Where environmental conditions are harsh, it is often desirable to use an 110V system. To reduce the risk of shock to earth the centre tapping of the source transformer or generator is connected to earth so that the voltage measured to earth is about 55V. As most shocks are to earth this will provide adequate shock protection in the majority of cases. The use of such a system requires double pole switches and fusing in both live poles of the supply. The system is particularly useful for portable equipment.