Root cause analysis of thermal relay misoperation: influence of ambient temperature, load profile and selection errors

1. Introduction: Syndrome of false activation of the thermal relay

Thermal relay misoperation is a common problem in industrial settings, resulting in unplanned equipment shutdowns, reduced productivity, and significant economic losses. Despite the fact that thermal relays are designed to protect electric motors from overloads and overheating, their incorrect operation - triggering in the absence of a real threat - requires a deep engineering analysis. This analysis must be systematic, evidence-based, and focused on root causes, not just symptoms. Typical consequences include: loss of working time of up to 3 hours per incident, diagnostic and restart costs of up to €500 per incident, and possible product damage in process chains.

2. Component overview: Thermal relay and its function

The thermal relay is a key element of the protection system of low-voltage electric motors, which ensures their long-term reliability by turning off the power when the permissible currents are exceeded. Overload protection is critical because excessive current leads to overheating of the motor windings, insulation degradation and, as a result, complete failure. Operating the motor at a temperature of windings 10°C above the nominal reduces its service life by approximately half. The mean time to failure (MTBF) for a quality three-phase motor is 50,000 to 70,000 hours, but this figure drops sharply with frequent overloads.

There are two main types of thermal relays:

• Bimetallic relays: They work on the principle of deformation of bimetallic plates under the action of heat released during current flow. Relatively simple, economical, but sensitive to ambient temperature.
• Electronic relays: Use thermistors or current transformers for measurement, microprocessors for analysis and precision protection. Provide higher accuracy, less sensitivity to external temperatures and advanced features including winding temperature monitoring (using PT100 sensors) and communication capabilities.

The selection and setting of the thermal relay are regulated by international and national standards, in particular DSTU EN IEC 60947-4-1:2020 "Complete low-voltage switchgear. Part 4-1: Contactors and starters of electric motors. Electromechanical contactors and starters", which defines the requirements for the design, characteristics and tests of these devices.

3. Докази відмови: Що спостерігає технік

In the case of false activation of the thermal relay, an experienced technician is faced with a paradoxical situation: the engine stops, but there are no visual and instrumental signs of overload. Typical evidence of refusal includes:

• Entries in logs: Systematic triggering of relays at certain times of the day or during certain production cycles, which do not correlate with peak loads. For example, triggering at 2:00 p.m. every day, when the sun heats up the workshop as much as possible.
• Current measurement: The clamp meter shows a current that is between 70-90% of the motor's rated value, while the relay is set at 110-120% of that value. The absence of asymmetry of currents in phases (deviation <5%) indicates load balance.
• Thermographic examination: Using a thermal imager (for example, Fluke TiS60+) does not detect abnormal heating of the motor windings (body surface temperature <60°C, output terminal temperature <70°C). However, an increased temperature inside the control cabinet, where the relay is located, exceeding +35°C may be detected.
• Vibration analysis: Portable vibrometers (e.g. SKF Microlog Analyzer) record vibration levels corresponding to class A or B according to DSTU ISO 10816-3 (typically <4.5 mm/s RMS for motors up to 300 kW), indicating the absence of mechanical problems that could cause overloading.
• Visual inspection: No smoke, burning smell, darkening of wires or contacts. The engine runs smoothly, without unusual noises or intermittent vibrations.

Example: A thermal relay with a range of 23-32 A, set to 31 A, is installed on a conveyor with a 15 kW electric motor, rated current 29 A (400 V). It operates systematically in the summer at a workshop temperature of +38°C. Current measurement shows 27 A, motor thermography +55°C, control cabinet +45°C. This clearly indicates the influence of external factors, and not an overload of the engine.

4. Root cause research: Systematic analysis

A systematic approach, such as the "5 Whys" analysis or the Ishikawa (fishbone) diagram, is used to identify the root causes of thermal relay misoperations. This allows you to deeply investigate the problem, taking into account all possible factors.

4.1. The "5 Why" method

1. Problem: Thermal relay trips, motor stalls with no apparent overload.
2. Why? The relay perceives normal operating current as an overload.
3. Why?
   • A. The ambient temperature where the relay is installed is elevated.
   • B. The motor load current has uncharacteristic peaks that exceed the operating class of the relay.
   • A. The relay was incorrectly selected or configured.
   • D. Relay elements have degraded or have increased transient resistance.
4. Чому (для кожного з пунктів А, Б, В, Г)?
   • А. Why is the temperature elevated? - Insufficient ventilation of the cabinet, direct exposure to sunlight, proximity of heat sources (stoves, compressors).
   • B. Why uncharacteristic current peaks? - Short-term technological overloads (jamming of material), high starting currents, changes in the characteristics of the working environment (liquid density).
   • Q. Why is it wrongly selected/configured? - Insufficient engineering qualifications, lack of accurate data on the engine load profile, failure to take into account the engine's operating class.
   • D. Why did the elements degrade? - Natural aging of materials, corrosion of contacts, vibration, pollution, exposure to aggressive environments.
5. Why (for further factors)? For example, Why is the qualification insufficient? - Lack of regular training, neglect of the manufacturer's instructions.

