Ground fault protection coordination

Since the ground fault current changes from several mA to thousands A, the selective protection coordination shall be examined in the remarkably wide range. When a selective breaking system is used, coordination cannot be achieved only based on the difference in rated current sensitivity. Fig. 9.23 shows the operating characteristics of ELCB by single lines. Selective coordination between ELCB1 with sensitivity of 30mA and high-order ELCB2 with sensitivity of 200mA can be achieved at a ground fault current of 150mA or less. But, at a ground fault current exceeding 150mA, there is a strong possibility that both ELCB1 and ELCB2 will operate because they are high speed circuit breakers with operating time of 0.1 sec or less. Therefore, the selectivity shall be examined in consideration of the rated current sensitivity and operating time.

Fig. 9. 23 Operating characteristics of high speed MCCB

Ground fault protection coordination by time delay circuit breakers

In many cases of prevention of electric shock and ground fault at terminals, circuit breakers with high sensitivity (30mA or so) having the highest protection effect are used. For long systems, since faults between transformer and branches can occur, MCCB are often used also on the main lines for protection between them. In such a case, if selective coordination is not achieved on the whole circuit, the high order ELCB will operate upon occurrence of ground fault, other sound circuits will be broken, and power supply cannot be continued. Therefore, it is necessary to examine the coordination. Fig. 9. 24 shows an example of a coordination system using time delay circuit breakers, and Fig. 9. 25 shows the coordination diagram of ELCB1, ELCB2, ELCB3 and ELCB4 on the system shown in Fig. 9. 24. If a ground fault of several A occurs at the point A, ELCB4, which is a high speed circuit breaker, will operate within 0.1 sec. The high-order ELCB3 which is a time delay circuit breaker with operating time of 0.3 sec has a longer  operating time than the total breaking time of ELCB4 at the branch. Therefore, selective trip can be ensured ly. On time delay circuit breakers, the inertial non-operating time is shown. The circuit breakers will not operate within the time even if ground fault current is flowing. The minimum value of the time is 0.1 sec. Therefore, coordination between the time delay and high speed types can be easily obtained as in this case.

Fig. 9. 24 Example of coordination system with time delay circuit breakers

Fig. 9. 25 Coordination characteristics of time delay circuit breakers

 Similarly, since the inertial non-operating time of ELCB1 and ELCB2 is longer than the total breaking time of ELCB4, ELCB3, ELCB2 and ELCB1 will not operate upon occurrence of fault. When a fault occurs at the point B, since the inertial non operating time of ELCB2 and ELCB1 is longer than the total breaking time of ELCB3, coordination between ELCB3 and ELCB2 and ELCB1 can be achieved, and power supply to ELCB6 can be continued. Also when a fault occurs at the point C, coordination between ELCB2 and ELCB1 can be achieved, and power supply to ELCB7 can be continued.