Excess-current protection by over-current relays on medium-voltage circuits

The paper reviews existing methods of protection from excess load current in medium-voltage circuits, and outlines problems encountered with the wide variation of industrial applications. The need is stressed for full co-operation between engineers, and also between engineers and the user, in the control-gear, motor and distribution fields. B.S. Specifications dealing with excess current are considered, suggestions are made to correlate them, and a review is made of the main factors which determine the protection required. Motor starting loads, starting times, and various duty cycles, are mentioned, coupled with faults which can be caused by the inherent characteristics of some electrical equipment. Existing basic designs of overload relays, and some of the difficulties experienced with them, are dealt with. The effect of temperature variation on thermal and oil devices is outlined; also excess current due to lost-phase faults, with suggestions for the type of protection required to meet various circuit conditions. General design points are reviewed in the light of past experience and recent developments. The relation of equipment to both excess load-current capacity and short-circuit rupturing capacity is considered; also the correlation of fuses and overload relay design with the thermal rating of electrical gear in the protected circuit.