Mains isolation with specialty transformer

[crutschow]

Here's my simulation of asdf44's circuit using a second TL431 (U3) as the clamp zener, set for 10V nominal.
The trigger level of U4 is set for about 288Vpk (206Vrms) so it gives output pulses for the 240Vrms input and no pulses for a 200Vrms input.

 

[crutschow]

Note that the U4 voltage setting above is arbitrary and can be set to any brownout voltage desired, by changing the value of R4.

The U4 trip point is where its control input terminal becomes slightly greater than 2.5V.