Eeyore wrote:

Sjouke Burry wrote:

Capacitive leakage from the HOT wire to the switch
wire?

So why doesn't this happen all the time ? Answer, the capacitance is very low,
as is the frequency.

Graham

A leak cap has very high impedance, and via
the input rectifier can charge the input cap(slowly).
Then when a threshold is passed, the circuit
produces a flash.
And it only happens, if the input capacity of the
rectifier circuit is low compared to the leak cap.