Suggestions Needed: Thermal switch on live MV heatsink

[AG-PPI]

I am currently working on putting together a design draft for an MV inverse-parallel thyristor controller assembly.
The assembly itself would be running at 7920Vac nominal, 50Hz 90A.
The thyristors we will be using are 'hockey puck' style, single side cooled on a suitable aluminium heatsink.
Our customer has a requirement for temperature sensing on the heatsink via a thermal switch (either N/O or N/C).
I have some ideas on how to isolate the standard run-of-the-mill bi-metallic thermal switches, but they are not ideal.

I need some suggestions on how else to approach this. Ideally I want something that can mount direct to the heatsink.
I would like to know if anyone out there is aware of any kind of temperature sensing device that would be easy to mount on a heatsink whilst being electrically isolated from almost 8kV?
I just need to know if I should cross off the idea of direct mounting on the heatsink rather than using some expensive isolation materials.
If there is some other kind of device that would work but does not operate like a bi-metallic switch, then please suggest that too as we may be able to work with that.

 

[LionelHutz]

Float it to the same circuit as the gate drive for the SCR cathode connected to that heatsink then convert to an optical signal to send back to the controller.

 

[EdStainless]

Our big power supplies used bimetallic switches that were electrically isolated.
Though I doubt that they would handle your voltages.
Not that it really matters since you are only looking for the circuit to open.