4.2. Ishikawa diagram factors

• Human (Man): Errors of personnel during selection, installation, adjustment and maintenance.
• Method: Absence of standardized control, diagnostic and preventive maintenance procedures.
• Machine: Wear or failure of the relay itself, wrong type of relay for the application.
• Material (Material): Degradation of wiring, contact surfaces, low-quality relay components.
• Environment: High ambient temperature, high humidity, dust pollution, aggressive chemical vapors.
• Measurement (Measurement): Use of uncalibrated instruments, lack of accurate data on engine parameters and load profile.

5. Identified root causes

On the basis of a systematic analysis, it is possible to identify the main root causes of false operation of thermal relays, ranked by probability and confirmed by evidence:

1. Increased ambient temperature (40% chance)

   Gist: Thermal relays, especially bimetallic ones, are calibrated to work at a standard temperature of +20°C. When the ambient temperature rises, the relay quickly heats up to a critical point, even if the motor current is normal. Each increase in the temperature in the control cabinet by 10°C above the nominal operating temperature of the relay (usually +20°C) can lead to a reduction of the permissible current by 5-10%. For example, a relay configured for 30 A at +20°C may operate at 27 A if the cabinet temperature reaches +40°C.

   Evidence: Thermographic examination of control cabinets reveals temperatures of +40°C...+55°C. Historical data shows an increase in the number of trips during the hot period of the year or in certain shops (for example, foundry, thermal).

2. Mismatch of the load profile and the class of relay operation (probability 30%)

   Gist: Some technological processes are characterized by short-term peak loads (for example, starting a heavy conveyor, cyclic operation of a press, loading a crusher). These peaks can be significant (150-200% of the rated current), but their duration is too short for the motor to overheat. However, if the thermal relay has a low tripping class (for example, class 10A, which means tripping according to ДСТУ EN IEC 60947-4-1:2020 for 2-10 seconds at 7.2 times the current), it can trip the motor during these peaks.

   Evidence: Analysis of current waveforms with a power quality recorder (such as a Fluke 435 Series II) shows regular short-term current spikes that exceed the relay's set point, but are not permanent overloads. The engine shows no signs of overheating.

3. Errors in relay selection and configuration (probability 20%)

   Gist: Incorrect selection of the thermal relay according to the current range or incorrect setting of its setting is a direct cause of false activations. The relay should be selected so that its adjustment range includes the rated current of the motor, and the setting is set at 1.05-1.15 of the rated current of the motor (depends on the service factor of the motor). A situation often occurs when the relay is designed for 100% of the rated current, which leaves no margin for normal operating fluctuations.

   Evidence: Checking the engine nameplate and comparing it to the relay settings reveals a discrepancy. For example, the rated current of the motor is 40 A, and the relay is set to 38 A. Lack of clear documentation on the selection and configuration of the equipment.

4. Degradation of insulation and transient resistances (probability 10%)

   Gist: Over time, especially in conditions of vibration, high humidity or chemically aggressive environments, relay contacts and terminals can oxidize, which leads to an increase in transient resistance. An increase in resistance causes local heating at the contact point, which additionally affects the bimetallic elements of the relay, forcing it to operate earlier. Insulation degradation in the wiring to the motor can also create leakage currents or localized heating.

   Evidence: Relay contact resistance measurement (microohmmeter) shows >100 µOhms, while for new contacts it is <50 мкОм. Візуальний огляд виявляє сліди окислення або перегріву на клемах. Заміри ізоляції мегомметром (наприклад, Fluke 1587 FC) можуть виявити пробої або зниження опору ізоляції до 1-5 МОм (норма >100 MΩ).

6. Corrective actions: Immediate and long-term solutions

In order to effectively eliminate false activations of the thermal relay, it is necessary to apply a set of corrective actions:

6.1. For increased ambient temperature:

• Immediate: Opening the control cabinet door (temporary solution that violates security), installing a temporary fan.
• Long-term: Installation of industrial air conditioning systems for control cabinets (eg Rittal Blue e+), use of fans with filters for forced air circulation. Moving cabinets away from heat sources. Use of thermal relays with temperature compensation.

6.2. For load profile mismatch:

• Immediate: Temporary increase of relay current setting (with mandatory subsequent adjustment), current monitoring.
• Long-term: Using thermal relays with a higher tripping class (eg class 20 or 30 for heavy starting motors). Installation of soft starters or frequency converters to reduce starting currents. Optimization of the technological process to smooth out peak loads.

6.3. For selection and configuration errors:

• Immediate: Checking and adjusting the relay current setting according to the rated motor current and its service factor (usually setting = rated motor current * 1.1).
• Long-term: Development of standardized instructions for the selection and adjustment of thermal relays for different types of motors and loads. Regular training of engineering and technical personnel. Conducting an energy audit to accurately determine the real load profile.

