1. Problem description & scope of application
False activations in safety systems represent a significant challenge in industrial production. These unplanned shutdowns of machines and systems, often without any apparent immediate reason, lead to significant losses in productivity, increased maintenance costs and can undermine the acceptance of safety devices by operating personnel. This guide addresses symptoms such as sporadic or unexplained machine downtimes triggered by safety door switches, light curtains, emergency stop buttons or other safety-related sensors and the associated safety relays. The affected system types include manufacturing machines, robotic cells, machining centers and assembly systems that are equipped with safety devices in accordance with the Machinery Directive 2006/42/EC and the DIN EN ISO 13849-1 or DIN EN 62061 standards.
The diagnosis focuses on the integrated chain from the sensors to the cabling to the safety relay in order to identify the exact cause of the false trip. The severity of these disruptions is classified as critical because they not only affect operational availability but, in the worst case, could indicate an impairment of the safety function.
2. Safety precautions
IMPORTANT: SAFETY FIRST!
Before starting any diagnostic or maintenance work on safety systems, the machine must be completely secured in accordance with the applicable regulations (e.g. DGUV regulation 3) and internal operating procedures. This includes:
- Lockout/Tagout (LOTO): The machine or the affected part of the system must be isolated from all energy sources (electrical, hydraulic, pneumatic, mechanical) and secured against being switched on again. Locking and labeling the energy isolators is mandatory.
- Check for absence of voltage: Before touching electrical components, ensure that there is no voltage using a suitable and tested voltage tester (e.g. according to VDE 0682).
- Residual energy: Particular caution is required when dealing with potential residual energy (e.g. stored hydraulic pressure, tensioned springs, rotating masses). This must be dismantled or secured in a controlled manner before work begins.
- Personal protective equipment (PPE): Always wear the prescribed PPE, such as safety glasses, safety shoes and, if necessary, insulating gloves, especially when working on electrical systems.
- Hazardous substances: Pay attention to the handling of potentially hazardous substances (e.g. coolants, lubricants) and wear appropriate protective gloves and respiratory masks if necessary.
3. Required diagnostic tools
| Tool | Specification/Model | Measuring range/setting | Purpose |
|---|---|---|---|
| Digital multimeter (DMM) | Min. CAT III 1000V, true RMS value | Voltage: 0-1000V AC/DC, current: 0-10A AC/DC, resistance: 0-40 MOhm, continuity test, capacity, frequency | Checking power supply (e.g. 24V DC ±10%), signal integrity, resistance, continuity of cables |
| Oscilloscope | Min. 100 MHz bandwidth, 2 channels, isolated channels preferred | Voltage: 1mV/Div to 50V/Div, time base: 5ns/Div to 1s/Div (for fast signals), trigger on edge change | Analysis of dynamic sensor signals, detection of EMC interference, glitches, bounce behavior of contacts, monitoring of signal levels |
| Laser alignment tool/alignment gauge | For optical sensors (e.g. light grids, retro-reflective sensors), precision gauges for mechanical switches | Alignment accuracy ±0.1 mm | Precise adjustment of optical sensors and reflective surfaces. Checking the correct approach path of mechanical switches. |
| Vibration meter | Handheld, measurement of acceleration, speed and distance (RMS) | Speed: 0-200 mm/s RMS (frequency range 10-1000 Hz) | Detection of mechanical resonances or vibrations that can affect sensors, cables or connectors. (Thresholds after VDI 2059) |
| Thermography camera | Resolution min. 160x120 pixels, sensitivity < 0.05 °C at 30 °C | Temperature range: -20 °C to +350 °C, set the emissivity correctly (e.g. 0.95 for painted surfaces) | Identification of overheating in electrical panels, terminals, cables, motor windings or poor contact points; Detection of hotspots |
| Insulation measuring device (megohmmeter) | Test voltage: 250V, 500V, 1000V DC (according to VDE 0100 Part 600) | Measuring range: 0-2 GOhm, limit > 2 MOhm for safe isolation | Checking the insulation quality of cables and components against earth or between conductors. Detection of leakage currents. |
| Safety relay diagnostic software/interface | Manufacturer-specific (e.g. Siemens Safety ES, Pilz PSSup, Festo Safety Editor) | Read out diagnostic buffer, compare/change parameterization, firmware updates | Reading out diagnostic buffers, error codes, status information; Comparison and reprogramming of the parameterization. |
| Endoscope / inspection camera | Flexible probe, min. 5 mm diameter, integrated lighting | Visual inspection of hard-to-reach areas (e.g. inside machines, cable ducts) for damage or contamination. |
4. Checklist for initial assessment
Before starting the detailed diagnosis, a careful assessment of the initial situation is crucial. This checklist helps gather important information that can significantly shorten the diagnostic process.
| Checkpoint | Description/Action | status | Notes |
|---|---|---|---|
| Machine status before fault | What was the machine doing immediately before it was triggered? (Production, setup process, standstill, partial load, full load, axis movement, etc.) | Document the specific operating mode. Was an operator in the danger zone? | |
| Error history / alarm messages | Are there recurring or similar alarm codes in the machine system or on the safety relay? (e.g. overflow counters for trips, specific error codes) | Note the date, time, exact error code (e.g. F031, E-STOP active) and the frequency of events. Read out diagnostic buffer. | |
| Environmental conditions | Abnormalities such as extreme temperature changes, vibrations, increased humidity, accumulation of dirt, strong external light (with optical sensors), electromagnetic fields (due to new devices nearby)? | Measurement of ambient temperature (°C), relative humidity (%), visual inspection for contamination. | |
| Final maintenance work | Have there been any recent changes to the machine, wiring, controls, sensors or guards? (e.g. sensor replacement, cable laying, software update) | Check maintenance logs and shift books. Contact the executive staff. | |
| Operator input/incorrect operation | Were there any indications of improper operation, bypassing of safety devices or unusual behavior by the operating personnel? | Surveying the operating staff (discreetly and systematically). | |
| Visual inspection | External damage to sensors, cables (insulation damage, pinch points), connectors, protective devices (protective doors, covers), control cabinet (doors, cooling)? | Visually check the entire course of the safety circuit from sensor to relay. | |
| Supply voltage | Stable supply voltage for safety circuit and sensors? (24V DC ±10% according to manufacturer's specifications) | Measurement with DMM directly on the safety relay and on a reference sensor. Documentation of voltage dips or fluctuations. |
5. Systematic diagnosis – decision tree
This decision tree guides the technician through a structured process to identify the root cause of false trips.
- Symptom: The security system triggers unplanned, for no apparent reason.<ol type=
