Switch is off but energy light flickers on dimly!

[kendor]

FWL you obviously have never come across induced voltages in floating cables from your statements above, read tricia's reply and what she says makes a lot of sense although i wouldn't condone the modification that is suggested as this alters the CFL from it's manufactured specifications.

 

[tricia]

Hi

Thanks Kendor for your input.

To [hopefully] conclude this matter, I did the following experiment which I believe
adequately explains my 91V issue and the energy saving light.

I took 3 metres of 1mm^2 twin and earth switch cable, and lay it on my floor.
I then connected on one end of it a light holder (ceiling rose type) and left it
empty (no bulb in it)
I then connected the other end of the cable as folllows.
The earth wire was connected to my house electrics earth and the
LIVE wire (red) to my house electrics LIVE wire. The black wire of this
cable was left disconnected at BOTH ends.

I then measured the p.d. beween the black and earth wire at the light holder
end and my meter displayed 29V. (It also read 242V between the red wire
and earth.) My meter is a Fluke 8020B, set to range 750V, A.C.

So, in 3m of this cable one can expect to *see* about 30Volts induced
in a free/floating disconnected [black] wire that is running parallel inside the cable
very closely next to the red wire which has a potential of 242V on it (not loaded)

So, for a longer switch cable, e.g. in my 2-way circuit, I conclude that 61V is
typical, and when both switches are switched off in the other direction, as two wires
then are at LIVE voltage, there is more indcued in the floating wire and one sees
91V as I did.

Of course, there would be negligible current flowing if one measured it between
the black floating wire and earth, as I did and saw 0.04 milliAmps. So this is not
a hazard, I would suggest.

I hope this info is helpful to others (and boy I wish I could have read this in a book
somewhere.)

Thanks again to all.

Tricia

 

[ban-all-sheds]

... saw 0.04 milliAmps. So this is not a hazard, I would suggest.

From memory that's less than what flows through you when you use a neon tester...

I hope this info is helpful to others (and boy I wish I could have read this in a book somewhere.)

Well I think we all knew that in theory you'd see an induced voltage, but I for one was very surprised to find that it could be so high in a relatively short, relatively straight run of cable, so thanks for doing the test. What I don't understand though is that there was no current flowing in the live conductor (other than maybe a few pA through the insulation to earth?), so how was there any magnetic field being generated?

Do you have access to a pico-ammeter and a gaussmeter, by any chance?

Another Q - what's the capacitance of the cable measured between L&E?

 

[tricia]

I hope this info is helpful to others (and boy I wish I could have read this in a book somewhere.)

Well I think we all knew that in theory you'd see an induced voltage, but I for one was very surprised to find that it could be so high in a relatively short, relatively straight run of cable, so thanks for doing the test.

My pleasure.

What I don't understand though is that there was no current flowing in the live conductor (other than maybe a few pA through the insulation to earth?),
so how was there any magnetic field being generated?

Yes, I pondered over this too. I cannot really add much here but the potential on
LIVE wire _is_ continually changing (sinusoidally) and so this must count for something.
Quite how I am not sure. Perhaps it is purely an electric field effect and not magnetic???

Do you have access to a pico-ammeter and a gaussmeter, by any chance?

No, I'm afraid not.

Another Q - what's the capacitance of the cable measured between L&E?

Again, I do not know and don't have the means to hand to find out. Sorry.

Thanks for you interest,

Tricia

 

[kendor]

Although the wire isn't connected into a circuit it is connected to a voltage potential that is continually rising and falling and a magnetic field will propogate around the wire and then collapse, on collapsing the field will cut the other wire inducing a voltage into it, in very simplified terms think of these open ended wires as aerials.

 

[ban-all-sheds]

I didn't think that you got a magnetic field unless there was current flowing - just having a voltage on a length of wire isn't enough.

Is it?

 

[flashbang]

This is a fascinating post to read.... I'm interested because my energy saver bulb, on my landing, with a two way switch, does exactly the same thing. The difference is, I don't care... I thought it was just an induced current from somewhere, I'm confident my electrics are safe. I have a smoke alarm approx 15" from the fitting, and suspected it was that...

My theory, and I have to admit to knowing very little... Is that as the voltage in the wire is AC, it must be building and collapsing 50 times a second. Therefore, no current flowing but enough changes of state to induce a current...
My only comparison, is with the coil of a car. When the primary coil is switched off, the secondary coil has to lose it's charge, which it does in the form of a bl***y great spark.. And lots and lots of volts...which were all induced from just a 12 volt source..

Just my two pennith..
Flashbang.

 

[ban-all-sheds]

Yes, but in that case a current flows through the primary winding, thus setting up a magnetic field. When it collapses you get the motion component needed to generate a voltage in the secondary coil. If you disconnect one end of the primary, you don't get any more sparks.

In the case of Tricia's experiment, "voltage in the wire building and collapsing 50 times a second" (100, actually) doesn't result in current flowing in the wire, so no field to grow and shrink 100 times a second.

Tricia - what do you see if you also leave the earth conductor floating (but still measure between neutral and installation earth)?

 

[kendor]

induced signals are a constant problem especially in the electronics field leaving wires unconnected can upset circuits and usually a grounding resistor will stop stray pickup. Ban think of one of those volt sense probes that will work even if there is no current flow in the circuit.