6.4. For degradation of insulation and transient resistances:

• Immediate: Thorough cleaning of contact groups, tightening of terminal connections with observance of the tightening moment (for example, 2.5 Nm for terminals up to 2.5 mm²).
• Long-term: Inclusion in the PPR plan of regular checks of the state of contacts, insulation and replacement of worn relays. Use of terminals with spring clamps for increased reliability.

7. Quick diagnostic checklist for field technicians

This checklist is designed for quick and effective diagnosis of the causes of false activations of the thermal relay directly at the place of operation. It is recommended to use a portable tablet for data entry.

1. Visual inspection: Check the control cabinet and motor for visible damage, contamination, traces of overheating, faulty indicators.
2. Environmental temperature: Measure the temperature inside the control cabinet and the ambient air near the engine (infrared thermometer).
3. Load current: Measure the actual operating current of the motor on all three phases (current clamp, eg Fluke 376 FC). Record the maximum and minimum values ​​for a full working cycle (at least 30 minutes).
4. Relay setting: Check the current current setting on the thermal relay.
5. Rated motor data: Read the rated current and insulation class from the motor nameplate. Calculate the recommended relay setting.
6. Power supply voltage: Measure the voltage between phases and between phases and zero (multimeter). Check for phase misalignment (<2%).
7. Contact state: Turn off the power! Check the reliability of the terminal connections, the absence of oxidation or weakening. За потреби, затягнути всі клеми.
8. Thermal pattern: Carry out thermography of the motor (housing, bearings), terminal box, power cables and the thermal relay itself under load. Look for "hot spots".
9. Trigger history: Collect information about previous triggers (date, time, operating conditions, changes in the technological process).
10. Vibration: Measure the overall level of engine vibration (vibrometer) to rule out mechanical causes of overload.
11. Ventilation check: Assess the effectiveness of ventilation of the control cabinet and engine cooling.
12. Harmonic distortion: If suspected, measure the current nonlinear distortion factor (THDi) using a power quality analyzer (must be <10%).

8. Prevention strategy: Increasing system reliability

An effective strategy for preventing false activation of thermal relays includes a set of measures aimed at optimizing design, installation, operation and maintenance:

• Design and selection:
  • Current margin: Always select a thermal relay with a small margin on the upper limit of the regulation range, so that the rated motor current is 85-95% of the upper limit.
  • Activation class: Take into account the motor activation class and the specific load. For motors with difficult starting or cyclic peak loads, use Class 20 or Class 30 relays.
  • Temperature compensation: Prefer bimetallic relays with built-in temperature compensation, or use electronic relays in conditions of significant fluctuations in ambient temperature.
• Assembly and installation:
  • Cabinet ventilation: Ensure proper air circulation inside the control cabinets. Install industrial fans with filters or air conditioning systems to maintain a temperature not higher than +30°C.
  • Separation of heat sources: Place thermal relays away from other components that generate significant heat (eg transformers, resistors).
  • Quality of connections: Use high-quality cable ends, use torque screwdrivers to tighten terminal connections according to the manufacturer's recommendations.
• Condition Monitoring:
  • SCADA systems: Implementation of SCADA systems or industrial controllers (PLC) for constant monitoring of the operating current of motors, winding temperature (through PT100 sensors), control cabinet temperature.
  • Thermographic surveys: Regular planned thermographic surveys of all control cabinets and electric motors (once every 6-12 months) for early detection of overheating.
  • Analysis of power quality: Periodic checking of the presence of harmonic distortions in the network, which can cause additional heating.
• Maintenance and training:
  • PPR: Include in the PPR plan regular inspection, cleaning and, if necessary, replacement of thermal relays (every 5-7 years for bimetallic ones, depending on operating conditions).
  • Calibration: Checking the calibration of the measuring instruments used to set up the relay.
  • Staff training: Conducting regular trainings for technicians and engineers on the principles of operation of thermal relays, diagnostic methods and corrective actions.

9. Conclusion

The efficient functioning of thermal relays is critically important for ensuring uninterrupted and safe operation of industrial equipment. A systematic approach to the analysis of the root causes of false positives, which includes thorough diagnostics, corrective actions and a prevention strategy, allows you to significantly increase the reliability of production processes. Інвестування в правильний вибір обладнання, оптимізацію умов експлуатації та кваліфіковане обслуговування окуповується зменшенням часу простоїв та продовженням терміну служби дороговартісного обладнання.

For more information on protection devices, electric motors and industrial automation components, visit UNITEC-D E-Catalog.

10. Links

• DSTU EN IEC 60947-4-1:2020. Low-voltage distribution devices are complete. Part 4-1: Contactors and starters of electric motors. Electromechanical contactors and starters.
• DSTU ISO 10816-3:2006. Vibration is mechanical. Evaluation of machine vibration based on measurement results on stationary parts. Part 3: Industrial machinery with a rated power exceeding 15 kW, operating at speeds between 120 rpm and 15,000 rpm, placed on flexible or rigid foundations.
• Manuals for operation and maintenance of electric motors and thermal relays from leading manufacturers (for example, Siemens, Schneider Electric, Eaton).
• Mankis, S. E. (2012). Guide to operation and repair of electric motors. Moscow: Energoatomizdat.