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