120 volts at light

[stedon1]

I was fitting a new light in my downstairs hallway. Using a voltage/continuity tester I was getting 240 volts between permanent live and neutral and 240 volts between permanent live and earth. When I went between switched live and neutral and switched live and earth I was only getting 120 volts. Its 2 way switching and one of the switches is a dimmer, could this be the problem?

 

[rebuke]

Firstly, I assume you'd turned the dimmer up to full before testing?

I'm not familiar with how domestic dimmer switches work (only theatrical lighting dimmers), so I don't know whether you'd see a lower voltage regardless or not.

Other possibilities are that the switch was actually off, and the 120V was a phantom voltage, that if you actually put a load on it would vanish, or that there are two fittings that have somehow been wired in series rather than parallel, so you're getting half the expected voltage...

 

[stedon1]

Yeah the dimmer was up and the switch was definately on. This particular light is the only one the two switches control. Does it sound like it could be dangerous due to some underlying fault?

 

[Spark123]

Replace the dimmer with a normal switch for testing - dimmers need a certain amount of current running through them to power the electronics to run the triac. Without this current anything can happen!
Also worth noting that if you are using a high impedance volt meter you can get odd readings caused by capacitive coupling of the cables. Inserting a lamp into the holder is enough to eliminate this effect.

 

[TicklyT]

Dimmers and other electronic controls may give unpredictable test results unless the correct load is connected to them.

The usual load connected to a dimmer is a lamp.

A voltmeter has a much higher impedance than a lamp, so that may adversely affect the test readings.