals can usually be found looped in the rose and the permanant lives are looped in the switches.

 

[plugwash]

Probablly this setup or some variant of it.

Note: I strongly reccomend isolating all lighting before working on it as this setup very often indicates a borrowed neutral.

 

[conroyster]

Thanks for the replies.

Am I right in thinking that a borrowed neutral is were live is taking from one circuit and the neutral from another? Is so why would you do this?

 

[ColJack]

you don't in general, but where a house was once wired with a single lighting circuit for both up and downstairs, and has subsequently been split into 2, they may have overlooked the fact that the landing light is fed from downstairs but takes it's neutral from upstairs..

it could also just be bad planning by the installer..

 

[1john]

correct, to save running an extra cable. also i suppose the dangers wern't apparant

 

[ColJack]

IS there an inherant danger with a borrowed neutral?
after all, every single neutral is connected together back at the CU anyway..
it's only the introduction of RCD's that complain when neutrals are borrowed..

 

[plugwash]

Borrowed neutrals do not present a hazard to those using the installation but they do present a hazard to those maintaining and/or modifying it.

When you disconnect the neutral to a load while leaving the live connected the disconnected neutral conductor will become live. While it will have a much higher impedance than a supply conductor it's impedance will still be more than low enough to deliver a fatal shock.

Now consider an installation with a borrowed neutral. Consider the situation where you are working on the circuit the neutral is borrowed from before the place the neutral is borrowed. You isolate the circuit and confirm it is dead, then you start disconnecting a loop-in wired fitting to replace it and in doing so break the neutral conductor to the fitting with a borrowed neutral. The neutral conductor to the next fitting in the circuit is now live.

 

[conroyster]

From my initial description what makes you think that the neutral is borrowed?


Thanks

 

[plugwash]

From your descrtption (which is a little vauge) it sounds like the live for the hall light and the live for the landing light come from the same place, most likely the downstairs lighting circuit. However unless someone has made special provision otherwise the neutral for the landing light will most likely be taken from the upstairs lighting circuit.