Flashing LED Lamps on Two Way Switched Circuits

This is an occasional observation with both LED and CFL lighting on two-way switched circuits and is a frequent topic on IEE and electrician’s discussion forums. It is not a fault with the lamps.

This issue is normally a result of capacitive coupling between the live and switched live wires running between the two-way switches. The capacitive coupling causes a build-up of charge within the lamp which is then released when it reaches a certain level causing the lamp to flash. The cycle then repeats. This is a safe if not ideal situation created by historically common wiring methods which enable capacitive coupling. The effect has not been apparent with filament lighting as the charge can dissipate without any noticeable illumination of the lamp but is highlighted by the sensitivity of modern low energy lamps.

The phenomenon does not affect all two-way switched circuits and is often reduced or non-existent when there is a connected earth conductor running between the strapper wires, or the wires are widely separated.

If there is more than one lamp on the circuit the diagnosis can be confirmed by replacing one of the lamps with a filament lamp as the additional resistive load will prevent the LED lamp from building-up a charge. In addition, with the switches off, there will probably be a detectable voltage on the switched live wire to the lamp in one of the off configurations.

There are two approaches to solving the problem:

1) A suppressor (or snubber) can be added to the circuit between the switched live and neutral in parallel with the lamps and as close to the last lamp as possible, possibly within one of the fitting itself or the ceiling rose. This a cheap solution and a suppressor such as this can be used:http://www.maplin.co.uk/contact-suppressor-498

2]A resistive load which mimics the action of the halogen lamp as mentioned above can be added in parallel with the lamps but this is a more expensive solution.