The design and performance of high- and low-voltage terminal boxes

Abstract

The failure of terminal boxes in service has led to new developments in two main respects. Firstly, connection arrangements are designed to be through-fault-proof and the possibility of a dielectric failure is minimized by using fully taped connection arrangements in a clean, dry enclosure. Secondly, as faults give rise to very dangerous conditions, where equipment is circuit-breaker protected and the supply has a large short-circuit capacity, excessive pressure build-up under short-circuit conditions is prevented by a suitable pressure-relief vent.Comprehensive test results under short-circuit conditions are described and a basis of calculation is presented. The alternative method of electrical protection by h.r.c. fuses is also considered, in conjunction with test results and appropriate theoretical predictions. The more generalized behaviour of arcs in enclosures and the effect of the surrounding medium, which is important in connection with the alternative methods of termination, e.g. under compound, and for oil-cooled equipment generally, is discussed briefly in terms of a limited special investigation. It is considered that the proposed designs ensure an absolute minimum likelihood of failures, without incurring any disadvantageous complications and at the same time ensuring the safety of personnel, if a fault should occur for any reason.