MIMS cable is invariably made from a Copper Sheath with a Chalk inner holding 2 or more wires in suspension.
The wires themselves, having a 200 degree C+ rating.
They are used in the Non-US Areas for providing circuits in Boiler-Rooms and other High Temperature areas.
Biggest thing that anyone that works with MIMS, will tell you, is moisture!.
Allow a wee bit of moisture into a MIMS cable and see the Insulation Resistance to drop like a stone!.