Abstract
This chapter reviews some of the fundamental points for the design engineer to watch for in planning the application of IDMTL OCEF protection to medium-voltage switchboards and networks. It also explains the protection design parameters on medium- and low-voltage networks. There are numerous installations where the performance of the current transformers has been overlooked or misunderstood. The most widely mistaken error has undoubtedly been the choice of CT ratio, which is invariably selected on the basis of full-load current without due regard to performance under fault conditions. The voltage developed across the CT secondary shall be sufficient to drive the transformed current through its own internal impedance plus the impedance of the relay and any other equipment to which it is connected. A number of instances arise where the load current demands high-ratio line current transformers, and the neutral current has been limited to a low value by the use of a remote high-impedance neutral earthing device for safety reasons.
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