Why is there a delay between when the APS-4C turns on and when the SC-40C comes on?

This delay between the APS-4C and the SC-40C is programmed into the system in order to reduce surge current. When the heaters are turned on they will have a large inrush of current for a few seconds. If all the heater circuits came on at the same time the inrush current could be large enough to trip the main breaker in supplying panel. This 5-second delay is meant to reduce this problem.

Click here to view the APS-C series manual

APS-4C connected to SC-40C