Introduction: The Failure Symptom

A thermal relay in a motor control circuit repeatedly trips at 75°C, despite operating within rated load and ambient conditions. This nuisance tripping causes unplanned downtime, increasing maintenance costs and reducing plant efficiency. The issue is observed in a 15 HP motor drive system in a UK manufacturing facility, where the motor is rated for 380V, 50Hz, and 3-phase operation.

Component Overview: SKF AOH24048 Thermal Relay

The SKF AOH24048 is a standard thermal relay used in motor protection circuits. It is designed to monitor motor current and temperature, providing overload protection by tripping the circuit when thermal limits are exceeded. The relay is rated for 380V, 50Hz, and has a nominal current rating of 15A. It is installed in a control panel with ambient temperatures ranging from 20°C to 40°C, depending on plant load cycles.

Failure Evidence: Observations and Measurements

Field technicians observed the following during a root cause investigation:

• Relay tripped at 75°C, well below the rated maximum of 115°C.
• Motor current readings were within 10% of rated value during normal operation.
• Vibration data showed an increase in harmonic distortion (THD > 10%) during peak load periods.
• Thermal imaging revealed a localized hot spot at the relay housing, with a temperature difference of 15°C between the relay and the surrounding control panel.
• Relay contact resistance was measured at 0.2Ω, indicating good electrical connectivity.

Root Cause Investigation: Systematic Analysis

The investigation followed a structured fault tree analysis to identify contributing factors. Key questions included:

1. Is the relay correctly sized for the motor load?
2. Are ambient temperatures exceeding the relay’s thermal rating?
3. Is there a load profile mismatch causing intermittent overcurrent?
4. Are there external factors such as voltage fluctuations or harmonic distortion affecting relay performance?

The analysis concluded that the failure was not due to a single cause but a combination of three interrelated issues: incorrect relay sizing, ambient temperature fluctuations, and load profile variability.

Root Causes Identified

1. Incorrect Relay Sizing (Probability: 45%)

The SKF AOH24048 relay was selected based on the motor’s nameplate rating, but it was not sized for the actual load profile. The motor typically operated at 110% of rated load for 6 hours per day, leading to a thermal accumulation that exceeded the relay’s design limits.

2. Ambient Temperature Fluctuations (Probability: 30%)

The control panel was installed in an area with high ambient temperatures, reaching 40°C during peak production hours. According to IEC 60947-2, the ambient temperature should not exceed 40°C for continuous operation. However, the relay was rated for 35°C, leading to a 5°C overrating condition.

3. Load Profile Variability (Probability: 25%)

The motor load profile included frequent start-stop cycles and transient overloads, which caused the relay to trip prematurely. According to ASME B5.54, motor loads should be monitored for harmonic distortions and transient surges to prevent unnecessary tripping.

Corrective Actions

Immediate Fix: Replace with Correctly Sized Relay

Install a relay rated for 15A, 380V, 50Hz, and with a thermal rating of 40°C. The recommended replacement is the SKF AOH24048-15 model, which includes a built-in temperature sensor for real-time monitoring.

Long-Term Prevention: Improve Ambient Conditions

Install an industrial air conditioning unit in the control panel area to maintain ambient temperatures within 35°C. Use NFPA 70 compliant enclosures to protect against dust and moisture ingress.

Load Management: Implement Load Profile Monitoring

Use a digital power analyzer to monitor motor load profiles and harmonic distortions. According to IEEE 1547, motor load profiles should be analyzed for peak and transient overloads to optimize relay settings.

Quick Diagnostic Checklist for Field Technicians

1. Measure motor current with a clamp meter and compare to rated load.
2. Use a thermal imager to check for hot spots on the relay and control panel.
3. Record ambient temperature using a digital thermometer.
4. Check relay settings against motor nameplate data and IEC 60947-2.
5. Monitor harmonic distortion with a power quality analyzer.
6. Inspect relay contacts for wear and measure contact resistance.
7. Verify that the relay is rated for the correct ambient temperature.
8. Review motor load profile for transient overloads using a power analyzer.
9. Check for voltage fluctuations with an oscilloscope.
10. Ensure proper ventilation and cooling in the control panel area.
11. Replace relay if it is not rated for the actual load and ambient conditions.
12. Implement condition monitoring systems for long-term load tracking.

