2W Light Wiring Diagram

[Dewi]

I am about to wire up 2 light fittings in my loft with a 2W switch. Can someone tell me if the diagram below is the correct way to do it. Basically I am planning on putting a 2W 1G switch at the bottom of the stairs and another at the top. These will switch 2 lights that are daisy chaines and will both come on together. I've checked that I won't be overloading the upstairs lighting circuit and it is ok. Many thanks.

 

[plugwash]

one of the earths in your diagram is disconnected but i presume thats just a drawing error.

for the run between the two switches you should use 3 and earth cable, don't use singles unless they are either double insulated or in counduit/trunking for thier entire run.

remember to mark the switched live with red sleeving

other than that your diagram looks fine.

 

[Dewi]

Many thanks for the fast reply plugwash. Good spot - you are right, I resized the drawing and somehow managed to disconnect one of the wires !. I was planning to use some double insulated cable and 1.0mm T/E that I had lying around rather than buy some 3 and earth, so it should be ok. Best Regards.

 

[Adam_151]

If you are using a DI single and T&E for the two way switching, as opposed to a piece of 3C&E, then make sure they both go through the some hole on the backbox

(doing otherwise would result in the sum of the currents through the hole not being zero, which would try and induce a current in the metal backbox (think of a transformer), with lights the results probably wouldn't be too bad, but when its done with the tails of a distrution board for example, it makes a bit of a mess of the cables)

 

[BJS_Spark]

If you are using a DI single and T&E for the two way switching, as opposed to a piece of 3C&E, then make sure they both go through the some hole on the backbox

(doing otherwise would result in the sum of the currents through the hole not being zero, which would try and induce a current in the metal backbox (think of a transformer), with lights the results probably wouldn't be too bad, but when its done with the tails of a distrution board for example, it makes a bit of a mess of the cables)

Surely that is nonsense and you just made that up. A transformer requires the wires to be made into turns and thus you get magnetic coupling between the 'windings'.
Also a 3C&E for two way switching is one earth and three live wires so where would an imbalance come from?

 

[Qedelec]

Adams right, BJ
But as he said the current used for lighting wouldn't amount to a hill o'beans.
As for two way lighting, the current flowing down one conductor will cancel out the current in the other resulting in nill flux.
Thats why its recommended to prevent noise, in buildings incorporating a hearing loop.

Large current carrying conductors entering a METAL enclosure must have their entrance holes joined up to prevent build up of magnetic flux resulting in the heating of the enclosure.

 

[BJS_Spark]

As for two way lighting, the current flowing down one conductor will cancel out the current in the other resulting in nill flux.

Sorry that is not true either. If as you say 'the current flowing down one conductor' (the live supply) 'will cancel out the current in the other' (the live return to the light) then no current would reach the light and it would not work!!!

 

[Adam_151]

Right, i'll try and explain it a different way:

If you had a metal cased CU, and it was loaded to 50A, you'd have a current of 50A on each tail, well technically 50A on one tail and -50A (because the current flow is in opposite directions), now if each tail went in through a different K/O, the sum of currents through each K/O would be 50A, and this would act like a current transformer, with the tail being the primary, and the metal of the hole surrounding it being the secondary.

if both tails went though the same hold, the sum of the currents through that hole would be 50A + -50A =0A, as there is no net current on the 'primary' coils, there is nothing induced in the 'secondary'

If there was no option but to put the tails through separate holes, then you could cut a thin slot between the two holes, effectivly transforming it into one hole.

Thinking of it in terms of a clamp meter is probably better than a transformer, clamp one tail, and you'll get 50A, clamp both and you'll get 0A

if you draw out the two way switching circuit spoken about here, you can see that if the DI single went through a separate hole than the T&E, then like in the CU example that the currents through each hole would not be balenced, but with it being a lighting circuit and currents being small, the effect is likely to be miniscule

 

[slippyr4]

As for two way lighting, the current flowing down one conductor will cancel out the current in the other resulting in nill flux.

Sorry that is not true either. If as you say 'the current flowing down one conductor' (the live supply) 'will cancel out the current in the other' (the live return to the light) then no current would reach the light and it would not work!!!

Because the currents are equal, but running in opposite physical directions, the magnetic fields induced around them will be equal and opposite, thus cancelling each other out.

To the OP: you really ought to buy some 3C+E - it's going to cost you less then a fiver, and produces a smarter and more proper job.

 

[BJS_Spark]

Basically then what should have been said was the induced field will cancel and not the current.

I am still not entirely convinced that this statement holds water

If you had a metal cased CU, and it was loaded to 50A, you'd have a current of 50A on each tail, well technically 50A on one tail and -50A (because the current flow is in opposite directions), now if each tail went in through a different K/O, the sum of currents through each K/O would be 50A, and this would act like a current transformer, with the tail being the primary, and the metal of the hole surrounding it being the secondary

The enclosure (your proposed secondary) would be at earth potential and the 'single turn' has such a minute resistance it is as good as a dead short. Even at 50 or more Amps I can imagine little current actually generated in it.

 

[Bylough]

Sounds like the deadly kuu-flux-flan !
But while on the subject of two way sw. Can you assist me in understanding how I get over the hazzard of:
I have a triple gang switch downstair- two parts of which are on the down fuse, and one part on the up fuse. I once drew out the down fuse to fix this switch mechanically- and, you've guessed it I imagine, nearly got deaded- well my screwdriver is!. I did the wiring many years ago.
Can I mark the switch or what? The switch came with the house!

 

[Adam_151]

It needs one of those labels that says isolate from multiple sources, as this would look ****e stuck to a switch in a house, the best thing to to would be to stick it to the bottom of the backbox, then chances are anyone doing work on it will see it before they touch any connections, you should be able to get the label from an electrical wholesalers

 

[plugwash]

best way is to use a grid system (mk grid plus is the best known but there are other brands too) then put the warning on the grid plate. That way it will be noticed before any connections are touched but its not unsightly and not prone to wear and tear.

 

[Bylough]

Much appreciated, guys- I will sort both out from suppliers. Just glad to know that it is a 'stock' problem. Thanks for replies. (Excuse my butting in- post originator)

 

[Lectrician]

EDDY CURRENTS.

This is a very important to thing to be aware of, and it amazes me how many people have no idea!

Particular attention needs to be paid when wiring galv trunking/conduit systems, and using metal clad accessories etc etc.