A thermal relay is an electrical protection device primarily used for overload protection of electrical equipment such as motors and transformers. It controls the opening and closing of circuits based on the equipment's operating current and the set overload current value, thereby ensuring safe operation of the equipment. In practical applications, the normally open and normally closed positions of a thermal relay have different applications.

First, Applications of the Normally Open Position
The normally open position refers to the open position of the thermal relay's contacts during normal operation. When an overload current occurs in the equipment, the thermal relay activates, closing the contacts, thereby disconnecting the circuit and protecting the equipment. The normally open position is mainly used in the following areas:
1. Overload Protection
The normally open position is the most common application of thermal relays. When an overload current occurs in the equipment, the thermal relay activates, closing the contacts, disconnecting the circuit and protecting the equipment. This method is simple and reliable, suitable for a wide range of electrical equipment.

2. Phase Loss Protection
Phase loss protection refers to a three-phase power system in which, when a phase fails or loses power, the relay detects current changes to determine whether a phase loss has occurred and promptly disconnects the circuit to protect the equipment. Normally open (NO) contacts can be used to implement phase loss protection. By connecting the NO contact of a thermal relay to a three-phase power supply, phase loss detection and protection can be achieved.

3. Overvoltage Protection
Overvoltage protection refers to the process of detecting voltage changes in electrical equipment when the voltage exceeds a set value, thereby detecting overvoltage and promptly shutting off the circuit to protect the equipment. The NO contact can also be used to implement overvoltage protection. By connecting the NO contact of a thermal relay to a voltage detector, overvoltage detection and protection can be achieved.

Second, Applications of Normally Closed (NC) Contacts
NNC contacts are closed during normal operation of a thermal relay. When an overload current flows into the equipment, the thermal relay activates, opening the contacts and shutting off the circuit to protect the equipment. NC contacts are primarily used in the following areas:
1. Starting Protection
Starting protection refers to the process of detecting current changes during motor startup to detect overload and promptly shut off the circuit to protect the motor. Normally closed (NC) points can be used for startup protection. By connecting the normally closed (NC) point of a thermal relay to a motor, overload detection and protection can be implemented during the motor startup process.

2. Reverse Current Protection
Reverse current protection refers to detecting current changes during motor operation when the power supply is reversed, and immediately shutting down the circuit to protect the motor. Normally closed (NC) points can be used for reverse current protection. By connecting the normally closed (NC) point of a thermal relay to a motor, reverse current detection and protection can be implemented.

3. Leakage Current Protection
Leakage current protection refers to detecting current changes in electrical equipment when a leakage occurs, and immediately shutting down the circuit to protect the equipment and personnel. Normally closed (NC) points can be used for leakage current protection. By connecting the normally closed (NC) point of a thermal relay to a leakage detector, leakage detection and protection can be implemented.

In short, the normally open and normally closed (NO) points of a thermal relay have different uses in different applications. Choosing the appropriate application method can improve the reliability and safety of electrical equipment and ensure its normal operation.