High limit controls are characterized as:

High limit controls are designed to ensure that the temperature within a system does not exceed a predetermined level, providing safety and preventing potential hazards such as overheating, which could lead to system failure or even fires. These controls operate substantially in a normally closed state, which means that under regular conditions, they maintain a closed circuit that allows operation. When the temperature exceeds the set limit, the control opens the circuit, interrupting the flow of energy to the system and effectively shutting it down until the temperature falls back to a safe level.

The normal closed operation is critical for safety, as it allows for immediate intervention when conditions become unsafe. This characteristic is essential for maintaining safe operating temperatures in oil heating systems, ensuring that they function only within designed parameters. By understanding the standard operational state of high limit controls, technicians can improve their troubleshooting and maintenance practices, leading to safer and more efficient heating system operations.