Dimmer Switch , Still Getting Voltage From The Switch Wire When The Switch Is Off.

[doodneylucky13]

Hi .

 Got called to a job to change a few light fittings.  First  fitting was on the ground floor , two way lighing. one switch was a dimmer and the other was just a bog standard switch. The switch wire was not marked up at the fitting so i did some live testing to find the switch wire.

I was finding that i was still getting voltage down the switch wire evern thoe the dimmer switch was in the off position . I was only getting about 50v but my tester was saying  there was voltage. testing between switch wire and neutral.

I was also asked to change a fitting up stairs which was not on a dimmer switch but the up stairs circuit did have a dimmer switchses on it. Again i was reading 50v on the switch wire  evern thoe the switch was off. 

Is this because the dimmer back feeding of the neutral ? or a faultly switch?  or does anybody know before i try and go through all my old books from college.

 

[Marvo]

If you were using a digital tester you might just be seeing a 'surface voltage' that's being induced or capacitively coupled from another circuit in close proximity. Best to repeat the test using an analogue tester in this kind of case, there's a good chance it will show no voltage because of it lower input impedance.

 

[doodneylucky13]

Yes it was a digital tester that i was using. will try a analogue test next time and see.

thankss for your help.

 

[Marvo]

It's also worth noting that the output from a dimmer module may not be registered correctly by some voltage testers. It's always prudent to prove dead on the supply side of a dimmer rather than the load side.

 

[Andy™]

 so i did some live testing to find the switch wire.

surely its quicker (& safer) to do the job properly and find the switch wire without turning the power on...

 

[ProDave]

If your job is to CHANGE some light fittings, you identify the wires BEFORE you disconnect the old fitting.

Judging by how many times this comes up, nobody seems to do that : headbang

Now if you had said someone had already removed the fitting and you were just presented with a bunch of wires hanging from the ceiling.......

 

[doodneylucky13]

It's also worth noting that the output from a dimmer module may not be registered correctly by some voltage testers. It's always prudent to prove dead on the supply side of a dimmer rather than the load side.

Hi , when i did turn the isolate the circuirt i did prove it was dead by testing at the switch on the suppy and load side as well as the fitting.

 

[doodneylucky13]

If your job is to CHANGE some light fittings, you identify the wires BEFORE you disconnect the old fitting.

Judging by how many times this comes up, nobody seems to do that : headbang

Now if you had said someone had already removed the fitting and you were just presented with a bunch of wires hanging from the ceiling.......

Hi,

the custermer had already taken the old fitting down and all the cable wired in corrnetor blocks there hanging from the ceiling as they tried to change the fitting them selfs and wre struggling, that why i had to test and find out what was what .

Of coure it make sence to know and lable what your discourntecting before you change the fitting. thats just common sence

 

[doodneylucky13]

surely its quicker (& safer) to do the job properly and find the switch wire without turning the power on...

Hi   i wouldnt say dead teasting was quicker but a compertent spark can  still work safely  and do the job properly doing live testing.

 

[Doc Hudson]

Hi   i wouldnt say dead teasting was quicker but a compertent spark can  still work safely  and do the job properly doing live testing.

As you say it is possible for a competent person to work on a live circuit. But I imagine that others may also expect a competent person to be well aware of induced voltages along parallel paths, especially lighting circuits and the appearance floating voltages on dead wires when using a digital multimeter. These "induced/floating" voltages are one of the causes of some low energy lamps glowing and or flickering when switched off, especially low power LED's.

Doc H.

 

[doodneylucky13]

hi , it something i just wanted to check as  this is something i deal with from day to day , this is why i asked the question and thought i would ask and see if i got a anwser straight away instead of spending ages on the internet and old college book and not getting anywhere. 

 

[doodneylucky13]

hi , it something i just wanted to check as  this is something i deal with from day to day , this is why i asked the question and thought i would ask and see if i got a anwser straight away instead of spending ages on the internet and old college book and not getting anywhere. 

  * something i dont deal with from day to day .