"Five Preventions" is an abbreviation for preventing five types of electrical misoperation in power systems. According to statistics, over 80% of misoperation accidents in power systems are caused by these five types of misoperation: accidentally opening or closing circuit breakers, operating disconnect switches under load, hooking up ground wires or closing ground switches while energized, closing switches with ground wires on, and accidentally entering a live compartment. Therefore, a specific operating sequence must be maintained between various switchgear devices; otherwise, serious consequences and even accidents can result. To this end, high-voltage switchgear should have these "five preventions."
1) Prevent opening or closing disconnect switches and disconnect plugs under load.
2) Prevent accidentally opening or closing circuit breakers, load switches, and contactors.
3) Prevent closing circuit breakers, load switches, etc. while the ground switch is in the closed position.
4) Prevent accidentally closing the ground switch while energized.
5) Prevent accidentally entering a live compartment.
For removable high-voltage switchgear, the following requirements must also be met:
1) Primary isolating contacts may be withdrawn or inserted only when the circuit breaker, load switch, or contactor is in the open position. Otherwise, the isolating contacts will interrupt or close the load current or short-circuit current, generating arcs between the contacts, causing severe burns to the contacts and other nearby components, and potentially causing short-circuit accidents.
2) Circuit breakers, load switches, and contactors may be opened or closed only when the trolley carrying the circuit breaker is in the operating position, test position, disconnected position, grounding position, or removed position.
3) The trolley carrying the circuit breaker may be pushed into the operating position only when the grounding switch is in the open position. Otherwise, if the circuit breaker is closed while the grounding switch is still in the closed position, an unnecessary short-circuit closing operation may occur.
4) The grounding switch may be closed only after the trolley carrying the circuit breaker has been extended to the test position or any subsequent position, i.e., after a sufficiently large insulation gap has been formed between the isolating contacts.
