This scheme is used to prevent the operation of the non-load break disconnect switch while the breaker is closed. Insuring the fuse compartment is isolated before it can be accessed.
Initial Condition: Line #1 feeds load #1 through switch #1 and breaker #1. Breaker #1 is closed with key "A" captive in its key interlock. Switch #1 is locked closed with no key present in its LOC position key interlock while its LO position key interlock has a "B" key captive. The fuse compartment is locked closed with no interlock key present.
Opening the Non-load break disconnect switch and gaining access to an isolated fuse compartment: open and lock breaker #1 thereby removing the "A" key. Use the "A" key to unlock and open switch #1. Lock switch #1 in the open position thereby releasing the "B" key. Use the "B" key to unlock and open the fuse compartment. Reverse to restore the initial condition.
Servicing/testing of the breaker: open and lock breaker #1 thereby removing the "A" key. Use the "A" key to unlock and open switch #1. Lock switch #1 in the open position using the LOC key interlock, use the "A" key to unlock the breaker for servicing or testing. Reverse to restore the initial condition.
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