Diagnostic Guide for Untimely Triggering of Security Systems

1. Description of the Problem and Scope

This guide is intended for maintenance technicians faced with untimely and unjustified triggering of safety systems on industrial equipment. These events, referred to as “nuisance triggers” or “novel”s, can result in unplanned production downtime, significant loss of time and potentially an erosion of confidence in the integrity of safety systems. Rigorous diagnosis of these anomalies is essential to maintain the availability of installations and the effectiveness of protections.

Affected equipment includes machines with light curtains, safe door switches, emergency stops, safety zone scanners and any component involved in a safety chain conforming to EN ISO 13849 (PLr) or EN/CEI 62061 (SIL). This guide is particularly aimed at installations in the aeronautical and energy sectors where the reliability of security systems is critical.

Severity Classification of Untimely Trips:

• Critical: Unplanned shutdown of essential equipment or production line, directly impacting safety or operational continuity. Requires immediate intervention.
• Major: Frequent or recurring shutdown of non-essential equipment, affecting overall productivity. Requires rapid diagnostic planning.
• Minor: Sporadic triggering, with no immediate impact on production, but indicating potential degradation of the system. Requires planned investigation.

2. Safety Precautions

WARNING: Before any intervention, it is CRITICAL to apply the lock-out and de-lockout procedures (LOTO - Lockout/Tagout) in accordance with standard NF C 18-510 or the company's internal directives. Failure to do so could result in serious injury or death due to unexpected start-up or release of residual energy.

PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear insulating gloves (NF EN 60903), safety glasses (NF EN 166), safety shoes (NF EN ISO 20345) and appropriate work clothing. In the presence of electrical hazards, ensure that you have appropriate discharge equipment for capacitors or other energy accumulators.

RESIDUAL ENERGY: Check and discharge all residual energy (electrical, pneumatic, hydraulic, mechanical) before handling the components. Use a certified multimeter to check for absence of voltage (VAT) before any electrical manipulation. The NF EN 50110-1 standard is applicable for electrical safety.

WORK AT HEIGHT: If the intervention requires work at height, use certified equipment (scaffolding, baskets) and a safety harness complying with the NF standard EN 361.

3. Required Diagnostic Tools

The following tools are essential for effective and safe diagnosis of security systems.

4. Initial Assessment Checklist

Before initiating an in-depth diagnosis, a rigorous initial assessment helps to define the problem and guide the investigation.

5. Systematic Diagnostic Flowchart

This flowchart offers a structured approach to diagnosing unwanted triggering of security systems.

