I've got three tools and I've been using them to test my lamppendants. I've got the faithful neon screwdriver, a digital multimeter (set to500V) and a 'digital circuit tester' from Blackspur which I bought fora quid. This last tool has two functions: (a) it shows if theterminal in contact is live; (b) what the voltage is. It has areading of 12-36-55-110-220v and illuminates the value below what theactual voltage is. It works by the user holding the non-contact endto complete the circuit a la the neon screwdriver.When the pendant is switched off, the phase terminal shows (in orderof the tool above):1. LED lit quite brightly2. (null reading)3. between 12V and 36v (36v is slightly illuminated).When the pendant is switched on, the phase terminal shows (in order ofthe tool above):1. LED lit very brightly2. 246-250v (depending upon pendant)3. 220V.This situation is present on each pendant I have tested. None of thependants appear to faulty in day-to-day use.I'm a little intrigued that the two cheaper tools are showing the thephase terminal is live when there should be no current. My realconcern is that these tools infer there is current flowing to theterminal with the pendant is swicthed off and that there's apossibility of getting electrocuted when changing a bulb.Are all my pendants wired incorrectly?Thanks

What you're seeing is inductive coupling between the 'live' wire and theswitched one running next to it. Put the bulb back in and you'll see zero,as the switched line will be connected to neutral via the much lowerimpedance of the bulb.Things like neon screwdrivers - and DVMs - have a very high inputimpedance so will read very low currents that the body wouldn't feel.It's a good idea to use a low value shunt which draws about 0.1 amp whenusing a high impedance test device on mains - or alternately make up atest lamp using a small mains bulb.

should unswitched live should as live on a neon screwdriver?

Nov 15, 2003 22 Replies

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Zymurgy 22 years ago

"Andy Wade" wrote

Well, we'll know soon enough, as i've X posted a mail over to sci.electronics for the definitive answer !!!!

HTH

Paul.

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Andy Wade 22 years ago

I've given you a definitive answer.

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Zymurgy 22 years ago

"Andy Wade" wrote

Hardly, if you consider a two word answer definitive !

But from what i've read so far, I concede, you're right ;-)

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