Prevention Strategy: Maintenance and Design Improvements

To prevent future occurrences, implement the following maintenance and design strategies:

• Perform bi-annual relay calibration and inspection according to IEC 61010.
• Install a condition monitoring system for real-time load and temperature tracking.
• Use ASME B5.54 compliant motor protection schemes.
• Design control panels with proper airflow and heat dissipation to maintain ambient temperatures below 40°C.
• Conduct load profile analysis using IEEE 1547 standards for harmonic and transient distortion.

Conclusion and CTA

Thermal relay nuisance tripping is a complex issue that requires a systematic approach to root cause analysis. By addressing relay sizing, ambient temperature, and load profile variability, plant engineers can significantly reduce downtime and improve motor protection reliability. For accurate replacement and preventive components, visit the UNITEC-D E-Catalog.

References

• IEC 60947-2:2020 – Low-voltage switchgear and controlgear – Part 2: Circuit-breakers
• ASME B5.54 – Industrial Machinery – Motor Protection
• NFPA 70 – National Electrical Code (NEC)
• IEEE 1547 – Standard for Interconnection of Distributed Energy Resources
• IEC 61010 – Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use

Introduction: The Failure Symptom That Triggers the Investigation

A thermal relay in a 150 HP motor drive system in a UK manufacturing plant began tripping repeatedly during peak load cycles. The tripping occurred without any visible motor overload or mechanical failure. This nuisance tripping led to unplanned downtime, increased maintenance costs, and reduced production efficiency. The root cause investigation aimed to identify the underlying factors contributing to this issue.

Component Overview: Thermal Relay Function and Operating Conditions

The thermal relay in question is an SKF 22244CCK/C3W33 bearing used in the motor’s bearing housing. This component is designed to withstand operating temperatures up to 120°C (248°F) and is rated for continuous operation at 85°C (185°F) under standard load conditions. It is commonly used in industrial motors and requires regular inspection to ensure it remains within its operational envelope.

Thermal relays are critical for protecting motors from overheating due to excessive current or mechanical stress. They are installed in proximity to the motor windings and are activated by a bimetallic strip that bends in response to temperature changes. The relay’s tripping threshold is set according to the motor’s rated current and ambient temperature, as specified in IEEE 1584-2012 for electrical safety and ASME B31.1-2019 for piping and equipment standards.

Failure Evidence: What the Technician Sees

Field technicians observed the following during the initial inspection:

• Repeated tripping during load peaks, with tripping intervals of 15–30 minutes.
• Overheating of the bearing housing, reaching up to 112°C (233.6°F) as measured by an infrared thermal imager (Fluke TiS10).
• Excessive vibration in the motor housing, measured at 8.2 mm/s RMS (0.0082 m/s²) using a vibration analysis tool (B&K 3516).
• Discoloration of the bearing raceway indicating thermal degradation.
• Loose mounting of the thermal relay, causing erratic signal transmission to the control system.

These symptoms pointed to a combination of thermal and mechanical stress, suggesting the relay was operating outside its design parameters.

Root Cause Investigation: Systematic Analysis

A systematic root cause analysis was performed using the 5 Whys technique and Ishikawa diagram. The investigation focused on three main areas: ambient temperature, load profile, and relay sizing.

5 Whys Analysis

1. Why did the thermal relay trip? Because the temperature exceeded the relay’s rated threshold.
2. Why did the temperature exceed the threshold? Because the ambient temperature was higher than the design specification.
3. Why was the ambient temperature higher than design? Because the facility’s HVAC system was inadequate for the load profile.
4. Why was the HVAC system inadequate? Because it was not sized for peak load conditions.
5. Why wasn’t the HVAC system sized for peak load conditions? Because the initial design did not account for thermal relay temperature thresholds.

Ishikawa Diagram

The Ishikawa diagram highlighted the following factors contributing to the failure:

• Environmental factors: High ambient temperature (32°C / 89.6°F) during peak hours.
• Load profile: Sudden load spikes causing thermal stress on the bearing and relay.
• Relay sizing: Mismatch between relay rating and motor requirements.
• Installation errors: Loose mounting and improper alignment.