1. Symptom: Unintentional Safety System Triggering
   • Action: Collect initial information (Section 4).
   • If the problem is intermittent and without a clear reason:
     • Proceed to check the integrity of the wiring (Step 2).
   • If the problem is reproducible under certain conditions or after a specific action:
     • Proceed to check the sensors and alignment (Step 3).
   • If the problem occurs randomly and sporadically:
     • Proceed to review environmental interference (Step 4).
   • If the problem is constant and immediate upon reset:
     • Proceed to check the safety relay and input wiring (Steps 2 and 5).
2. Wiring Integrity Check
   • Action: Megohmmeter and Multimeter (Section 3).
   • Test:
     1. Check the continuity of all the conductors of the safety chain (sensor → relay → controller). Expected value: < 1 Ω.
     2. Measure the insulation resistance between each conductor and ground, as well as between adjacent conductors. Expected value: > 2 MΩ (ideally > 10 MΩ).
     3. Check the tightness of the connections in all terminal blocks, cable glands, and on sensor and relay connectors.
   • If continuity or insulation fault:
     • Probable Cause: Damaged wiring, poor connection, partial short circuit to ground.
     • Resolution: Replace faulty cable, tighten or clean connections.
   • If wiring is intact:
     • Proceed to check sensors and alignment (Step 3).
3. Sensor and Alignment Check
   • Action: Visual inspection, multimeter, safety loop tester, masking tape (for barriers).
   • Test:
     1. For position sensors (door switches, limit switches):
        • Check manual activation and deactivation. Measure the voltage across the sensor in active and inactive states. Expected: 24V DC / 0V DC or vice versa for OSSD outputs.
        • Ensure that the actuator travels correctly and that there is no play.
     2. For light curtains/laser scanners:
        • Check the alignment of the transmitter and receiver (LED indicators or specific manufacturer tools). The alignment must be perfect.
        • Clean the optics.
        • Test detection with reference objects. Check that the protection zone is clear.
        • Use the Safety Loop Tester to validate OSSD outputs.
     3. For all sensors: Examine the mounting bracket for vibrations or loosening (using the vibrascope).
   • If sensor misalignment, dirt or damage:
     • Probable Cause: Mechanical misalignment, optical contamination, physical damage of the sensor.
     • Resolution: Realign, clean, replace the sensor.
   • If sensor and alignment correct:
     • Proceed to review for environmental interference (Step 4).
4. Review of Environmental Interference
   • Action: Multimeter, oscilloscope, thermal camera, environmental monitoring.
   • Test:
     1. Electromagnetic Disturbances (EMI/RFI):
        • Use the oscilloscope to observe sensor signals and power supplies. Look for electrical noise (spikes, harmonics).
        • Identify the proximity of noise sources (frequency variators, motors, unshielded power cables, welding).
     2. Thermal Variations:
        • Use the thermal camera to identify hot spots on relays or in electrical cabinets.
        • Check the ambient temperature of the cabinet and sensors compared to the manufacturer's specifications (e.g. Class B according to NF EN 60068-2-1/-2-2).
     3. Vibration:
        • Use the vibration analyzer to quantify vibration levels on sensor mounts and cabinets. (Critical threshold > 5 mm/s RMS at 100 Hz).
     4. Humidity/Condensation:
        • Check the presence of condensation or humidity in the cabinets, sensor boxes (IP protection rating conforming to EN 60529).
   • If interference detected:
     • Probable Cause: EMI/RFI, local overheating, excessive vibration, humidity.
     • Resolution: Improve shielding, cable routing, grounding (NF C 15-100), ventilation, vibration isolation.
   • If no significant interference:
     • Proceed to safety relay check (Step 5).
5. Safety Relay or Controller Diagnosis
   • Action: Diagnostic software, multimeter, relay test replacement.
   • Test:
     1. Reading Diagnostics: Use the manufacturer's software to read error codes, input/output statuses and internal diagnostic messages from the safety relay/controller.
     2. Input Measurement: Check the input voltage on the safety relay terminals coming from the sensors. Ensure that voltage levels (eg: 24V DC ± 10%) comply with specifications (NF EN 60204-1).
     3. Output Test: For relays with OSSD outputs, measure the output signals in safe state and in working state.
     4. Test by Substitution: If possible and all other causes have been ruled out, replace the safety relay with a new one or a certified test relay to eliminate an internal fault.
   • If internal error code, voltages out of specification or malfunction after replacement:
     • Probable Cause: Internal failure of the safety relay, micro power outages, safety controller fault.
     • Resolution: Replace the safety relay, check the power supply to the cabinet.
   • If everything is compliant:
     • Revisit the previous steps with increased attention to detail. Consider consulting the equipment manufacturer.

6. Cause-Fault Matrix

This matrix summarizes the symptoms, probable causes and associated diagnostic tests, ordered by probability.

7. Root Cause Analysis for Each Defect

Understanding why a defect occurs is the key to lasting resolution and effective preventative measures.

7.1. Faulty Wiring and Connections

• Explanation: Cables subjected to repeated mechanical stress (bending, vibration) may suffer partial breakage of the conductors or degradation of the insulation. Loose or corroded connections increase electrical resistance, causing voltage drop or overheating. Insulation failure can create temporary short circuits or ground leaks, simulating a safe condition. Non-compliance with standards NF C 15-100 (electrical installations) or NF EN 60204-1 (electrical equipment of machines) can also be a cause.
• How to Confirm: Measure the continuity and insulation resistance (with megger at 500V DC or 1000V DC) on the cables and connectors. An insulation resistance less than 2 MΩ between conductor and ground indicates critical degradation. Observe voltage drops (multimeter) along the cable under load. Perform vibration tests (vibrascope) on cables passing over machines.
• Damage if not corrected: Risk of fire from overheating, complete safety circuit failure, premature wear of electronic components due to voltage fluctuations, and potentially loss of critical safety function.

7.2. Misalignment or Damage to Sensors

• Explanation: Safety sensors (light curtains, door switches) require precise alignment and physical integrity. A misalignment, even minimal (typically > 2-3 mm for a barrier), can result in loss of detection. Mechanical shocks, vibrations or contamination of the optics reduce their reliability. A faulty sensor may return incorrect information or be unresponsive, causing a trip. The NF EN 61496 (optoelectronic protection devices) and NF EN ISO 14119 (interlocking) standards are essential.
• How to Confirm: For optical sensors, check the alignment indicators (LEDs) and clean the optical surfaces. Use a test bench or reflector (for barriers) to validate range and detection. For door switches, check the mechanical play of the actuator. Measure the sensor OSSD outputs with a multimeter or oscilloscope to validate switching (safety voltage 24V DC ± 10%, safe state 0V).
• Damage if not resolved: Frequent production stoppages, possible bypass of the safety system (if disabled to avoid unwanted triggering), or in the worst case, a failure of the protection leading to personal accidents.

7.3. Environmental Interferences

• Explanation: Electromagnetic fields (EMI/RFI) generated by frequency converters, powerful motors or unshielded power cables can induce spurious signals in safety cables. Extreme variations in temperature or humidity can impair the performance of sensors and electronic components, or result in condensation creating conductive paths. Excessive vibration can cause false contact. These phenomena are all the more critical in difficult industrial environments, such as those encountered in aeronautics or energy.
• How to Confirm: Use an oscilloscope to view noise on safety signals. Measure electromagnetic fields with a suitable probe. Check ambient temperature and humidity with recorders. Monitor vibrations using an analyzer. The acceptable thresholds for electromagnetic emissions are defined by the NF standard EN 61000-6-4.
• Damage if not resolved: Random and recurring triggering, reduction in the life of electronic components, and in extreme cases, disruption of safety communications.

7.4. Safety Relay or Controller Failure

• Explanation: Safety relays or controllers are critical components that monitor sensor inputs and activate safety outputs. An internal failure (electronic component, software, contact wear for electromechanical relays) can generate a safe state for no reason. Micro power outages can disrupt their operation or cause them to go into fault mode. Aging of components, overvoltages, or incorrect programming (initial commissioning or modification) can also be causes. Compliance with the NF EN 60947-5-1 (low voltage switchgear) and EN ISO 13849-1 standards is essential.
• How to Confirm: Use the manufacturer's diagnostic software to query internal error codes. Check the relay supply voltages. For relays with contacts, check the voltage and continuity of the contacts (with the multimeter) when they are loaded. If all else is ruled out, temporary replacement with a test relay is the most reliable method.
• Damage in the event of non-resolution: Unjustified machine stoppages, inability to reset the system, and in the event of an unsafe failure, risk of loss of protection.

8. Step-by-Step Resolution Procedures

The following procedures detail corrective actions for each identified root cause.

8.1. Troubleshooting Wiring and Connections

1. Confirmation of the Lockout: Check the disconnection of all energy sources and the absence of voltage (VAT). [SAFETY: NF C 18-510]
2. Inspection and Cleaning: Visually inspect the entire safety cable routing. Clean the terminal blocks and connectors with a suitable dielectric cleaner (e.g. KONTAKT CHEMIE KONTAKT 60).
3. Tightening the Connections: Using a torque screwdriver, tighten all the screws on the terminal blocks and connectors. Typical tightening torque: 0.5 Nm to 1.2 Nm for small sections, according to the terminal manufacturer's recommendations.
4. Replacement of Defective Wiring: If a cable has an insulation resistance < 2 MΩ or signs of physical damage, replace it with a shielded cable of type LiYCY or equivalent, with a section of 0.75 mm² minimum, and a braided shield connected to ground at one end (cabinet side).
5. Routing Optimization: Separate security cables from power cables (minimum distance of 30 cm) or use separate cable trays to reduce interference. Use earthed metal trunking.
6. Post-Repair Check: Carry out a new continuity and insulation measurement on the repaired circuit.

8.2. Troubleshooting Sensors and Alignment Issues

1. Confirmation of Lockout: Check the disconnection of all energy sources. [SAFETY: NF C 18-510]
2. Cleaning the Sensors: For optical sensors, gently clean the optical surfaces of the transmitter and receiver with a soft cloth and a non-abrasive optical cleaner.
3. Realignment of Light Curtains/Scanners:
   • Use the sensor's alignment indicators (LEDs) to achieve optimal alignment. For SICK barriers, the alignment must be "solid green".
   • Adjust the mounting brackets with millimeter precision. Use a laser level if necessary.
   • Check the admissible angular deviation (generally ± 2.5° for optical barriers).
4. Adjusting the Door Switches/Position:
   • Adjust the position of the switch or its actuator to ensure proper and complete engagement.
   • Verify that the actuator travel is sufficient for both active and inactive states, without hard spots or excessive play.
   • Check contact overtravel if specified by the manufacturer (typically 1-2 mm).
5. Replacing the Damaged Sensor: If the sensor is physically damaged or if it does not respond correctly to the tests, replace it with an identical or equivalent model, CE certified and compliant with EN 60947-5-1 and EN ISO 13849. standards
6. Post-Repair Check: Carry out a complete functional test of the sensor (object detection, door opening/closing) and validate the feedback to the safety relay.

8.3. Troubleshooting Environmental Interference Issues

1. Confirmation of Lockout: Check the disconnection of all energy sources. [SAFETY: NF C 18-510]
2. Shielding Improvement:
   • Replace unshielded cables with high quality shielded cables for security signals. The shield must be grounded at only one end (cabinet side) to avoid ground loops.
   • Install EMC (Electromagnetic Compatibility) filters on the power supplies of frequency variators or other disturbing equipment.
3. Optimization of Grounding:
   • Check the quality of the grounding of the equipment and electrical cabinets. The earth resistance must be less than 1 Ω (NF C 15-100).
   • Use short, large-section ground braids to connect the equipment to the main earth.
4. Thermal Management:
   • Install ventilation systems, air conditioning or heat exchangers in electrical cabinets to maintain the temperature below 35°C, in accordance with the recommendations of electronics manufacturers.
   • Clear cabinet vents.
5. Vibration Isolation: Install anti-vibration rubber or polymer supports under the sensors and their supports to absorb shocks. Ensure that the vibration level on the sensor does not exceed 2 mm/s RMS.
6. Humidity Control: Install heaters or dehumidifiers in cabinets in humid environments to avoid condensation. Ensure the integrity of the seals (IP54 minimum).
7. Post-Repair Verification:Remeasure the signals with the oscilloscope after modifications to confirm noise reduction.

8.4. Troubleshooting Safety Relay or Controller Problems

1. Confirmation of Lockout: Check the disconnection of all energy sources. [SAFETY: NF C 18-510]
2. Replacement of the Safety Relay: If the diagnosis (via software or manual tests) indicates an internal failure, replace the safety relay with a strictly identical model or a functional and certified equivalent (CE, TÜV, UL), respecting the required performance level (PLr) or SIL.
3. Checking the Power Supply: Measure the power supply voltage of the safety relay. It must be stable and conform to the manufacturer's specifications (eg: 24V DC ± 10%, without excessive ripple < 50mVpp). Install an inverter (UPS) or filter if the power supply is unstable.
4. Programming Verification: If a programmable safety controller (safety PLC) is used, check the safety logic in the software. Ensure that no unauthorized changes or configuration errors are present. Compare the current version with the last validated version.
5. Firmware Update: In some cases, updating the relay or controller firmware may resolve known bugs. Consult the manufacturer for the latest versions and update procedures.
6. Post-Repair Verification: Perform a complete test sequence of the safety system after replacement or modification, including emergency stop tests, light curtain tests, and rearm tests. Check for new error codes.

9. Preventive Measures

The application of preventive measures is essential to reduce the frequency of nuisance triggering and extend the lifespan of security systems.

10. Spare Parts and Components

The availability of certified spare parts is essential to minimize downtime during a failure.

For a complete catalog of our certified spare parts and to order, please consult our e-catalog UNITEC-D.

11. References

• NF C 18-510: Operations on electrical works and installations.
• NF C 15-100: Low voltage electrical installations.
• EN ISO 13849-1: Safety of machinery - Safety-related parts of control systems - Part 1: General design principles. (Performance Level, PLr)
• EN/CEI 62061: Machine safety - Functional safety of safety-related electrical, electronic and programmable electronic control systems. (Safety Integrity Level, SIL)
• NF EN 60204-1: Safety of machines - Electrical equipment of machines - Part 1: General requirements.
• NF EN 60947-5-1: Low voltage switchgear - Part 5-1: Switching devices and elements for control circuits - Electromechanical devices for control circuits.
• NF EN 61496: Machinery safety - Electrosensitive protective equipment (EPE).
• NF EN ISO 14119: Safety of machines - Interlocking devices associated with guards - Principles of design and choice.
• NF EN 60068-2-1/-2-2: Environmental tests - Cold / Dry heat.
• NF EN 60529: Degrees of protection provided by the enclosures (IP code).
• NF EN 61000-6-4: Electromagnetic compatibility (EMC) - Part 6-4: Generic standards - Immunity for industrial environments.
• Diagnostic and installation manuals from manufacturers (PILZ, SICK, Rockwell Automation, Siemens, Leuze, SCHMERSAL).