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NEPSI's Metal-Enclosed Capacitor Banks and Harmonic Filter Banks come equipped with a key interlock system that prevents the following:

Unsafe entry into enclosure while the bank is energized.

Out-of-sequence operation of non-loadbreak devices, such as air-disconnect switches.

Unsafe operation of ground switch(s) while equipment is energized.


The figure below shows a typical "advanced" key interlock scheme offered by NEPSI. Schemes vary based on equipment supply and customer requirements. For the scheme shown below, the sequence of operation is as follows:

  1. Turn all vacuum switches to the off position. In doing this, a 5-minute timer is started to allow the capacitors to discharge. After a 5 minute waiting period, the A1 key can be released. (Note: once A1 is removed, all vacuum switches are disabled.)
  2. Unlock and open the air disconnect switch (this provides a visible break and releases a mechanical interlock between the air disconnect switch and the ground switch).
  3. Close the ground switch. After closing the ground switch, remove the A2 key.
  4. Open door 1 with the A2 key. When this door is opened, the vacuum switches are activated to effectively ground all capacitors within the bank. Remove the A3 key and proceed to door 2.
  5. Open door 2 with the A3 key and remove the A4 key.
  6. Open door 3 with the A4 key and remove the A5 key.
  7. Open door 4 with the A5 key. (Note: The A5 key will remain captive in door 4 until it is closed).
  8. Reverse procedure to energize bank.


Kirk Key Interlock System


  1. As standard, NEPSI does not provide the 5-minute timer and automatic activation of vacuum switches when opening door 1. The "advanced scheme" needs to be specified to obtain these functions.
  2. The swing out barrier behind door 1 may be interlocked with an upstream breaker or disconnect switch. This would prevent entry into termination compartment while cables are energized. Please specify this option if desired.
  3. When working around capacitors, it is always prudent to assume capacitors are charged unless shorted at the terminals. Therefore, NEPSI recommends local ground leads be utilized before coming in contact with capacitors.

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