Electrical Cell Maintenance: Arc Chamber Inspection, Contact Resistance Measurement and Trip Unit Testing

1. Scope and Purpose

This practical guide details the essential procedures for preventive and predictive maintenance of medium and low voltage electrical cells. It focuses on three critical tasks: inspection of arc quenching chambers, resistance measurement of main contacts, and functional verification of trip units. The primary objective is to ensure the operational reliability of the equipment, prolong its useful life and prevent catastrophic failures that could result in production interruptions or accidents. This maintenance is critical in routine inspection programs, after a failure (trip) event, or prior to commissioning of new or relocated equipment. The application of these procedures contributes to compliance with electrical safety regulations such as EN 50110-1 (Operation of electrical installations) and UNE 20460-100 (Low voltage electrical installations).

2. Safety Precautions

🚨 DANGER: Lockout/Tagout (LOTO) Mandatory. Before starting any work, the equipment must be de-energized, locked and tagged in accordance with internal plant procedures and UNE-regulationsEN 50110. Verify the absence of voltage in all phases and control circuits with a calibrated voltage detector.

⚡ ELECTRICAL RISK: Capacitor Discharge. Make sure that all capacitors in the control or power circuits are completely discharged. Use insulated tools and temporary grounding equipment.

⚠️ CRITICAL PERSONAL PROTECTIVE EQUIPMENT (PPE): Wear dielectric gloves (class appropriate to rated voltage), safety glasses, safety helmet, steel-toed, dielectric-soled safety shoes, and flame-retardant clothing (ARC Flash category appropriate) at all times.

🚫 DO NOT WORK UNDER VOLTAGE: Unless absolutely essential and under a safe work procedure approved and supervised by qualified personnel, avoid working on energized circuits.

3. Tools and Materials Required

The correct execution of tasks depends on the availability and calibration of the appropriate tools.

4. Pre-Maintenance Inspection Checklist

Before de-energizing the equipment, perform a preliminary visual inspection to identify obvious problems.

5. Step by Step Procedure

Step 1: Area Preparation and Safety

1. De-energization: Follow the plant LOTO procedure to completely de-energize the switch or electrical cell. This includes opening circuit breakers on the primary and secondary side, and disconnecting any control power sources.
2. Verification of Absence of Voltage: Use a calibrated voltage detector to verify the absence of voltage in all phases and connection points of the equipment. Check the detector before and after use on a known voltage source.
3. Discharging Capacitors: If the equipment contains capacitors (e.g. in control circuits or trip mechanisms), ensure they are completely discharged by using a suitable discharge rod and checking with a voltmeter.
4. Temporary Grounding: Install temporary grounds on all phases and at appropriate connection points to protect against accidental re-energization or induced voltage.
5. Delimitation of the Area: Mark and delimit the work area to prevent access by unauthorized personnel.

Step 2: Inspection and Cleaning of Arc Extinguishing Chambers (Arc Chutes)

Arc chambers are critical components that dissipate arc flash energy during current interruption. Its condition is essential for the performance of the switch.

1. Access to the Arc Chambers: Remove the protective covers or remove the switch depending on the equipment design. Always consult the manufacturer's manual.
2. Detailed Visual Inspection: Examine each arc chamber with a powerful flashlight. Look for signs of:
   • Severe Wear or Erosion: Burned, melted or warped metal plates.
   • Cracks or Fractures: In insulating materials or plates.
   • Carbon and Residue Accumulation: Black, powdery deposits on the insulating surfaces and between the plates.
   • Misalignment or Fixation: Loose, bent or poorly spaced plates.
   Common mistakes: Ignoring small accumulations of soot; not inspecting all slots between plates; force the plates during disassembly.
3. Cleaning:
   1. Use an industrial vacuum cleaner with a HEPA filter to remove dust and loose debris.
   2. Apply dry, clean (oil-free) compressed air at low pressure to dislodge particles from narrow grooves.
   3. Spray a quick-drying dielectric cleaner on contaminated surfaces and gently wipe with lint-free cloths until carbon deposits are removed. Make sure the cleaner is compatible with the camera materials.
4. Distance and Alignment Check: Make sure the plates are properly spaced and aligned. Use a clearance gauge if the manufacturer's manual specifies clearance values.
5. Replacement: If severe wear, cracks, or significant structural damage is observed, the entire arc chamber or its damaged components should be replaced.

Step 3: Contact Resistance Measurement

The resistance of the main contacts is a critical indicator of their condition. High resistance can cause overheating and failure.

