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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Fault? CFL light flashes when switched off https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1279664438 Message started by Gavona on Jul 20^th, 2010, 11:20pm

Title: Fault? CFL light flashes when switched off
Post by Gavona on Jul 20^th, 2010, 11:20pm

I had several new circuits and a new consumer unit installed and tested as part of an extension project last year. I have two problems, possibly/probably unconnected.

1. I have one CFL light which when switched off is observed to flash slightly (more of a glow than full on) and regularly. Till happens if I change the bulb. Does this indicate a wiring fault? The light is in the hall and is subject to two way switching. Could such a fault have escaped the testing? In case it helps, one of the banks of switches that controls it also controls lights on other circuits.

2. I am swapping some outlets and switches in parts of the house where new circuits were not installed for newer ones. I switch of the MCB for the relevant circuit and my multimeter records no voltage between earth and live or neutral. However if I happen to touch some of the wires together the RCD trips. Is this anything to be concerned about? Never come across it before but as MCBs are single pole I can understand that this could happen. Am I working safely merely to disconnect the circuit using the MCB?

Thanks for your advice. (I am no longer in contact with the original electrician so can't ask him.

Title: Re: Fault? CFL light flashes when switched off
Post by Lectrician on Jul 21^st, 2010, 5:27pm

Both your questions are not issues.

A CFL lamp required very little voltage to cause it to flicker. Induced volts can be enough to cause this to happen. Induced voltage is more predominant in two way switching due to the longer lengths of cable and the way the cables sit parallel with each other in the same direction (if that makes sence).

An RCD can trip when neutral and earth are shorted together even with an MCB off. This is due to load from the other protected circuits being able to flow from the neutral to the earth when you short it together.

Title: Re: Fault? CFL light flashes when switched off
Post by Gavona on Jul 21^st, 2010, 8:23pm

Thanks that's reassuring. :)

Title: Re: Fault? CFL light flashes when switched off
Post by CWatters on Jul 22^nd, 2010, 7:47am

I did wonder if the low level flickering could effect the life of CFL bulbs. Would be interesting to know how you get on.

Title: Re: Fault? CFL light flashes when switched off
Post by Gavona on Jul 23^rd, 2010, 10:15pm

Quote:

I did wonder if the low level flickering could effect the life of CFL bulbs.

Hmm... I'll try to remember to come back if I ever draw a conclusion on that. I've been swapping the bulb to try to eliminate a bulb fault so don't really have anything to compare with.... and I don't know how consisent cheaper bulbs are (are they cheaper or merely subsidised?)

As I recall swithcing lamps on annd off shortens the life. Here's hoping that this one will last really long time 'cos its never fully off!!!

Quote:

Induced voltage is more predominant in two way switching due to the longer lengths of cable and the way the cables sit parallel with each other in the same direction (if that makes sense)

I have been thinking about that. Intellectually I would like to understand. I thought that to induce a current then there must be a current flowing in the other wire. Is that right? I notice the problem at night when the only thing that I can think of that is drawing a current is my smoke detector (very close to the light and the wire probably follows the same path). Indeed the smoke detector (one of several linked ones) was installed with the extension and I never had the problem before so I guess I could consider that as a possible cause but surely the currents would not be significant enough to cause that induction effect?
What voltage is needed to make a cfl flicker?

Looking at it another way.. Do you experts see this issue reguarly?

Best regards,
Gavin

Looking at it another way, do you guys
In any event I can't do anything about it. Indeed

Title: Re: Fault? CFL light flashes when switched off
Post by Gavona on Jul 23^rd, 2010, 10:54pm

OK realised I should at least try to research the issue:

http://forums.whirlpool.net.au/forum-replies-archive.cfm/990109.html
Quote:

I've heard of CFLs dimly flashing (only visible in the dark) which was caused not by induction, but rather capacitance in a long run of twin-core wire, which combined with the CFL's rectifier would allow the voltage to rise until it fired the bulb, discharging the stray capacitance and then repeating, causing a continuous series of flashes.

http://forums.whirlpool.net.au/forum-replies-archive.cfm/920047.html
Quote:

This has been discussed a few times. It all comes down to a capacitor in the CFL. This capacitor accumulates all the tiny induced voltages in the wiring, and over time it charges up enough to trigger the the light, but is then instantly discharged, so it starts the cycle again. The flash is usually pretty weak, and can only be observed when looking at the light in a dark room. This is generally more of an issue with 2-way switched lights, but the same thing can happen with single switch wired lights.

Title: Re: Fault? CFL light flashes when switched off
Post by CWatters on Jul 24^th, 2010, 9:44am

Here is an example of two way switching...

The two common wires are frequently run in the same cable so they are close together. The longer the run and the closer they are together the higher the capacitance between them.

The mains is AC and capacitors "pass" AC. The exact amount of current flowing depends many factors including the frequency and size of the capacitor and other elements in the circuit such as the light. If the mains was DC it wouldn't be an issue as capacitors "block" DC.

Capacitance can cause other interesting effects... Consider what happens if an earth wire became disconnected at the consumer unit. You would be left with a long length of copper wire parallel with a live wire in the ring main. The capacitance between the live wire and the "floating" earth wire and can cause 50V or more to appear on all "earthed" metalwork in the house. So if someone complains of mild shocks from a cooker or other metalwork it might not be a faulty live but a loose earth connection somewhere. Dangerous either way.

Title: Re: Fault? CFL light flashes when switched off
Post by Gavona on Jul 24^th, 2010, 10:32pm

Thanks CWatters - really clear. Capacitance rather than inductance gave me the clue as to why and then your post saved me thinking through how capacitance might do that. The 50v earth example helps to scale.

Thanks for taking the time. :)