Switch Return

[FrankTrent]

i am replacing a light fitting but the switch return has not been marked and I cannot get fingers up through plasterboard to tell which cable goes where.
Will it make any difference if they are reversed on the fitting ? (it is metal but will be earthed ).r

 

[Paul_C]

Yes, it will make a difference!

You will need to identify which cable goes to the switch. If you have a test meter, then with the cables all separated (and power switched off), use your meter on a low ohms range and measure across the red/black (or brown/blue) conductors of each cable in turn. When you find the one which gives a near-zero reading with the switch on and open-circuit with the switch off, that's the switch cable.

 

[GaryMo]

If what is reversed? The switch feed and switch return?

 

[wingcoax]

If you know which is the switch feed then WITH THE POWER OFF pull or push this wire and the switch return should move with it.

 

[FrankTrent]

It is 5 amp lighting circuit with two 3 core cables hanging from the ceiling.
The old 1965 light fitting had 3 terminals, all identical, none marked. The wiring was done so that the fitting could be utilised for the switch wire. The replacement light has a connector with live, neutral and earth connections marked. it is metal and will be earthed
The 2 reds were twisted together both in one terminal.
The 2 black were each to the other 2 terminals.
The 2 Earths were just twisted together (without sleeving) and stuffed in the ceiling.
The switch has been extended through a wall and polyfilled so I cannot jiggle it.
The ceiling rose is under a fitted bath and floor so I can't access it to check where the cable runs.

To summarise, I know that the switch will be correctly on the live wire, but i don't know which of the 2 black wires is the live switch return or which is the neutral. If I get it wrong, the light will have the neutral on the live and vice versa. What exactly will happen /

 

[flameport]

If the new light fitting is just a standard bayonet lampholder, then there is no problem.

However other types of fitting will probably require that the L and N are connected properly. Getting this wrong could result in them not working at all, not working properly, or in some cases could result in live parts being exposed (such as for some screw fitting lampholders).

You need to get a multimeter or some other device to confirm what the wires are. The switched live will need a piece of red sleeving on it.

 

[ban-all-sheds]

If the new light fitting is just a standard bayonet lampholder, then there is no problem.

Or if it has a built-in transformer

However other types of fitting will probably require that the L and N are connected properly. Getting this wrong could result in them not working at all, not working properly,

I can't see how polarity reversal could do that...

or in some cases could result in live parts being exposed (such as for some screw fitting lampholders).

That's the real issue, but not a problem as long as the lampholder is an E14 or E27 complying with BS EN 60238.

You need to get a multimeter or some other device to confirm what the wires are.

Frank - this is quite true - a multimeter is an essential tool if you want to work on your electrics, and you should no more consider it acceptable to not have one than you would consider it acceptable to not have screwdrivers.

 

[FrankTrent]

Thanks everybody. It's now sorted.
I just connected one cable to the old fixing with a bulb and it worked so obviously that was the ring and the other cable was the switch.