1. Equipment: Use a 4-wire micro-ohmmeter (DLRO - Digital Low Resistance Ohmmeter). This method compensates for the resistance of the test leads, providing an accurate measurement.
2. Contact Preparation: If possible, clean the surfaces of the moving and fixed contacts with a dielectric cleaner and a soft cloth. In some designs, the contacts may be inaccessible without major disassembly.
3. Micro-ohmmeter connection:
   1. Connect the current terminals (generally the thickest ones) to both sides of the main contact to be measured.
   2. Connect the potential (thinner) terminals as close as possible to the actual contact surfaces, inside the switch terminals.
   3. Ensure a firm and clean connection to avoid measurement errors.
   Common errors: Loose or dirty connections; do not use the 4-wire method; measure at incorrect points including unwanted wiring resistance.
4. Performing Measurement:
   1. Select the appropriate test current range (e.g. 10A, 100A or 200A depending on the DLRO and switch size). A higher current helps break down surface oxide films.
   2. Activate the measurement and wait for the value to stabilize. Record the value in micro-ohms (µΩ) for each phase.
5. Results Analysis:
   • Typical Values: For well-maintained switch main contacts, values ​​are usually below 100 µΩ. However, always refer to the values ​​specified by the manufacturer for your specific model.
   • Comparison: Compare the values ​​obtained with the factory data, previous tests or with the values ​​of the other phases of the same switch. A deviation of more than 20-30% from these reference values ​​indicates a potential problem.
   • Corrective Action: If resistance is high, inspect contacts for corrosion, pitting, or misalignment. Consider cleaning, retightening connections, or replacing contacts if necessary.
   Common errors: Interpreting a high value without verifying the integrity of the test connections; do not consider the ambient temperature in the measurement.

Step 4: Trip Unit Testing

The trip unit is the "brain" of the switch, responsible for protecting the system against overloads and short circuits. Its correct operation is vital.

1. Equipment: Use a calibrated trip unit test kit that can inject primary or secondary current.
2. Connection: Connect the test kit to the trip unit according to the kit and switch manufacturer's instructions. This may involve connection to the primary current injection terminals or to the secondary test terminals on the trip unit itself.
3. Test Settings: Set the test parameters based on the manufacturer's trip curves for the specific breaker model. This includes:
   • Long Time (LT): Current and trip time for sustained overloads.
   • Short Time (ST): Current and trip time for delayed short circuit faults.
   • Instantaneous (Instantaneous - I): Trip current for severe short circuits without delay.
   • Ground Fault (GF): If applicable.
4. Performing Tests and Recording:
   1. Start the test sequence, injecting the specified current and measuring the actual trip time of the unit.
   2. Carefully record the current and trip time values ​​for each type of protection.
   3. Verify that the switch trip mechanism activates correctly.
5. Results Analysis:
   • Compare measured tripping times and currents with manufacturer's tripping curves and programmed settings.
   • Significant deviations (e.g. more than ±10% in trigger times) require investigation.
   • Corrective Action: If the trip unit does not operate within tolerances, check programming, clean internal contacts (if possible and authorized by the manufacturer), or consider replacing the unit.
   Common errors: Failure to calibrate the test kit; failure to follow the manufacturer's test sequence; not recording data; do not check the status of the trip mechanism after the test.

Step 5: Tighten Electrical Connections

Loose connections are a leading cause of hot spots, which can lead to failure and severe damage.

1. Identification of Connections: Check all screwed connections in the power and control circuit: input/output terminals, bus connections, relay connections, etc.
2. Use of Torque Wrench: Use a calibrated torque wrench. Adjust the wrench to the torque value specified by the equipment manufacturer or by applicable standards (e.g. UNE-EN 60947 for low voltage switchgear).
3. Typical Torque Values (for reference, always consult the manufacturer):
   • M6 screws: 8-12 Nm
   • M8 screws: 20-25 Nm
   • M10 screws: 40-50 Nm
   • M12 screws: 70-80 Nm
   Common errors: Overtightening (thread damage or component deformation); under-tightening (high-strength connection); do not use a calibrated torque wrench.
4. Marked: Once a connection is tightened to the correct torque, consider marking it with a safety marker to indicate that it has been verified.

Step 6: Re-assembly and Commissioning

1. Re-assembly: Reassemble all protective covers, barriers and panels that were removed. Make sure all safety interlocks are working properly.
2. Removal of Grounds: Remove all temporary grounds in the reverse order of their installation.
3. Final Checks: Perform a final visual inspection to ensure that no tools or materials have been left inside the cell. Check that all doors are closed and secured.
4. Re-energize: Follow the plant start-up procedure to gradually re-energize the equipment. Monitor initial currents, voltages and temperatures.

6. Post-Maintenance Verification Checklist

After re-energizing the equipment, perform the following checks to ensure proper operation.

7. Troubleshooting Guide

Table to diagnose and correct common faults.

8. Recommended Maintenance Schedule

Maintenance frequency may vary depending on the type of equipment, operating environment, and criticality of the application. This table provides a general guide.

9. Spare Parts Reference

Having the right spare parts minimizes downtime. Always consult the manufacturer's manual for exact part codes.

Visit https://www.unitecd.com/e-catalog/ for the complete UNITEC spare parts catalog, where you can find the specific parts for your equipment.

10. References

• UNE-EN 50110-1: Operation of electrical installations.
• UNE-EN 60947 (series): Low voltage switchgear.
• Technical Documentation and Operation and Maintenance Manuals of the Manufacturer (OEM) of the Switch/Cell.
• IEEE Std C37.010: IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis.
• NEMA AB 4: Guidelines for Inspection and Preventative Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications.