bulbelectricalkitchensSecurity
I have a energy saving spiral light bulb in my kitchen. In exactly every 27 seconds, the first segment of the bulb faintly flashes when the switch is off. And not once, repeatedly every 27 seconds. This weird situation occurs for more than half and a year. What can be causing these flashes? How can I prevent it if it spends energy or has safety risks?
PS: A new spiral bulb does the same thing. And the bulb doesn't flash when I remove from socket.
TL;DR: the dimmers aren't switching off completely: they're allowing some current to leak through, which is why you're seeing a voltage across the CFL. A different make of bulb may behave better with the leakage current that you're getting. Or perhaps a different brand of fan (if you haven't installed them all already).
I do know that operating CFLs in those sort of conditions will shorten their lives considerably, so you might actually be cheaper for you to use incandescents instead (a quick calculation says about 12 kWh per year for a 60 W bulb).
Read on for the technical explanation...
The voltage across the bulbs, Vb is determined by the formula:
Vb = Vin * Rbulb / (Rdimmer + Rbulb)
where:
The dimmer is a solid-state electronic circuit, so it has a very high effective resistance -- 10s of megohms is not unreasonable. Ditto for the control circuitry in the CFL. An incandescent bulb is a simple piece of resistive wire; a 60 W / 120 V bulb will have a resistance of 240 ohms.
Now, suppose the dimmer has a resistance of 50 MOhms and the CFL has a resistance of 10 MOhms; plugging the numbers into the equation above gives you 20 V across the bulb. OTOH, the voltage across a 60 W incandescent bulb will be about 600 microVolts, nowhere near enough to make the bulb glow.
If you have two bulbs in the light fixture, the resistance, R, of the two in parallel is given by:
R = R1*R2/(R1 + R2)
So if you have a CFL and an incandescent installed, the effective resistance is going to be very close to that of the incandescent alone:
R = 10,000,000 * 240 / (10,000,000 + 240) = 239.99 Ohms
Again, not enough to turn on either bulb.
With two incandescent bulbs, the effective resistance is half that of a single incandescent, so you have half the voltage across them.
The flickering you see with two CFLs is because the light you see is basically a high-voltage spark through the tube. The CFL contains circuitry to amplify the incoming voltage up to the point where the spark can occur. Under normal circumstances, the input voltage is enough to cause this spark 100 or 120 times per second (depending on mains frequency), which is far too frequent for the human eye to notice. With the reduced input voltage, it takes longer to reach the required voltage, so you notice the flicker. No two bulbs will be exactly identical, so they'll flicker at different rates and take different times to recover between discharges.
The red wire that goes to the second light (that is bundled with the rest of the reds at the first light), needs to be connected to the black with red sleeve wire. Since it's currently bundled with the rest of the red (hot) conductors, the light is always getting power. The black wire with a red sleeve is the switched hot lead, and only has power when the switch is on.
Best Answer
For this to happen there must be some voltage to the light socket. It may not be noticeable with a standard incandescent bulb because it apparently is not the full voltage and current. If you are comfortable in what you are doing, you could measure the voltage across the two wires and ground to see what you get. Incorrect wiring could be the problem, such as an improper shared neutral leg, or the neutral being switch instead of the hot leg. This can be found using a voltmeter and referencing to a known ground. If your not sure, leave it to an electrician. Wiring is not a hobby, and can be lethal!