Why does the use of CFL bulbs cause the wall switches to spark?

Answer 1

CFL's have tiny switching powersupplies in the base of the la(m)p to drive the tube. The first thing the switcher does is rectify the line to DC to power the rest of the circuit. Because the manufacturer is trying to cram a lot into a tiny lamp base, they sometimes leave out precharge and power factor correction circuits. Because of this cheap CFLs have the line feeding a bridge rectifier into a capacitor bank directy. This means that when the capacitors are empty, they will look like a short to the line until they charge (I = c dv/dt c is constant, dv/dt is a function of line voltage and the charge on the capacitor). This instant of high current draw is the same instant the contacts on the switch are closing, causing the spark.

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Inductance of Circuit can cause spark aswell.

In the CFL Circuit, is also an Auto-transformer of sorts, to transform the High Frequency, AC into a High Voltage to cause the tube to reach strike voltage, and then maintain a limit on the current, becasue as the ions in the tube collide and react with each other, they release electrons. As those electrons are converted into photons by the reaction, there is the replenishment of the electrons by the source ac current.

When an Inductor, has its current, the magnetic field wants to keep on going, but now, across the contacts is a high resistance, and the instantaneous impedance is several megohms, but with the trailing current of the tube still running, a slight charge is available to reflect back into the circuit.

[Similar to a spark coil in an automobile (pre electronic ignition)]

This causes a high-voltage to be impressed across the two contacts of the switch, and causes the contacts to pit, as the high voltage is dissipated into the pole (or ground mount) transformer secondary windings.

This can cause harmonics to be reflected back into the primary but the duration is very short. One or two cycles (16.6 - 33.2 mSec) or less.