Final answer:
The NO and NC switches in a time initiation / temperature termination defrost system change their positions at the start and end of the defrost cycle. The NO switch closes to activate the defrost heater, while the NC switch opens to stop refrigeration. At the end of the cycle, the positions reverse to resume normal operation.
Step-by-step explanation:
In a time initiation / temperature termination defrost system, the defrost time clock utilizes a Normally Open (NO) and a Normally Closed (NC) switch to control the defrost cycle in refrigeration equipment such as freezers or coolers. The NO and NC switches change positions based on the defrost timer's settings. When the defrost cycle is initiated by the timer, the NO switch closes to start the defrost heater and the NC switch opens to terminate refrigeration or fan operation. The switches remain in this state for the duration of the defrost cycle which is preset.
Once either the time preset for defrost is reached or the temperature termination control senses that the defrost is complete (whichever comes first), the NO switch opens to stop the defrost heater, and the NC switch closes to resume refrigeration or fan operation. This design ensures energy efficiency and prevents frost build-up, which could otherwise impair the system's performance.