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I am doing some work on a ceiling light and I notice that, even though the light-switch is off, my voltage tester shows voltage present.

Answers to other questions (below) explain about phantom voltages but how exactly do I test for these and know what voltages are real and what are phantom so that I can work safely?


Phantom voltage questions

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Unexpected voltage

Just to recap, a "phantom voltage" is where a wire from a switch to a light or outlet runs next to a permanently live wire, when the switch is off, the AC voltage in the other wire can induce a voltage in the switched-off wire. However, there is very little energy transferred so the phantom voltage is usually not dangerous because it can produce almost no current. However it can cause confusion and confusion can cause dangerous mistakes.

Most voltage testers and most multimeters are not useful for telling the difference between real and phantom voltages.

In When doing electrical work, what do I use to check wires are safe? I have a long answer that covers the different types of tester and some of their advantages and disadvantages - however it is very long and doesn't show how to use a tester.

Here's how I used mine when replacing a ceiling light:

First, with the breaker on and the switch on, I test that my voltage-tester is working.

enter image description here

Good, it shows 230 volts as expected.

Now I turn the switch off and check the voltage again. At this point you'd expect no voltage.

enter image description here

But no, it is reading 50 volts! what does this mean?

As explained above, this is a phantom voltage. How can we be sure?

The tester has a button marked "test". This puts a lower resistance across the live connection and allows a small current to flow (only about 3 thousandths of an amp)

enter image description here

When pressing the test button, the 50V dissapears. That shows that this is a phantom voltage with no real power behind it.

I can now turn off the breaker, check my tester again elsewhere and proceed to work on the wiring in safety.


UK wiring

Some notes about the wiring and light fittings shown above.

Don't worry if your wire-colours, connectors and light look nothing like this. The same principles apply wherever you are in the world, whether your voltage is 120 or 230 and no matter what colours your wires are.

If you are interested, read on, otherwise ignore what follows.

What is shown is a Swedish light fitting in a UK home. It uses a "2D" fluorescent bulb - which are no longer very popular and might not be found in your part of the world. However it works like any light bulb in that it lights up when mains/live/hot voltage is applied to it.

A common UK practice is to use the light fitting as a junction box for wiring fromthe supply, to and from the light-switch and onward to the next light fitting.

enter image description here

The three red wires on the right are

  • live in from supply
  • live out to switch
  • live out to next light fitting

The black wire with the red-tape on it is a "switched live" coming back from the switch. This is what gets connected to one side of the light itself.

The two black wires are

  • neutral in from supply
  • neutral out to next light fitting

the green/yellow wires are three bare-copper ground wires which the electrician covered with a yellow-green sleeve.

  • ground from supply
  • ground to switch
  • ground to next light fitting

In some countries you wont have any ground wires.

In some countries or some homes you won't have all the other wiring coming to the light fitting, you'll just have one one live/hot wire and one neutral wire. The colours depend on country. In the UK the colours have changed for European harmonisation. These are the old colours.

In other parts of the world, different types of connector are used. In the USA you will find "wire-nuts" twisted onto groups of wires.

Note that the connector blocks used in the UK allow you to fairly easily test voltages without undoing the connectors. The plastic insulation protrudes far enough from the metal inside that it is safe to handle the connectors with your fingers with wires in place. Howver there is room to get either an insulated screwdriver or a voltage-tester probe into the connector and touching the clamp-screw.

  • WAGO lever nuts have some of the features of the UK connectors. I don't understand why these haven't swept twist on connectors from the market. wago.us/wago/press/press-releases/overview/detail-1122.jsp – Jim Stewart Mar 11 '17 at 13:42
  • @jim Stewart, I started using lever locks on light fixtures a while back for low current devices that may get new ballast or direct wire led retro fit I like them. I think all the problems we have had with back stabs is the reason they have not caught on the ones I use are listed for 30 amps but I only use them on light fixtures. Similar to the reason the other side of the pond hates wire nuts, they had bad experiences in the past so they think they are a problem the same as many electricians feel like back stabs are bad and lever locks look similar. – Ed Beal Feb 18 at 15:29
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You need to check leakage voltage in your building, Check proper earthing, it might resolve your issue, otherwise, check wire connection once again, Faulty MCB will also make issue, Check MCB (OLP), i found these kind of issues in Table grounding.

  • Hello, and welcome to Home Improvement. What's "MCB" and "OLP"? Would you edit your answer to clarify? Thanks. – Daniel Griscom Feb 18 at 11:33
  • This doesn't sound like an earthing problem at all.... – ThreePhaseEel Feb 18 at 17:22

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