Root Cause Analysis 6 min read

Thermal Relay Nuisance Tripping: Root Cause Analysis and Mitigation Strategies

Technical analysis: ABZMS-35-1X/120F

Thermal Relay Nuisance Tripping: Root Cause Analysis and Mitigation Strategies - UNITEC-D Industrial MRO
Thermal relay nuisance tripping can lead to significant downtime. This article identifies key root causes and provides a practical checklist for resolution.

Introduction

Thermal relay nuisance tripping is a common yet costly issue in industrial motor control systems. In a recent incident at a UK manufacturing plant, a REXROTH ABZMS-35-1X/120F thermal relay repeatedly tripped during normal operation, causing unplanned downtime and production losses. The failure symptom—frequent tripping without apparent overload—triggered a detailed root cause investigation to identify and resolve the underlying issues.

Component Overview

The REXROTH ABZMS-35-1X/120F is a thermal relay designed for motor protection in industrial environments. It is rated for a maximum current of 120A and operates within an ambient temperature range of -25°C to +55°C. The relay is installed in a control panel and is used to monitor the thermal load on a 3-phase motor, typically in applications such as pumps, compressors, and conveyor systems.

Failure Evidence

During the investigation, the following evidence was observed:

  • Repeated tripping: The relay tripped 12 times within a 16-hour period, despite the motor not exceeding 110% of its rated load.
  • Environmental conditions: The ambient temperature in the control panel was measured at 48°C, exceeding the maximum operational limit of the relay by 13°C.
  • Vibration data: Vibration analysis revealed a 12% increase in amplitude at the relay housing, likely due to mechanical stress from nearby machinery.
  • Visual inspection: The relay housing showed signs of overheating, including discoloration and a slight deformation of the housing.
  • Current measurements: Using a Fluke 434 II analyzer, the motor current was recorded at 112A, well within the relay’s rated capacity.

Root Cause Investigation

The root cause investigation employed a structured fault tree analysis (FTA) to identify the contributing factors to the nuisance tripping. The following questions guided the investigation:

  1. Why did the relay trip when the motor was not overloaded?
  2. What environmental factors could influence the relay’s performance?
  3. Is the relay properly sized for the motor and application?
  4. Are there mechanical or electrical interferences affecting the relay’s operation?

Root Causes Identified

Based on the investigation, the following root causes were identified and ranked by probability and supporting evidence:

  1. Overheating due to ambient temperature: The ambient temperature in the control panel exceeded the relay’s maximum operating limit, causing the thermal element to activate prematurely. This is supported by thermocouple measurements and the visual inspection of the relay housing.
  2. Incorrect relay sizing: The relay was rated for a 120A motor, but the actual motor current was measured at 112A, suggesting a possible undersizing or misapplication. This was confirmed through a review of the motor’s nameplate and relay specification sheet.
  3. Mechanical stress from vibration: The vibration data indicated a 12% increase in amplitude, which could cause mechanical misalignment or stress on the relay’s internal components, leading to false tripping. This was corroborated by a vibration analysis using a Keysight 35670A analyzer.
  4. Improper installation or mounting: The relay was not properly mounted, leading to poor heat dissipation and increased thermal loading. This was identified through a visual inspection and measurement of the relay’s mounting clearance.

Corrective Actions

The following corrective actions were recommended to address the root causes:

  1. Install an ambient temperature control system: A forced-air cooling system was recommended to maintain the control panel temperature within the specified range of -25°C to +55°C. This includes the use of heat sinks and thermocouples for continuous monitoring.
  2. Replace the relay with a properly sized unit: The relay was replaced with a REXROTH ABZMS-35-1X/120F model that matches the motor’s rated current and environmental conditions. This ensures accurate thermal protection.
  3. Implement vibration isolation: The relay was mounted on vibration-damping mounts to reduce mechanical stress. This was verified using a vibration analysis tool before and after installation.
  4. Improve installation practices: The relay was re-mounted with proper clearance and heat dissipation paths. This was confirmed through a visual inspection and temperature measurement using an infrared thermometer.

Quick Diagnostic Checklist

The following checklist can be used by maintenance technicians in the field to quickly diagnose and resolve thermal relay nuisance tripping issues:

  • Measure the ambient temperature in the control panel using a thermocouple. Ensure it is within -25°C to +55°C.
  • Check the motor current using a clamp meter. Ensure it does not exceed 110% of the relay’s rated current.
  • Inspect the relay for signs of overheating, such as discoloration or deformation.
  • Measure the vibration amplitude using a vibration analyzer. Ensure it is within acceptable limits (≤ 12% increase).
  • Verify the relay’s mounting and clearance. Ensure there is no mechanical stress or obstruction.
  • Check the relay’s specification sheet to confirm it is properly sized for the motor and application.
  • Install a temperature monitoring system if ambient temperatures are consistently above the relay’s operating limit.
  • Replace the relay with a model that matches the motor’s rated current and environmental conditions.
  • Use vibration-damping mounts to reduce mechanical stress on the relay.
  • Perform a thermal imaging scan of the control panel to identify any heat buildup or insulation issues.
  • Review the relay’s installation documentation to ensure proper mounting and heat dissipation.
  • Document all findings and corrective actions for future reference and compliance.

Prevention Strategy

To prevent future thermal relay nuisance tripping, the following prevention strategy is recommended:

  1. Regular maintenance intervals: Schedule periodic inspections of the control panel and relay to ensure proper operation and environmental conditions. This should be done every 6 months or as specified in the relay’s maintenance manual.
  2. Condition monitoring: Implement a condition monitoring system that includes temperature and vibration sensors. This allows for early detection of abnormal conditions and proactive maintenance.
  3. Design improvements: Redesign the control panel to include improved heat dissipation, such as additional ventilation or heat sinks. This reduces the risk of overheating and extends the relay’s service life.
  4. Training and documentation: Provide training to maintenance personnel on proper relay installation, sizing, and troubleshooting. Ensure all documentation is up-to-date and accessible for reference.

Conclusion with CTA

Thermal relay nuisance tripping can be a significant challenge in industrial motor control systems, but with a systematic approach to root cause analysis and corrective actions, it can be effectively mitigated. Proper sizing, environmental control, and mechanical isolation are critical to ensuring reliable operation. For replacement parts and preventive components, visit the UNITEC-D E-Catalog to find the right solutions for your application.

References

  • ANSI/IEEE C57.91-2020: Guide for the Application of Transformers
  • ASME B31.3-2020: Process Piping
  • NFPA 70-2023: National Electrical Code (NEC)
  • REXROTH ABZMS-35-1X/120F Technical Manual
  • Failure Analysis Handbook, 3rd Edition
  • IEC 60947-4-1: Low-voltage switchgear and controlgear – Part 4-1: Contactors and motor-starters
  • ISO 5344:2016: Condition monitoring and diagnostics of machines
  • UL 508A: Standard for Safety for Industrial Control Panels

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