View Full Version : Electrical conundrum

7th Jan 2018, 02:40
We have been fitting new kitchen lights. As part of the renovation we replaced the switch with a dimmer.

I wanted to put in the new light the other day and found the wire was live even after the switch was turned off using a voltage detector. I put a meter on it and it registered 110 volts. I spent an hour checking the wiring and couldn't find anything wrong so I turned the breaker off and installed the light.

The light, the dimmer and the switch work just fine so I wonder if there is some sort of latent voltage left in a dimmer after the switch it is turned off. Damned confusing.

Anyone know?

7th Jan 2018, 03:01
Sounds like the active and neutral wires may have been swapped somewhere in the circuit.

7th Jan 2018, 04:48
Sounds like the active and neutral wires may have been swapped somewhere in the circuit.

Agree sounds like someone is switching neutral. Think the latest standards 3000 in Australia require switching both active and neutral in new installations, old installs often just switched active.

7th Jan 2018, 06:03
If you are using a non-contact, pen type voltage detector, you could be getting a "false positive". Have you tried a meter?

7th Jan 2018, 06:48
Using a good meter; check all voltages from supposed live, neutral and earth to a good known earth.
Then you might have some facts to work from.

7th Jan 2018, 08:06
Or, if it's a fancy dimmer such as one might need for LED lights, it quite likely has a built-in DC power supply. This would involve rectifying the incoming mains and smoothing this with a capacitor. The voltage stored in the capacitor can remain a while after the power is removed.

You can often get a "pisser", as we used to call them, if you pull the power cable out of the back of a PC and immediately touch the mains terminals in the soxket.

Pontius Navigator
7th Jan 2018, 08:43
Or possibly a previously undiscovered fault. We had a live earth and neutral. The earth on the light circuit and neutral on the power. I think my description is right.

When our consumer unit had been replaced the fitter created the cross over. It had not affected the light or the power, it was simply dangerous.

7th Jan 2018, 08:48
Possibly a voltage induced in the suspect wire by running a live wire very close to the open circuit wire in the same conduit/trunking.

7th Jan 2018, 09:38
ChrisVJ, perhaps you could advise what make/model of dimmer you have and what type of lights it is driving ?

If all else fails, and you are not a licenced electrician, call one to investigate :)

7th Jan 2018, 10:21
Modern digital multimeters are so sensitive they will give false positive readings of induced voltages on disconnected cables from parallel cables running alongside. Low wattage LEDs can also glow when the circuit is disconnected from induced voltage in the wiring.

LED circuits generally require a trailing edge dimmer switch, rather than the cheaper leading edge dimmer switches commonly sold. The same applies to any transformer fed lighting circuit that is capable of being dimmed, otherwise you get buzzing from the lighting transformers or sometimes a faint continuous glow from LED lights when they are turned off.

Check you have the correct type of dimmer switch installed, manually trace the wiring of the circuit to confirm correct connection, or get a certified electrician to verify the circuit is wired correctly and the correct components are being used.

7th Jan 2018, 10:26
There is a lot to be said for using the old fashioned AVO meter......even more so if working anything producing RF.

7th Jan 2018, 14:06
Dimmers aren't perfect and will have some leakage. Digital meters have a very high input impedance and will give some reading. Make sure the dimmer is in the live leg and load it with a lamp across the meter leads, preferably incandescent, the reading should now be zero, or very small. As you turn on the dimmer and increase it the reading should also increase.

7th Jan 2018, 16:29
We have an extractor fan were the wireing gives the same voltage when off. But if you apply a resistance it drops to zero. It would seem that the permanent live feed for the timer is causing a conduction in the switched live.

8th Jan 2018, 02:35
Yes, checked it with a digital meter.

Gouli's answer confirms my suspicions.

Wiring is checked. Lives are live and Neutrals are neutral.

I have noticed the pen type voltage detectors often give a false positive. Seems they do it a lot with aluminium wiring too.

8th Jan 2018, 08:33
There is a lot to be said for using the old fashioned AVO meter......even more so if working anything producing RF.

I've still got and use my AVO 8, the best meter ever built. :ok:

Though the AVO 7 had a 'capacity' mode that was occasionaly useful.

Loose rivets
8th Jan 2018, 10:37
I love aluminium wiring.

I rip it out, strip the plastic off it and use it for dozens of DIY jobs. Very bendy Carry a reel in your car to fish car keys out of gullies and drains. Make things to show the kids. Then, get the plastic - which of course you've taken off without splitting - and use it as insulators or Systoflex.

Gosh, that name brings back memories.


I've got my old AVO 8 here, but the 'new' one is in Texas. Doubt I'll ever see that again. :{

Most times I use a DVM which I bought several of in Texas. $1.49, with battery. No, that's not a mistake. The display is clearer than my Fluke which cost more than a hundred times as much. Strange world.

8th Jan 2018, 10:56
Carry a reel in your car to fish car keys out of gullies and drains.

LR: Is this a habit of yours?