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Electrical Riddle No.57 - Flicker of fluorescent lighting
I was replacing the switch (1-way) controlling the fluorescent lighting in my garage.  The lamps were initially off from the switch but when I switched the circuit off at the MCB to isolate the circuit, the lamps came on albeit dimly.

There is no voltage indicated at the wires in the switch assembly so I was able to replace the switch safely regardless.  I just moved into this property.

How can this be?

The wiring is bunched up with lots of other circuits around the garage upper spaces given the distribution board is located there.  Could this be a case of electromagnetic induction?  Can someone expand on this phenomenon?  What is the complete circuit to allow the lamps to be lit?

Any other explanation?

Author : Mike - From: UK
Sat, February 9th, 2013 - 18:23
As you know the fluorescent bulb contains a gas that produces invisible ultraviolet light (UV) when the gas is excited by electricity. The UV light hits the white coating inside the fluorescent bulb and the coating changes it into light you can see. Because fluorescent bulbs don't use heat to create light, they are far more energy-efficient than regular incandescent bulbs, therefore the bulb could still be burning residual gas inside the bulb with a little stray current, hence the flickering.
The stray current can be created due to tuning of inductance/resistance of circuit (ballast and bulb electrical discharge path) with variable stray capacitance. Stray capacitance amount is very complex and varied by different switching in circuit.

Author : Hamid - From: Iran
Wed, February 13th, 2013 - 20:47
Thank you Hamid.  Does that mean left alone for a long enough time, the induced voltage will decay to zero according to the characteristics of the equivalent circuit?   
Author : Mike - From: UK
Sat, February 16th, 2013 - 07:51
No, it is not induced voltage and it doesn't decay; because we face to an AC system. 
Author : Hamid - From: Iran
Thu, November 5th, 2015 - 12:05
Indeed bulb remaining thermal energy decay and be cold after several flicking when main switch turn to off.

Conductive current in bulb flickering condition is unstable and could not be continue, because it doesn't have enough energy for remaining. When the through conductive little current  reach to zero, stray current via stray capacitor, increase recovery voltage on bulb again and ignition repeat interval several time.
The decay additional resistor to circuit (red resistor in above figure) could be a solution for prevention of bulb flickering after switch off.

Author : Hamid - From: Iran
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