confused

hello,
im more of a theatre electrican. so i know a little, im having a problem with my lights at home. meatering across the rose i get 240v when the light is on. when the light is off i meter 120v ??????? can someone please diagnose this for me. i am so confused

cheers david wimpenny

Does the light stay dim when off?

What sort of meter are you using? DVM or moving coil?

If it's a DVM, double-check that you are reading 120V and not 120mV.

Are you testing between phase-earth or phase-neutral ?
What other lights do have on the same circuit ?

hello

im using a dvm meter set for 120V the lights go off but energy saving ones flicker.
theres a couple of lights on the same circuit that do the same but not all.
and im metering between the two outer connections in the ceiling rose

hope i've given you the info you need david wimpenny

I had a look at a lighting spur with a fluke DVM. I get 240VAC when on, 6VAC when the lightswitch is off but MCB and main-isolator on, 4VAC when the MCB is off but the main-isolator is on, and 0V when the main isolator is off... what's that all about then?

I dismantled an "energy-saver" bulb a few days ago. There is quite a complex power-supply circuit in them which includes a capacitor. This *could* be the source of your 120V reading if a bulb has gone faulty.

try metering again with all bulbs in your installation removed & post your findings.

Swap your low energy bulb for a standard filament bulb and your meter will show zero when it's off (give or take a few millivolts). So what's going on?

With the switch off you still have a connection from your bulb to live. It's called cable capacitance and it forms a potential divider with the bulb resistance. Now a cold filament has a very small resistance, about thirteen times smaller than ohms law predicts, so very little voltage appears across it. A low energy bulb is different. It has a very high resistance until you exceed its striking voltage. This means that a substantial voltage can appear across it, enough to make it flicker in your case. The longer the switch wires the greater the capacitance and the greater the voltage at the bulb. If it's big enough the bulb will try to strike, hence the flickering.

I had a rather ominous thought last night. I was puzzled as to why I'd never had any problems with flickering low energy bulbs, even though some of them have long switch wires. Then it struck me. The capacitive coupling across a flat twin and earth cable is very small because the earth wire is in the way - UNLESS IT'S NOT CONNECTED. I think you should check yours ASAP!

I'd like to add to the comment by Felix, this is absolutely true, as far as it goes, except for 2 way switch circuits where depending on the internals of the three core and earth, the capacitance to ground can be ~half that of the capacitance between the two cores that are both on the same side of the earth.
The attack of the flickers on a properly earthed house are then usually only seen on the landing light circuit where the 10m of three core plus earth is about a nanofarad to live, and 500pF to deck..
Incidentally this is a high tec version of the neon lamp relaxation oscillator 'neon timebase' of days of old.

The easy fix is to fit a mains rated polypropylene motor suppression capacitor (I'd use a class X type at least) of 100nF or so across the lamp holder, creating a loss-less potential divider with the leakage capacitance. One could of course use a normal lamp or resistor, but it rather defeats the point of the low energy lamp.

regards M.

Felix, can you post/link a diagram that indicates this capacitive link idea? I can't seem to get my head around it.
cheers

Some more words on capacitance and discharge lamp flicker. I have a drawing at home, and if you email me (see profile) I can send you a copy.

This is a relaxation oscillation, similar to the old neon timebase, (search google for neon timebase gets you this ..http://www.vego.nl/4/n/n_029.htm) but basically the voltage rises when the lamp is off, as the impedance is many megohms. Once threshold is reached the gas breaks down and the lamp lights, but the source impedance is far too high to sustain action for more than a brief flicker, so the volts on the capacitor plummet and the light goes out, and back to high impedance state. Capacitor starts to fill again....

Now leaping forward from 1931 to the present day, and AC mains, the high impedance supply is the capicitance of the wiring, 100p/metre for 1mm T+E, so ~30 jMeg ohms for every metre, so say 10m, say 3meg.. say 100uA of charging current, and the old neon is now replaced by a rectifier and oscillator circuit of the compact flourescent lamp (CFL). The reservoir capacitance downstream of the bridge in the lamp will need to reach say 50V to start, and will be perhaps 100uF, so flicker rate of half a second, say. result irritating flicker, worse when tube is warm as strike voltage is lower.

If you set this up on the bench with nothing other than an open ended length of cable (as if to go to a switch , but don't fit one) and the CFL, by varying the length of the cable, and therfore the charging current, you can alter the flicker rate.
If it is 10m or so of wire in the switch circuit, then a permanent fix is either to place low current resistive load (perhaps 100K or so - best made up of two resistors in series, to prevent a single failure), or a nanofarad or 10 of suitibly mains rated capacitor in parallel with affected device, to pull the voltage accross the lamp firmly down when off. If you prefer ready made componenents you might find a switch suppressor accross the lamp (not the switch -that would increase the leakage) is all you need. I'd suggest building these into the lumiaire if there is room, or a proper joint box, as dangling live components in the breeze will attract the wrath of the rightous (though its OK inside washing machines and TVs..).
I suppose if you were feeling cheapskate then the parallell capacitance could just be an extra long cable feeding the lamp.
Rest assured this is a well known problem, and need not indicate wiring fault. (though in switched neutral, rather than switched live, the effect can be much worse, as now the total capacitance to earth adds to rather than reduces the leakage current.)

If this makes no sense, come back to me
Mike P_J

Felix's comments as to the reason for the oddity is correct, the 120V is the result of Compact fluorescent lamps in the circuit.

If any circuit is to be tested, then you should remove all such lamps prior to testing.

The voltage has nothing to do with the capacitive inductance of the cable, it is all to do with the voltage output of the DVM and the reaction to this of the electronic control gear in the CF lamp. The charge is induced across the lamp by the capacitors in the control gear charging up and discharging to the cathode at the end of the lamp, this spike of charge induces electrons to flow from the strontium or similar metal coating of the cathode, and this is the cause of the flickering that is observed. However this creates a slightly bigger charge due electron inductance across the lamp, this ends up being the 120V that you read on the circuit.

It should be remembered that no circuit with CF lamps on should ever be subjected to an Insulation Resistance meter set above 230V, you will destroy the control gear, and you could cause the lamp to implode, thus causing a small explosion (about 10 times the violence of droping the lamp). Youi should never meggar these things out anyway!!

it is all to do with the voltage output of the DVM

Click to expand...

What...? please elaborate! My fluke DVM doesn't show a voltage on the probes when I'm measuring V-AC. Or did I understand this wrong?