switch wiring oddity
 

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

"Witchy" wrote in message
...
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

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

Witchy wrote:
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


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.

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

Any clues?

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

On 13 Sep 2004 04:44:09 -0700, (Dave) wrote:


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

Any clues?

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.

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

On Mon, 13 Sep 2004 22:45:16 +0100, Witchy
wrote:



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

cheers!

--
wtchy/binarydinosaurs


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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