Which is better epoxy or Mylar insulation ?

For Low Voltage up to 600 Volt, Which is better for bus duct bus bars ( Busway ) insulation Epoxy or polyster film / Mylar? & why?

Note that, Specs in Gulf area @ ambient temperature is 50 Deg C.

GE, Siemens & Cuttler Hammer are using epoxy but most other manufacturers are using double polyester films ( Mylar ) due to its simple know how.

Our working area ; Saudi Arabia, Jeddah