Maintenance Best Practices for Industrial Electrical Switchgear

Industrial switchgear fails for predictable reasons: contaminated contacts, degraded insulation, corroded connections, and protection relays that have never been tested against the coordination study they were supposed to implement. Every one of those failure modes is detectable and preventable with a structured maintenance programme.

This article is written for plant engineers, electrical managers, and procurement professionals at South African industrial facilities who are responsible for the upkeep of low-voltage (LV) and medium-voltage (MV) switchgear. It covers the specific tasks that belong in a maintenance programme, the instruments needed to perform them, and the schedule framework that keeps the work manageable.

---

Why Maintenance Matters

The cost of an unplanned switchgear failure extends well beyond the equipment itself. A main incomer breaker that fails to clear a fault can propagate damage through an entire distribution board. A failed MCC section can trip multiple production lines simultaneously. In severe cases — particularly where arc flash energy is high or fire suppression is absent — equipment damage becomes a safety emergency.

SANS 10142-1 (the Wiring Code of Practice) and the Occupational Health and Safety Act, Section 9 both impose an affirmative obligation on employers to maintain electrical equipment in a safe working condition. The obligation is not qualified by cost: an employer who defers maintenance and sustains an injury as a result is liable under the OHS Act regardless of the reason for deferral. The Electrical Installation Regulations under the OHS Act also require that electrical installations be maintained by suitably qualified and registered persons.

Beyond compliance, the economics are straightforward. A maintenance programme for a mid-size industrial switchboard typically costs a fraction of a single failure event. The asymmetry is pronounced for MV equipment, where fault energy is high and replacement lead times can stretch to months.

---

Visual and Physical Inspection

Visual inspection is the foundation of any maintenance programme and requires no specialist instruments. It can be performed by the in-house electrical team during planned access windows.

What to look for:

• Corrosion and moisture ingress — check the interior of switchboard enclosures for rust on structural steel, white oxidation deposits on aluminium busbars, and condensation inside cable entry sections. Cable entry seals and bottom plate gaskets are common ingress points.
• Busbar condition — inspect busbar joints and tapings for discolouration (bronze or black tarnishing on copper indicates thermal stress), physical distortion, or loose bolting.
• Cable termination condition — look for heat discolouration around lug barrels and conductor insulation adjacent to terminals. Cracked or brittle insulation near terminals indicates that terminations have run hot.
• Door seals and enclosure integrity — check that door seals are intact and undistorted. Gaps allow dust ingress that accelerates insulation tracking.
• Interlocks and safety shutters — verify that mechanical interlocks between circuit breaker, isolator, and earthing positions operate correctly. Safety shutters on withdrawable units must close fully when the unit is withdrawn.
• Labels and documentation — ensure that circuit labels, voltage and current ratings, and arc flash hazard labels are legible and match the current configuration. A mislabelled feeder is a safety hazard during fault response.

---

Insulation Resistance (IR) Testing

Insulation resistance testing measures the dielectric integrity of cable and equipment insulation. Moisture, contamination, mechanical damage, and thermal ageing all degrade insulation resistance over time; IR testing detects degradation before it progresses to a flashover.

Instruments: A calibrated insulation resistance tester (commonly called a Megger after the dominant brand) is standard. Use 1 000 V DC test voltage for 415 V circuits and 2 500 V DC for 3.3 kV or higher.

Acceptable values: As a general reference, insulation resistance should be at least 1 MΩ per kilovolt of system voltage. For a 415 V circuit, this means a minimum of approximately 1 MΩ; for an 11 kV cable, at least 10 MΩ. Trend data matters more than a single reading. A result of 50 MΩ that has declined from 500 MΩ over three successive annual tests warrants investigation, even though 50 MΩ is technically above the minimum.

What degradation looks like: Values below the minimum threshold, values that fall rapidly between consecutive tests, or significant asymmetry between phases on the same circuit. Note that ambient temperature affects IR readings — normalise results to 20°C using correction factors if test conditions vary.

Record results with date, temperature, test voltage, duration (typically 1 minute), and instrument calibration reference. This data is what turns a test into a trending asset.

---

Contact Resistance Measurement

Electrical contacts — busbar joints, circuit breaker main contacts, cable lug terminations — develop resistance over time due to oxidation, fretting, and contamination. Rising contact resistance causes localised heating, which accelerates further degradation in a feedback loop that ends in connection failure.

Instruments: A micro-ohmmeter (commonly called a Ductor or DLRO — Digital Low-Resistance Ohmmeter) injects a precise DC test current and measures the voltage drop across the contact. Test current should be at least 100 A for main busbar joints and circuit breaker contacts; some standards specify higher currents for certain applications.

The Ductor test procedure: Isolate and earth the circuit. Connect the current injection and voltage measurement leads on either side of the contact under test. Record the resistance in micro-ohms (μΩ) or milli-ohms (mΩ).

Acceptable limits: Manufacturer specifications vary, but typical references include:

• LV busbar joints: < 50 μΩ for bolted joints
• LV moulded case circuit breaker main contacts: compare against manufacturer data and previous measurements rather than a universal threshold
• MV vacuum circuit breaker main contacts: manufacturer's specified value, typically in the range of 50–150 μΩ depending on frame size — rising values indicate contact erosion

When to act: Any result significantly above the manufacturer specification, or above the previous measurement on the same contact, requires investigation before return to service.

---

Protection Relay Testing

Protection relay testing verifies that the relay will operate correctly when a real fault occurs. This is arguably the most safety-critical element of a switchgear maintenance programme — a relay that fails to operate on a genuine fault allows fault energy to propagate unchecked.