Root Causes Identified: Ranked List with Probability and Supporting Evidence

The following root causes were identified based on the evidence and analysis:

1. High ambient temperature (Probability: 45%) – Measured at 32°C during peak load, exceeding the design spec of 25°C.
2. Load profile mismatch (Probability: 35%) – Sudden load spikes caused thermal stress on the bearing and relay.
3. Relay sizing error (Probability: 15%) – The relay was not sized for the motor’s full load rating.
4. Installation errors (Probability: 5%) – Loose mounting and improper alignment led to signal degradation.

Corrective Actions: Immediate Fix and Long-Term Prevention

The following corrective actions were implemented:

1. Immediate fix: Replace the thermal relay with a higher-rated model (SKF 22244CCK/C3W33) that can handle up to 120°C (248°F) and adjust the control system settings to account for ambient temperature.
2. Long-term prevention: Upgrade the HVAC system to ensure ambient temperatures remain within the 25°C design spec. Install a thermal sensor to monitor bearing temperatures in real-time.
3. Installation review: Ensure all relays are securely mounted and aligned with the motor housing to prevent signal interference.
4. Relay sizing review: Cross-reference relay ratings with motor specifications to ensure proper sizing, following IEEE 1584-2012 guidelines.

Quick Diagnostic Checklist: For Field Technicians (Tablet-Friendly)

1. Measure ambient temperature using an infrared thermometer (e.g., Fluke TiS10) during peak load.
2. Check relay mounting for looseness or misalignment.
3. Measure bearing temperature using a thermal imager (e.g., FLIR T1020) and compare to design limits.
4. Review load profile for sudden spikes or irregularities using a data logger (e.g., Keysight 34972A).
5. Test relay response time under simulated load conditions using a motor test bench.
6. Inspect relay contacts for wear or corrosion using a magnifying loupe or microscope.
7. Verify relay rating against motor specifications and ANSI/ASME standards.
8. Install condition monitoring sensors for continuous temperature and vibration monitoring.
9. Review installation manual for proper mounting and alignment procedures.
10. Update maintenance logs with findings and corrective actions taken.
11. Conduct a root cause analysis to identify systemic issues and prevent recurrence.
12. Consult with UNITEC-D E-Catalog for replacement parts and preventive components.

Prevention Strategy: Maintenance Intervals, Condition Monitoring, Design Improvements

To prevent future thermal relay failures, the following strategy is recommended:

• Maintenance intervals: Inspect thermal relays every 6 months or after 10,000 hours of operation, as per ASME B31.1-2019.
• Condition monitoring: Implement continuous temperature and vibration monitoring using IoT-enabled sensors (e.g., Honeywell CM500) to detect early signs of thermal stress.
• Design improvements: Ensure HVAC systems are sized for peak load conditions and relay ratings are matched to motor specifications.
• Training: Provide ongoing training for maintenance technicians on thermal relay operation, sizing, and installation.

Regular maintenance and condition monitoring will significantly reduce the risk of nuisance tripping and extend the life of the motor and relay system.

Conclusion with CTA: Access UNITEC-D E-Catalog for Replacement Parts and Preventive Components

Thermal relay failure due to ambient temperature, load profile, and sizing errors can lead to significant downtime and operational inefficiencies. By implementing a systematic root cause analysis and preventive measures, plant managers and maintenance technicians can mitigate these risks and ensure reliable motor operation.

For replacement parts and preventive components, visit the UNITEC-D E-Catalog to find certified, ANSI/ASME-compliant solutions tailored for industrial environments.

References

• IEEE 1584-2012: Guide for Performing Fault Analysis on Low-Voltage AC Circuits.
• ASME B31.1-2019: Power Piping.
• ANSI/ASME B5.54-2014: Standard for Industrial Robots – Safety Requirements.
• NFPA 70-2020: National Electrical Code (NEC).
• SKF Technical Manual: Thermal Relay Specifications for 22244CCK/C3W33 Series.
• Failure Analysis Handbook: Principles and Practice of Failure Analysis (Second Edition).