1. Description of the problem and scope of application
This manual is intended for diagnosing erratic or jumpy signals from industrial sensors (4-20mA, 0-10V, HART, digital buses). The problem is classified as critical, as it leads to false activations of automation, stoppage of production lines and the risk of equipment damage. The manual covers pressure, temperature, flow and level sensors integrated into PLC-based ACS systems.
2. Safety measures
WARNING: Working with electrical systems poses a risk of electric shock. Before starting diagnostics, it is necessary to perform the LOTO (Lockout/Tagout) procedure. Use personal protective equipment (PPE): dielectric gloves (class 0), safety shoes and glasses. Remember the presence of stored energy in capacitors and power batteries.
3. Necessary diagnostic tools
| Tool | Characteristics | Purpose |
|---|---|---|
| Digital multimeter | True RMS, CAT III 1000V/CAT IV 600V | Measurement of voltage, current and resistance |
| Megaohmmeter | The test voltage is 50-500 V DC | Checking the integrity of cable insulation |
| Oscilloscope | Portable, minimum 2 channels, 100 MHz | Detection of EMI/RFI interference |
| Thermal imager | Resolution from 160x120 | Search for hot spots in connectors and terminals |
4. Initial assessment checklist
| Parameter | What to check/record |
|---|---|
| Working conditions | Does the instability coincide with starting large motors or variable frequency drives (VFDs)? |
| Recent changes | Has cable work, installation of new equipment or maintenance been carried out? |
| History of accidents | Are there any errors in the PLC event log (timeouts, output range)? |
| External review | The presence of mechanical damage to the cables or corrosion in the terminal boxes. |
5. Systematic diagnosis
- Checking the power supply:
- Measure the supply voltage of the sensor. If ripple > 100 mV, check the power supply filters.
- Ground integrity analysis:
- Check the resistance between the sensor housing and the PE bus. Resistance should be < 1 ohm. High resistance indicates oxidation or a break in the ground circuit.
- EMI/RFI detection:
- Connect the oscilloscope parallel to the signal line. If high-frequency spikes > 500 mV are present, look for sources of interference (VFD, power cables near signal cables).
- Cable line diagnosis:
- Unplug the cable from both sides. Check the insulation resistance (to the screen and between the cores). A value < 100 MΩ indicates insulation degradation.
6. Malfunction matrix
| Symptom | Probable reasons | Diagnostic test | Expected result |
|---|---|---|---|
| Noisy signal | EMI/RFI interference | Oscilloscope on the signal line | Disappearance of VFD shutdown noises |
| Zero/Scale Shift | Oxidation of contacts | Measurement of loop resistance (Loop resistance) | < 20 Ohms (for standard loop) |
| Periodic jumps | Ground loop (Ground loop) | Current measurement between screens | The current should be 0 mA |
| Complete rejection | Insulation degradation | Megaohmmeter | > 100 MΩ |
7. Analysis of root causes
Electromagnetic interference (EMI/RFI): Occur when laying signal cables in common cable channels with power lines (over 400 V). High-frequency switching of frequency converters induce voltage in the signal pairs. Confirmation: Oscillogram analysis during VFD operation.
Ground loops (Ground loops): Occur when the cable screen is grounded on both sides at points with different potentials. This creates a current through the screen that distorts the signal. Confirmation: Current measurement in the cable screen.
Cable degradation: Under the influence of an aggressive environment (oil, moisture, vibration), the insulation loses its dielectric properties. Confirmation: The megohmmeter shows low insulation resistance.
8. Troubleshooting procedures
- Shield ground: If a ground loop is detected, disconnect the shield from one side (usually the sensor side). Make sure the shield is securely connected to the PE at only one point (on the controller).
- Cabling: Ensure a minimum distance of 300 mm between signal and power cables (according to EN 61000).
- Cable replacement: When low insulation resistance is detected, replace the cable with a shielded cable of the appropriate cross-section (for example, 2x2x0.75 mm²). Use only certified hermetic cable glands.
9. Preventive measures
| The reason | Strategy | Monitoring method | Periodicity |
|---|---|---|---|
| Oxidation | Use of anti-corrosion sprays | Visual inspection of the terminals | 6 months |
| Vibration | Checking the tightening torques | Checking with a torque wrench | 12 months |
| Guidance | Distribution of cable routes | Signal spectrum analysis | During modernization |
10. Spare parts and components
| Description of the part | Specification | When to replace | Category UNITEC |
|---|---|---|---|
| Shielded cable | LiYCY 2x0.75mm² | When the insulation is damaged | Cable products |
| Hermovvod | M20 IP68 | In case of loss of tightness | Mounting components |
| Terminal blocks | DIN-rail, spring | When the contacts are oxidized | Electrical engineering |
The complete range of components is available in our catalog: https://www.unitecd.com/e-catalog/
11. Links
DSTU EN 61000-6-2: Electromagnetic compatibility. Part 6-2. Standards for general requirements. Resistance to interference.
ISO 9001: Quality management system.
