The requirements of the power distribution industry are becoming more demanding in terms of the quality of supply, and the increase in loads in both rural and urban situations places ever increasing needs on the ability to provide automatic protection for load centres.

However, isolated load centres up to, say 1 or 2MVA may not justify the use of circuit breakers, and yet be beyond the performance levels of the basic drop-out fuse/isolator combination because of the inability to trip all 3 phases or switch off "on-load".

Engineers associated with cable networks have had the benefit of this facility for fifty or more years in terms of ring main units and similar devices, but now the option is available for overhead reticulation engineers to make significant improvements in network performance.