Thermal Relay Protection Parameter Settings

The primary standard for setting parameters of thermal relay protection is IEC 60255-149:2013, complemented by IEC 60947-4-1 for motor starters.

Overview of IEC 60255-149:2013

IEC 60255-149:2013 specifies minimum requirements for thermal protection relays, including the protection function, measurement characteristics, and test methodologies. It establishes a common and reproducible reference for evaluating dependent time relays that protect equipment from thermal damage by monitoring AC current through the equipment. The standard also allows for complementary input quantities such as ambient temperature, coolant, top oil, and winding temperature to be considered in the thermal protection settings . This standard covers relays based on a thermal model with memory function, meaning the relay accounts for both the current thermal state and the history of previous overloads. Test methodologies are included to verify the performance and accuracy of the thermal protection function .

Key Parameters for Setting Thermal Relays

1. Thermally Permissible Continuous Current (K-Factor) The K-factor represents the maximum continuous current the equipment can handle without overheating. It is used to scale the relay settings to match the motor or equipment rating .
2. Time Constant (Tau.th) The thermal time constant defines how quickly the relay responds to temperature changes. It is derived from the thermal model of the equipment and is used to calculate the tripping threshold based on the exponential progression of temperature rise .
3. Initial Thermal State The relay may be set to account for the initial thermal condition of the equipment, expressed as a percentage of the thermal state relative to rated current. This ensures accurate protection even if the equipment is already partially heated .
4. Inverse Time Characteristics Thermal relays typically operate on an inverse time principle, where higher currents cause faster tripping. This ensures protection against prolonged overloads while avoiding unnecessary trips for short-term current spikes .

Complementary Standards

• IEC 60947-4-1: Governs thermal overload protection for low-voltage motor starters, specifying trip classes, performance requirements, and testing procedures. It ensures consistency across electromechanical and electronic relays .
• IEC 60255-1xx series: Provides functional standards for protection relays, including measurement, testing, and performance evaluation, which can guide relay selection, commissioning, and operation .

Practical Application

When setting a thermal relay:

1. Determine the rated current and K-factor of the equipment.
2. Calculate the thermal time constant based on the motor or transformer thermal characteristics.
3. Set the initial thermal state if the equipment is not at ambient temperature.
4. Configure the relay according to inverse time characteristics to balance protection and operational continuity.
5. Verify settings using the test methodologies outlined in IEC 60255-149 to ensure compliance and reliability . By following these standards, engineers can ensure reliable thermal protection, prevent equipment damage, and maintain compliance with international safety requirements.