Contactor Interrupting Rating And Protection Settings
The Medium voltage contactor has limited interrupting capacity during a fault. The contractors are serially installed with the fuse to meet or exceed the maximum available fault current. It is important that protection relays operating the contractors are properly set so that contractors properly coordinate with the fuse and do not operate during a fault current higher than the interrupting capacity.
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Figure 1 shows the contactor interrupting rating. The HCV-6KAU has 7200 Amp rating.
Figure 2 shows the phase coordination between the contactor (Protection Relay) and fuse. The protection relay overcurrent element is delayed by 100 msec so that contactor does not operate before the fuse when the fault current is higher than 7200 Amps. In a high-impedance grounding system, when the Line-Ground fault current is limited to 200 Amp or less, the Line-Line-Ground current can be significantly higher than the contractor's interrupting capacity. So It is important that the ground element of the protection relay is properly coordinated with the fuse even when the Line-Ground fault current is very limited.
