CPLT (complete) appears on digital controllers after a kiln has finished firing. If your controller shows a low temperature, the CPLT message assures you that the kiln fired to completion while you were away, and the kiln is now cooling down.
"My digital kiln fired normally and showed the CPLT message," someone asked me. "But how do I know that the elements have turned off after firing to completion? At what point should I disconnect the power to the kiln?"
It is okay to turn off the power to the kiln after the CPLT message appears. The power to the elements has turned off, anyway.
Indications that the heating elements have shut off:
1) The kiln temperature is dropping.
2) You no longer hear a clicking sound.
3) You no longer hear a humming sound.
If you hear an intermittent clicking, the elements are still powered. Even most kilns with silent mercury relays use a driver relay that clicks. Whenever the elements turn on, you will probably hear a faint humming. That is another sign that they are under power.
You should disconnect the power after the kiln has cooled to room temperature. The easiest way is to have a power disconnect box near the kiln. Otherwise you will need to unplug the kiln or turn off the circuit breaker each time.