①, current protection: (according to the setting principle of protection, protection range and principle characteristics)
A, overcurrent protection – is to avoid the maximum load current that may appear in the protected equipment or line to set. Non-fault current, such as large motor starting current (short time) and traversing short circuit current, to ensure the normal operation of equipment and lines. In order to make the upper and lower overcurrent protection selective, a corresponding step difference is set in the time limit.
B, the current rate off protection — is in accordance with the protected equipment or the end of the line may appear the maximum short circuit current or transformer secondary side three-phase short circuit current and set. Quick break protection action, theoretically current current break protection has no time limit. That is to cut off the circuit breaker with zero seconds or less time limit action.
Overcurrent protection and current current break protection are used together as the main protection of equipment or lines and the backup protection of adjacent lines.
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