OBTAINING ADEQUATE SENSITIVITY
Apart from making sure that associated equipment is suitable for the application, the
principal step in the application procedure is to determine if the available adjustments of
the relaying equipment are such that the necessary sensitivity and speed are assured.
Manufacturers bulletins describe how to do this when one knows the maximum and
minimum fault-current magnitudes for phase and ground faults at either end of the line.
It is advisable not to adjust the equipment to have much greater sensitivity than is required
or else, in so doing, the CTs may be burdened excessively. With excessive burden, the overall
sensitivity may be poorer, as illustrated by Problem 2 of Chapter 7.
If the phase-fault currents are high enough to permit it, it is advisable to adjust the phasefault
pickup to be at least 25% higher than maximum load current. Then, the equipment
will not trip its breakers undesirably on load current should the pilot wires become opencircuited
or short-circuited. Undesirable tripping could still occur for an external fault,
unless supervising equipment is used to forestall such tripping.