Introduction: failure symptoms in Ukrainian plant facilities
In heavy industry and machinery manufacturing plants in Ukraine, emergency stoppages of equipment due to false tripping of protective devices account for up to 35% of all unplanned line downtimes. Typical scenario: the main drive of a conveyor or pump station suddenly stops during a technological cycle without visible signs of overcurrent. The shift electrician notes tripping of a bimetallic thermal relay or an electronic overload relay. Measurement of the operating current with current clamps shows values not exceeding the motor's nominal current ($I_n$). Attempts at immediate restart often result in repeated emergency shutdowns within 3–5 minutes. Similar incidents lead to loss of productivity, material waste in continuous processes, and premature wear of switching devices.
In this technical document, we will conduct a technical expert analysis of the causes of similar failures using the example of the interaction between power circuits and position sensors, particularly inductive sensors such as Schneider Electric XS212BLPBM12, which operate in harsh conditions of Ukrainian workshops with temperature fluctuations from $-25\,^\circ{\text{C}}$ to $+55\,^\circ{\text{C}}$.
Component Overview and Its Operating Environment
Overload relays are designed to protect squirrel cage induction motors from prolonged overloads, phase loss and current asymmetry. They are installed directly after the contactor in the AC power circuit (standards DSTU EN 60947-4-1, IEC 60947-4-1).
The standard operating cycle involves operation in an environment with nominal ambient temperature of $+20\,^\circ ext{C}$ or $+30\,^\circ ext{C}$ depending on the manufacturer's specification. However, actual operating conditions on Ukrainian plants (metallurgical combines in Zaporizhzhia, mining and enrichment combines in Kryvyi Rih, enterprises in the chemical industry in Cherkasy) significantly differ from laboratory conditions. Enclosed electrical panels installed near furnaces or hot pipelines create a microclimate with internal temperature above $+50\,^\circ ext{C}$. Under such conditions, calibration of bimetallic plates loses accuracy.
| Parameter | Nominal value | Actual value in the workshop | Limiting permissible value |
|---|---|---|---|
| Ambient temperature of the enclosure | +20 °C | $+42\,^\circ{\text{C}}$...$+58\,^\circ{\text{C}}$ | $+60\,^\circ{\text{C}}$ (IEC 60947) |
| Working current ($I_{load}$) | $24.5\ \text{A}$ | $24.0\text{ A}$ | $25.0\ \text{A}$ ($I_n$ двигатуна) |
| Engine start time ($t_{start}$) | $2.5\text{ s}$ | $7.8 ext{ s}$ | $10.0\text{ A}$ (class of cutter 10A) |
| Phase Asymmetry | < 1% | 4.5% | < 2% |
Diagnostic Data and Evidence Base
During the on-site inspection of the compressor station in the Dnipro region, a series of false trips of the main compressor motor with power $37 ext{ kW}$ ($I_n = 68 ext{ A}$) were recorded. Analysis of the event logs of the controller and monitoring system revealed the following facts:
- Vibration on the engine housing: $2.8 ext{ mm/s}$ RMS (within normal limits according to ISO 10816-3 for class II).
- Temperature inside the distribution cabinet at the level of the thermal relay: $+52.4\,^\circ ext{C}$.
- Starting current oscillograms showed an extended acceleration time due to voltage drop in the plant network to $372 ext{ V}$ (a drop of $14\%$ from nominal $400 ext{ V}$).
- Position sensors of the mechanism, specifically inductive switches Schneider Electric XS212BLPBM12, signaling the opening of the relief valve, were affected by harmonic distortions from the frequency converter ($THD_u > 8\%$), causing micro-pulses in the control circuits and erroneous reverse commands.
Fixed mean time between failures (MTBF) for the protection unit under uncompensated temperature conditions decreased from the rated $150\,000$ hours to $1\,200$ hours.
Systematic analysis of root causes (Ishikawa Methodology)
To identify the root causes of degradation of the protection system, the Ishikawa diagram was applied with four main vectors: environment, equipment, personnel, and calculation methodology.
1. Environmental Factors
Thermal relay responds to total heat: generated by internal current and obtained from the surrounding environment. An increase in temperature in the enclosure above $+40\,^\circ ext{C}$ causes the bimetal to bend before reaching the nominal current. The current trip correction factor at $+50\,^\circ ext{C}$ is approximately $0.88$ of the nominal value.
2. Design and Sizing Errors
Engineers often select thermal relays based on the nominal motor current, without considering the starting profile (class of interruption 10, 20, 30 according to DSTU EN 60947-4-1), the presence of frequent start-stop cycles (more than 15 starts per hour) and the load characteristics (inertial fans, heavy crushers).
3. Power Quality (Power Quality)
Uneven loading across phases (voltage asymmetry) creates a reverse sequence of currents in the stator windings. Even with a small voltage asymmetry of $3\%$, current asymmetry can reach $15-20\%$, overheating one of the phase bimetallic plates.
Root Causes Identified (Ranked List)
- Control cabinet overheating (Probability: 45%). Absence of forced ventilation and conditioning in power cabinets when the external temperature in the workshop exceeds $+35\,^\circ ext{C}$.
