Gas-insulated switchgear is very compact. It requires only one-sixth of the area needed for air-insulated switchgear. ABB Switchgear in Ludvika, Sweden has developed a new generation of 145 kilovolt gas-insulated switchgear. The development work involved the task of assessing new materials, including disconnector contacts which needed a new type of insulating rod. The earlier rods sometimes failed, partly due to their brittleness. So a strong but more resilient material was needed.

Wirsbo-inPEX proved to offer the right solution, due to its insulating properties and also its strength. "But the material posed some difficulties in the early stages," says Kerstin Skogh, a Development Engineer who worked on the new gas-insulated switchgear. 
"We had to find out how the material behaved, but once we did that, the rest was plain sailing," she said. Glass-fibre reinforced materials and other plastics were tried for the rods, but bonding would then have been necessary. So Wirsbo-inPEX with its thermal memory was better from the safety aspect. The pipes were secured by allowing them to expand into a cast aluminium plate. A silver-plated copper contact was secured to the other side of each pipe, resulting in a fairly heavy arrangement. The disconnector enclosure contains three pairs of contacts - one for each phase. The loading is high, and the contacts must be capable of withstanding several thousand make and break operations 10 000 times initially, during mechanical testing!

How many Thermobites does PEX contain? 

The material derives its thermal memory from the cross-linking of the polyethylene, which is achieved by subjecting the material to pressure and temperature in the production process. The internal structure of the material remembers the shape it was given in production. In this case, the pipe is forced into a new shape before cooling. The user then fits the pipe into the relevant equipment and heats it until the tube becomes transparent (133C). As a result of cross-linking, the heat causes the pipe to resume its original shape. In the ABB Switchgear application, the pipes are fitted into the cast aluminium plate, and the contacts are fitted to the pipe ends. The entire assembly is then heated, the pipes expand, and all of the parts are firmly locked in position. The pipe exerts many thermobites in the process. 'megathermobites' perhaps.