switches

[rydany]

hi i have 2 switches that work 1 lamp,but seem to have no power to either switch.any clues on what to check(fuses ok)

cheers

[badams]

Presumably you've done the obvious and changd the bulb?  It is possible the circuit is wired incorrectly, and is switched in the neutral rather than the live, in which case if the bulb blows, it will look s though you have no power.  Are you talking about a conventional two-way circuit as would be used on a staircase?  How did you test for power?

[rydany]

Hi badhams yes i have checked the bulb,i tested 4 power with a meter,is poss the fault is at the lamp holder end.

cheers

[badams]

At what point did you check with the meter?  On a conventional 2-way circuit there is no neutral at the switch.  Depending on how the opposite end switch is positioned, you may or may not be able to read across the contacts.

[rydany]

I tested at the switch both upstairs and down with no reading. 

[badams]

OK - when you tested, what readings were you trying to take?  Between the switch contacts and earth, or what?  If you are using a digital multimeter, you should ne able to get a reading between live and earth when no neutral is available - most two way switches do not have a neutral available at them.  It is normal on such a circuit that you will definitely get no readings across the switch at one end, and you can only get a reading between one of the strapper circuits and the switching contact at the other end.

[Tim Baxter]

If you're talking about two-way switches controlling the one lamp then only one switch's supply will be live.  It is impossible to say by visual inspection or meter readings which switch has the supply.  There is generally no earth/neutral return on either switch because the 1st switch's output side feeds the power through either conductor to the next switch and then on to the lamp.  You can check for v'age between the supply and a known earth/neutral (say in a mains socket).  Better still is to use a neon tester which will give you a light when touched to supply because you are the earth return.  So you can test for v'age on all wires attached to both switches.  If supply is present you can follow it's path through both switches, after the lamp it will not light because there is no v'age present.  If the rest of your lights work ok it would be unusual for the fuse to be faulty.  If you can ascertain that there is no v'age present then you should check for continuity, between lamp and earth (zero ohms) supply side of lamp (a few  'x' ohms) output of 2nd switch ('x' ohms) and so on tracing back along the circuit until there is a reading of infinity ohms which indicates a break in the cable.

[badams]

On any installation with steel wallboxes, the box should be earthed. The neon-tester (or phase-tester) is okay, but be aware that it may not glow much, or at all, if the person using it is not well earthed.  Someone with plastic soled trainers, standing on a nylon carpet, on a wooden floor, on the 1st floor of the building, has a resistance of many MegaOhms to earth, and might not pass enough current to light the neon!

If the neon does light, carry out the tests as Tim suggests, but NEVER assume that because a neon does not light, that a circuit is not live!  Tim - I think your metering description needs a slight elaboration.  "Between lamp and earth" - I presume you mean between the casing of the lamp and earth - not one of the contacts!  Zero ohms between a contact and earth would be very bad!

Be very careful about tracing circuits by reading ohms - if you are not familiar with the wiring, always check for voltage before taking an ohms reading - most electrical/electronic engineers manage to blow up at least one ohmeter in their life by forgetting this!

Another thought on this particular problem - rydany, is there any chance that there is a consumer unit in the building switched off, other than the one which supplies the live to this lamp?  It is very bad practise, but I have seen installations where a live is taken from one board, and the neutral is taken from another one. (Because it is convenient to pick up from another light etc.)  In this instance, the lamp can still have a live at it, and not light, because the neutral is removed when the 2nd consumer unit is switched off.