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Some Typical Key Interlock Schemes

Scheme A: To prevent the paralleling of two independent sources.

Initial Conditions: The load is fed from line 2 through switch 2 which is normally closed with its respective interlock key "A" captive. Switch 1 is locked open with no interlock key present.

Switching the load to line 1 through switch 1: Open switch 2 and lock thereby releasing the "A" key. Use the "A" key to unlock and close switch 1. Reverse to restore the initial condition.

Scheme H: Preventing the paralleling of two independent sources through the tie breaker while allowing the switching of sources to the load(s).

Initial Condition: Line A feeds load A through breaker #1 which is normally closed with its interlock key "A" captive. Line B feeds load B through breaker #2 which is normally closed with its interlock key "A" captive. Breaker #3 is locked open with no interlock key present.

Switching scenario #1: Line A feeding both loads through breaker #1 and breaker #3: Open and lock breaker #2 thereby removing the "A" key. Use the "A" key to unlock and close breaker #3. Reverse to restore the initial condition.

Switching scenario #2: Line B feeding both loads through breaker #2 and breaker #3: Open and lock breaker #1 thereby removing the "A" key. Use the "A" key to unlock and close breaker #2. Reverse to restore the initial condition.

Scheme H2: Preventing the paralleling of any of the sources while allowing the switching of the loads to alternate sources.

Initial conditions: Utility feed A supplies load A through breaker #1 which is closed with its interlock key "A" captive. Utility feed B supplies load B through breaker #4 which is closed with its interlock key "B" captive. Breakers #2, #3, and #5 are locked open with no keys present in their respective interlocks.

Switching scenario #1: Supplying loads A and B by utility feed A(B) through breaker #1(#4) and breaker #5: Open and lock breaker #4(#1) thereby removing the "B"("A") key. Use the "B" ("A") key to unlock and close breaker #5. Reverse to restore the initial condition.

Switching scenario #2: Supplying loads A and B by generator 1(2) through breaker #2(3) and breaker #5: Open and lock breaker #1(#4) thereby removing the "A"("B") key. Use the "A" ("B") key to unlock and close breaker #2(#3). Open and lock breaker #4(#1) thereby removing the "B"("A") key. Use the "B"("A") key to unlock and close breaker #5. Reverse to restore the initial condition.

Switching scenario #3: Generator 1(2) supplying load A(B) through breaker #2(#3): Open and lock breaker #1(#4) thereby removing the "A"("B") key. Use the "A"("B") key to unlock and close breaker #2(#3). Open and lock breaker #4(#1) thereby removing the "B"("A") key. Use the "B"("A") key to unlock and close breaker #3(#2). Reverse to restore the initial condition.

Note: The key interlock on breaker #5 is dual combinated and can accept either an "A" or a "B" key.

Uninterrupted power source scheme 1: To insure the proper sequential operation of UPS related equipment.

Initial Condition: The critical load is fed through the UPS through the UPS output breaker. The Solenoid Key Release Unit (SKRU) has its respective interlock key "A" captive with its internal form C contacts in position A. The maintenance bypass breaker is locked open with no "A" key present while the "B" key is captive. The UPS output breaker is closed with no "B" key present while the "A" key is captive.

Switching Scenario: Supply of the critical load through the UPS maintenance bypass breaker: The UPS is placed in the bypass mode. Upon completion a signal is sent to the SKRU enabling release of the "A" key while placing the internal form C contacts into position B. Use the "A" key to unlock and close the maintenance bypass breaker thereby trapping the "A" key and releasing the "B" key. Open and lock the UPS output breaker using the "B" key which will trap the "B" key and release the "A" key. Re-insert and "A" key in the SKRU and turn, thereby trapping key A and returning the form C contacts to position A. Reverse to restore the initial condition.

Uninterrupted power source scheme 2: To insure the proper sequential operation of UPS related equipment.

Initial Condition: The critical load is fed through the UPS through the UPS output breaker. The Solenoid Key Release Unit (SKRU) has its respective interlock key "A" captive with its internal form C contacts in position A. The maintenance bypass breaker is locked open with no key present. The UPS output breaker is closed with the "B"key captive.

Switching Scenario: Supply of the critical load through the UPS maintenance bypass breaker: The UPS is placed in the bypass mode. Upon completion a signal is sent to the SKRU enabling release of the "A" key while placing the internal form C contacts into position B. Use the "A" key to unlock and close the maintenance bypass breaker. Open and lock the UPS output breaker thereby releasing the "B" key. Insert the "B" key in the SKRU and turn, thereby trapping key "B" and returning the form C contacts to position A. Reverse to restore the initial condition.

Note: The SKRU is a dual combinated interlock and can accept either an "A" or a "B" key.

Key Interlock Ringbuss Scheme: to prevent the paralleling of feed #1 and #2; to enable switching the loads to the desired feeds; to prevent access to an energized fuse compartment.

Initial Conditions: Feeder #1 supplies loads A and B through switch #1,A1 and A2 which are all closed with their respective interlock keys captive. Feeder #2 supplies loads C,D,E, and F through switch #2,C2,C1, and B2 which are all closed with their respective interlock keys captive. Switch B1 is locked open with no key present. All fuse access doors are locked closed with no keys present. The key transfer blocks have their respective keys trapped or key positions vacant as shown.

Switching scenario (general): Supplying additional loads through one of the feeders (shedding loads from one of the feeders): Lock open the feeder switch desired (switch #1 or switch #2) thereby releasing the "R" key. Insert the "R" key into the 2-key transfer block vacant "R" position to obtain the key corresponding to the presently locked open switch. Use this key to unlock and close the corresponding switch. Open and lock the switch desired to supply or shed loads as required. Insert this key in its respective 2-key transfer block position to obtain the "R" key. Use the "R" key to unlock and close the feeder switch initially opened. Reverse to restore the initial condition.

Access to Fuse Enclosure: Open and lock the switch adjacent to the fuse enclosure to be accessed, and follow the general switching scenario, this will allow retrieval of the keys to access the fuse compartment desired without a major service interruption. Use these keys to unlock the 2-key fuse compartment access interlock. These keys will remain captive in the interlock unit until the door is re-closed and locked. Reverse to restore the initial condition.