What to test: Overcurrent (ANSI 51), earth fault (ANSI 51N), and where installed, differential (ANSI 87) and directional overcurrent (ANSI 67) functions.

The correct reference: Testing must be carried out against the original protection coordination study for the installation, not against the relay's current settings as-found. The coordination study defines the required settings and time-multiplier values; the relay settings must match, and the relay must operate within the specified time at the specified current multiples.

Test the whole protection chain: Secondary injection into the relay is not sufficient on its own. The complete chain — current transformer, pilot wiring, relay, trip coil, circuit breaker mechanism — must be tested. A relay that operates correctly but whose trip circuit has a broken wire or a failed trip coil capacitor will not clear the fault. Confirm breaker trip time from relay output to breaker open position.

Instruments: A relay test set capable of generating precise AC current and voltage at specified phase angles. Test record should document the pickup value, time to operate at each current multiple tested, and the as-found versus as-left settings for each protection element.

Update after network changes: Protection settings are only valid for the network configuration they were studied against. Whenever a significant change is made — a new transformer, a change in generator interconnection, a change in maximum fault level — the coordination study must be reviewed and relay settings updated accordingly.

---

Thermographic (Infrared) Inspection

Thermal imaging — using an infrared camera to detect temperature anomalies under live conditions — is the most effective non-invasive tool for finding loose connections, overloaded conductors, and failing components before they cause a failure.

How it works: Any electrical component with elevated resistance relative to adjacent identical components dissipates more heat under the same current. A loose lug, an oxidised busbar joint, or a breaker with worn contacts will run hotter than its neighbours. An IR camera detects this temperature difference as a visible gradient in the thermal image.

Inspection protocol: Thermal imaging should be performed with the switchgear under as close to normal operating load as practicable — temperature differentials are load-dependent and will not be visible at low loads. The engineer opens each switchboard door in turn, images the interior from a safe distance, and flags any component showing a temperature differential of more than 10°C above similar adjacent components.

Annual inspections: Thermal imaging surveys should be conducted annually as a minimum, typically as part of the annual maintenance programme. Many facilities perform them more frequently on critical supply paths.

Follow-up protocol: A hotspot found in a thermal survey is not a "monitor and review" item — it requires a corrective action scheduled within a defined timeframe. Minor hotspots (10–20°C differential) should be scheduled for correction within 90 days; severe hotspots (>40°C differential) should be de-energised or load-reduced until repaired.

---

MV-Specific Maintenance Tasks

Medium voltage switchgear carries additional maintenance requirements beyond LV inspection and testing.

SF6 gas-pressure checks: Ring main units (RMUs) and gas-insulated switchgear use sulphur hexafluoride (SF6) gas as both insulation and arc-quenching medium. SF6 pressure must be checked against the manufacturer's specification at each maintenance visit. Pronutec RMUs use sealed gas compartments rated to specific minimum pressures; a pressure loss indicator or gauge provides the check value. SF6 below minimum pressure compromises the insulation and interruption capability of the switchgear — the unit must be taken out of service and returned to the supplier for refilling.

Vacuum circuit breaker (VCB) mechanical operation tests: The vacuum interrupter in a VCB relies on the integrity of the ceramic-to-metal seals around the vacuum bottle. While a high-pot dielectric test is the definitive test of vacuum integrity, mechanical operation tests — opening and closing the breaker manually and electrically, verifying minimum operating voltages for close and trip coils — confirm that the operating mechanism functions correctly and within the manufacturer's specified operating times.

VCB timing measurements: Record the contact travel time (close and open), the contact wipe and overtravel, and the total operating time from coil energisation to contact separation/closure. Compare against manufacturer specification and previous records. Timing outside tolerance indicates spring wear, lubrication loss, or mechanical degradation of the operating linkage.

---

Maintenance Schedule

"On-event" means that the task must be performed following a specific triggering event regardless of the standard schedule cycle.

---

Preventive Tips

• Keep records — every test, every date, every instrument. A test result without a date and calibration reference is not a maintenance record; it is a number.
• Date and stamp results at the point of measurement. Do not transcribe results from field notes days later; transcription introduces errors.
• Never skip maintenance after a through-fault. A through-fault subjects switchgear to electromagnetic forces and thermal stress that can loosen connections, degrade contacts, and crack insulation without causing an immediate trip. A post-fault inspection is mandatory, not optional.
• Always update the coordination study when the network changes. New embedded generation, transformer replacements, and changes in Eskom supply fault level all affect the validity of existing relay settings. An unreviewed study is a silent protection gap.
• Use Telergon isolation switches at LV isolation points. Telergon fuse-switch-disconnectors and isolators provide a visible break point with high making and breaking capacity, simplifying LOTO (lockout/tagout) compliance at the panel level.

---

Conclusion

A structured switchgear maintenance programme is not optional under South African law, and it is not optional under sound engineering practice. SANS 10142-1 and the OHS Act set the floor; a properly maintained installation performs far better than the minimum that regulation requires.

The tasks described here — visual inspection, IR testing, contact resistance measurement, protection relay testing, thermographic surveys, and MV-specific checks — form a defensible programme when executed on the schedules outlined and documented to a standard that supports trending analysis.

GNTech supplies arc-resistant switchgear, spare parts and contact sets, Pronutec MV ring main units, and Telergon LV isolation switches, and can assist in specifying a maintenance programme matched to your facility's installed equipment. Contact the GNTech team to discuss switchgear maintenance support, spare parts supply, or a full maintenance programme for your facility.