- Misclassification of the coupling class to the operating mode (Probability: 25%). Use of class 10 relays on mechanisms with heavy start (start time over $10$ seconds).
- Voltage drop in the network and harmonics (Probability: 20%). Increase in engine acceleration time due to voltage sags in plant networks $0.4 ext{ kV}$.
- Mounting errors and transition resistances at terminals (Probability: 10%). Oxidation or insufficient tightening torque of power terminals ($M_A < 2.5 ext{ Nm}$), leading to local heating due to transition resistance and heat transfer to the bimetal relay.
Corrective and Preventive Actions
To eliminate the detected defects and prevent accidents on production lines, it is necessary to implement a complex set of technical solutions:
- Emergency (Immediate Actions): Reduce the operating relay setting taking into account the ambient temperature compensation according to the manufacturer's nomogram or install a relay with ambient temperature compensation (range from $-20\,^\circ ext{C}$ to $+60\,^\circ ext{C}$). Check the tightening torque of all power contacts using a torque wrench ($2.5 ext{ Nm}$ for standard M4 screws).
- Довгострокова модернізація: Встановити в розподільних шафах системи клімат-контролю (фільтровентиляційні модулі з термостатами або промислові кондиціонери) для підтримки стабільної температури $+25\,^\circ ext{C} \pm 5\,^\circ ext{C}$. Замінити застарілі електромеханічні реле на сучасні цифрові блоки захисту двигунів з вимірюванням справжніх середньоквадратичних значень (True RMS) та можливістю підключення через протоколи Modbus RTU для інтеграції в АСТП.
- Replacement of related components: In case of failure of control system elements, use CE certified industrial sensors and switching devices selected through UNITEC-D E-Catalog to ensure compliance with European and Ukrainian regulations.
Quick diagnostic card for shop floor personnel (Tablet-compatible)
This checklist is intended for engineers and maintenance electricians during an express diagnosis on site:
- [ ] Step 1. Cabinet temperature check: Measure the air temperature inside the cabinet at the level of the upper edge of the starter (norm: $< +40\,^\circ ext{C}$).
- [ ] Step 2. Load current check: Measure the currents in all three phases (A, B, C) using current clamps (the degree of asymmetry should not exceed $2-3\%$).
- [ ] Step 3. Terminal connection inspection: Perform thermal imaging inspection of contact connections for the presence of hot spots ($\Delta T > 15\,^\circ ext{C}$ relative to the busbar).
- [ ] Step 4. Start-up characteristic analysis: Measure the duration of the start-up period of the motor ($t_{start}$) using a digital oscilloscope.
- [ ] Step 5. Control circuit check: Test the stability of signals from position sensors for false triggering caused by electromagnetic interference.
- [ ] Step 6. Overcurrent Check: Check the conformity of the current rating of the thermal relay to the motor's nameplate current, taking into account the service factor (SF).
- [ ] Step 7. Inspection of the bimetal: Check for the absence of mechanical damage or corrosion traces on the housing of the protective device.
- [ ] Step 8. Testing the Test/Reset button: Check the mechanical operation of the signal contact group (97-98).
Failure Prevention Strategy
To ensure the continuity of industrial production processes in Ukrainian enterprises, it is necessary to implement a three-level maintenance system:
- Preventive Maintenance (PM): Quarterly inspection and torqueing of power terminals with torque wrench, cleaning of ventilation filters in distribution cabinets.
- Condition monitoring (PdM): Installation of temperature sensors inside cabinets with transmission of alarm signals to the upper level SCADA system upon reaching the limiting temperature $+45\,^\circ ext{C}$. Periodic thermal imaging inspection of electrical equipment (no less than 2 times per year).
- Spares Management: Constant availability of critical protection and switching elements on stock, selected taking into account real operating conditions through UNITEC-D E-Catalog.
Conclusion
False tripping of thermal relays is not an inevitable consequence of equipment wear. They occur due to non-compliance with temperature regimes inside switchgear, errors during initial selection of equipment, and neglect of actual parameters of the power supply network. A systematic approach to diagnostics, including thermal imaging, harmonic analysis, and adjustment of settings taking into account the influence of the environment, allows reducing the number of emergency stoppages by $80\%$. For selection of reliable components and parts, replace outdated units using UNITEC-D E-Catalog.
Normative references and literature
- DSTU EN 60947-4-1:2014 «Low-voltage switchgear and controlgear. Part 4-1. Electromechanical contactors and motor starters».
- IEC 60947-4-1:2018 «Low-voltage switchgear and controlgear - Part 4-1: Contactors and motor-starters - Electromechanical contactors and motor-starters»
- ISO 10816-3:2009 «Mechanical vibration - Evaluation of machine vibration by measurements on non-rotating parts».
- Technical documentation and operating instructions from Schneider Electric regarding protection of TeSys series electric motors.
- Industrial Equipment Failure Analysis Handbook, UNITEC-D Engineering Handbook, 2023.
