The time interval that must be allowed between the operation of two adjacent relays in order to achieve correct discrimination between them is called the grading margin. If a grading margin is not provided, or is insufficient, more than one relay will operate for a fault, leading to difficulties in determining the location of the fault and unnecessary loss of supply to some consumers. The grading margin depends on a number of factors:
1- the fault current interrupting time of the circuit breaker
2- relay timing errors
3- the overshoot time of the relay
4- CT errors
5- final margin on completion of operation
Factors (2) and (3) above depend to a certain extent on the relay technology used – an electromechanical relay, for instance, will have a larger overshoot time than a numerical relay.
Grading is initially carried out for the maximum fault level at the relaying point under consideration, but a check is also made that the required grading margin exists for all current levels between relay pick-up current and maximum fault level.