switch wiring oddity

[Witchy]

Hi folks,

Not been here for ages 'cos of a new job Down South, hence no diy apart from weekends!

Anyhoo, got an oddity at a mate's house with the bathroom light. The switch is outside the door and contains 3 cables, one of which looks like it contains 2 red wires?! 2 blacks are joined in a choc-block, one red doesn't go anywhere and the other 3 reds are in the switch.

Ceiling rose has 3 sets of cables, one of which goes to another light which is being removed.

Now the conundrum - with the light OFF there's 240V across the swtich. Turn the light ON and the voltage vanishes.

Any clues?

cheers

adrian/witchy, now in cambridgeshire.

-- cheers,

witchy/binarydinosaurs

[mrcheerful]

This is completely normal, when the switch is off, one side is live and the other earths through the bulb, when the switch is on the switch has less resistance than the meter so the meter shows nothing all the current goes through the switch. To prove this remove the bulb, the readings should remain the same, also if you go one side of meter to earth, the other to the switch you should see 240 on one side with switch off and both sides with switch on.

mrcheerful

[AJ]

When you switch the light on both sides of the switch are at the same potential - both at 240V - volmeters measure the potential difference between two points, with them both at 240V the reading will be zero..

Measure between earth and the conductors they will both be 240V when the switch is on and just the feed side will be at 240 v when the switch is off.

[Dave]

Yep, you're reading the voltage across the switch, rather than the voltage between the terminals and neutral - so you'll get the situation you describe. If you attach one side of your meter to neutral or earth and measure the terminal voltages again you'll see

230V on both terminals with the switch ON and 230V on one with 0V on the other with the switch OFF.

Trust me, the voltage hasn't 'vanished'!!

:-)

Dave

[Witchy]

Ta, the 3 of you, I guess I'd not tested that before, and having it explained like that makes sense.

I'll have to revisit it next weekend if he hasn't sorted it out by then :)

cheers!

-- wtchy/binarydinosaurs

-- cheers,

witchy/binarydinosaurs

[Andy Hall]

So what happened to the house? In the last episode, the ground floors didn't exist and you were building sleeper walls etc. IIRC....

Enquiring minds need to know.... :-)

.andy

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