Flashing Light bulb when switchs are off...

[generalbosh]

I consider myself a fairly experienced electrician for my age (22), qualified to 2330, nvq3, am2, 2391 (with a more industrial background) but my domestic fault diagnosis is fairly limited so would appreciate any comments/help on the following situation.

I've come across a single pendant fitting fitted with a energy efficiant bulb that flashes intermitantly when switched off. The circuit is wired as follows:

Pendant is top of stairs & switched from 3 locations (2 way at top & bottom of stairs & an intermediate at one end of the landing). Normal loop in/out arrangement in the rose. No junctions/alterations evident.

I have changed all switches as a matter of course (as they're around 15/20 years old) as a potential source of the problem. I've carried out insulation resistance testing across all conductors in the circuit which all confirm good readings.

When checking the voltage present to the switch return in the ceiling rose when switches are off (in every possible comination) a reading of around 30 volts is gained. I guess this 30 volts is enough to allow the energy efficient bulb to flash now & again.

Can anybody shed any light on the subject? (bad gag) any suggestions of further testing i can carry out?

Many thanks

 

[Guest123]

Ah yes, the old induced voltage in the strappers issue.

There are a couple of threads on here regarding this issue, I'll see if I can find them.

 

[Guest123]

http://www.electriciansforums.net/electrical-forum-general-electrical-forum/15152-landing-light.html


http://www.electriciansforums.net/e...when-i-turn-off-switch.html?highlight=snubber


Theres a couple.

 

[generalbosh]

Thanks ... first post, did trawl through a few threads & coudln't find anything directly relevant

 

[nickblake]

Have you checked the polarity if you have incorrect polarity you could well get flickering lights

 

[generalbosh]

yes polarity was checked ealry on when i found this 'induced voltage'.

But thanks for the heads up.

I think after reading through lennys suggested threads i'll try a different buld first. It seems odd though that for 20 years this has never been a problem, i can only assume the customer had been using old GLS lamps in the pendant untill now & the effect was either less or non existant with GLS lamps ...

 

[Guest123]

Correct, there is no such problem with the humble GLS lamp.

 

[MarkieSparkie]

This phenomenon is caused by the final circuit cable capacitance. It is more common in conventionally wired two way and intermediate switching circuits due to the wide spread use of 3+E cable and the extra parallel capacitance that creates.
The CFL lamp is essentially a fluorescent tube driven by a high frequency oscillator powered by a mains rectifier and smoothing capacitor.
Even though the live feed to the CLF is switched OFF the cable capacitance allows leakage current to flow to the CLF slowly charging the smoothing capacitor in the CLF until the potential is sufficient for the oscillator run and "fire" the CLF tube. This discharges the smoothing capacitor, the oscillator stops, the tube extinguishes and the process of charging starts again ... if conditions are suitable this continues indefinitely.
The problem may be temporary however, only occurring shortly after switch OFF, when the gas in the tube is warm and more easily ignited. As the gas cools ignition become increasingly difficult, the energy from the oscillator become insufficient to heat the filaments and ignite the gas in the tube before the smoothing capacitor becomes discharged.
A simple solution is to permanently discharge the cable capacitance by wiring two 160K ohm, 1/4W, resistors rated at >=250V, in series across the lamp holder terminals. Do not try to use a single 330K ohm resistor because this may fail due to voltage breakdown.
If this solution is implemented a notice should be placed at the DB stating which final circuits are effected and which lamp holders have been modified and how.
A conventional GLS filament lamp doesn't suffer this problem because the filament is permanently loading the cable capacitance and any acquired charge is unable to overcome the thermal inertia.