Carbon fibre conducts electricity really well.

Are there any types of polymers that can withstand temp of around 850C?
I thought carbon fiber was a good conductor, but wasn't quite sure.  "carbon" is a good hint

>Are there any types of polymers that can withstand temp of around 850C?

No, not even close.

http://www.starfiresystems.com/products/matrix_polymer.cfm

".......I'm researching alternative materials (to ceramic) for  high voltage electrical insulation.........."
I believe mica and composites made with it are still used for high voltage electrical insulation.
Pure mica has unrivalled electrical insulation properties and temperature withstand above 850 deg. C.
There is a company in UK called Arlex that made a tough composite from fusing glass and mica. Used for arc chutes and stuff. Might be worth trying to contact for information as well as others like them.
Why do you want to look at alternatives to ceramic insulators?

Thanks for the information.  Ceramic supply is one reason I'm researching alternative materials.

Fiberglass booms have been used in aerial device applications for years. The problem is not the insulation qualities of the glass but other considerations. ANSI has extensive testing procedures on fiberglass booms.

When using any materials in high voltage applications air conductivity is one of the major problems. 100,000 volts requires a 3 foot gap of non conductive material. As voltages increase air gaps must in increase. Other considerations is non contiguous water streams when exposed to rain. No air voids in the glass for condensation to collect in the boom. Soot from engine exhaust cannot be allowed to collect on the inside of the boom. Hydraulic lines must always have a positive pressure meaning no vacuum as a vacuum will conduct.

Cross phase accidents have occurred because of the metal shafts used to attach the platforms were too close to a conductor on the side away from the operator and the operator working on the the other side of the shaft working on another conductor.