Parameters and Functions of Thermal Relay Protectors

Thermal relay protectors safeguard motors by monitoring current and tripping on prolonged overloads, with key parameters including full-load current, overload settings, trip class, and time/current characteristics.Key Parameters1. Full-Load Current (FLC / Rated Current): The thermal relay is set to match the motor's full-load current, slightly above the normal operating current to avoid nuisance tripping during startup or minor load fluctuations . 2. Overload Setting (OL): This defines the current threshold at which the relay begins to trip. It is usually expressed as a percentage of the motor's rated current. The relay calculates thermal damage using the I²t principle, integrating the square of the current over time to determine when to trip . 3. Trip Class / Time-Current Characteristic: Thermal relays have different trip classes (e.g., Class 10, Class 20), which define the maximum time allowed to trip at a specified multiple of rated current. This ensures protection against prolonged overloads while allowing short-term inrush currents during motor startup . 4. Time Setting Multiplier (TSM): TSM scales the base operating time derived from the relay's characteristic curve. Lower TSM values result in faster tripping, while higher values provide delayed response for coordination with upstream protection . 5. Plug Setting Multiplier (PSM): PSM indicates how many times the relay's pickup current is exceeded. It is used in inverse-time relays to calculate operating time according to the IDMT curve. Higher PSM values lead to faster tripping . 6. Ambient Temperature Compensation: Thermal relays may require adjustment for ambient temperatures above 30°C, as higher temperatures reduce the motor's cooling efficiency, potentially causing earlier tripping . 7. Auxiliary Contacts and Reset Modes: Thermal relays typically include normally closed (NC) auxiliary contacts that open on trip to control the motor contactor. Reset can be manual or automatic, depending on the relay design . 8. Additional Settings (Optional): Some relays include settings for phase loss, phase imbalance, or earth leakage (EL), which enhance protection in three-phase systems and prevent damage from single-phase faults .Standards and CoordinationThermal relay settings and operation are guided by IEC 60255-8 and IEC 60947-4-1, which define overload characteristics, I²t curves, and coordination with upstream protection devices like circuit breakers or fuses . Proper calibration ensures reliable motor protection, prevents insulation damage, and extends equipment life.SummaryIn practice, configuring a thermal relay protector involves:Setting the full-load current and overload thresholdSelecting the appropriate trip classAdjusting TSM and PSM for coordinationCompensating for ambient temperatureEnsuring proper auxiliary contact operation and reset mode These parameters collectively ensure that the relay trips accurately under prolonged overloads while avoiding false trips during normal motor operation.