Surge arresters may appear inconspicuous and remain completely silent during normal operation, yet they play a critical protective role in power systems. Whether for transformers, switchgear, or transmission lines, the system depends on them when overvoltages occur. Metal-oxide surge arresters are now widely used and have largely replaced older silicon-carbide gapped arresters. However, long-term operation leads to aging, moisture ingress, and degradation of the varistor discs. Minor failures manifest as increased leakage current and abnormal heating; in severe cases, the arrester may fail to operate during an overvoltage event, or even explode and cause a phase-to-phase short circuit. Routine testing and maintenance are therefore not formalities but essential steps to eliminate faults at an early stage - detecting internal moisture, reduced insulation, or varistor degradation, and then repairing or replacing components to prevent widespread outages caused by a single failed arrester. In heavily polluted areas, regions with frequent lightning activity, or whenever online monitoring data shows anomalies, the inspection interval should be shortened.
Common faults fall into several categories. Internal moisture is the most prevalent problem, usually resulting from seal failure that allows moisture to penetrate the porcelain housing or composite polymer housing, leading to a sharp drop in insulation resistance, a significant rise in leakage current, and in severe cases, internal flashover. Varistor disc aging is another common issue; sustained operating voltage and repeated overvoltage surges cause the voltage-current characteristic to drift, with the resistive current component gradually increasing until heat generation exceeds the thermal stability limit. External insulation failures are often caused by pollution - dirt accumulation on porcelain surfaces or damaged sheds can easily lead to surface flashover under damp weather conditions. Grounding system problems are easily overlooked: corroded or broken down conductors, loose connections, or excessive grounding resistance can prevent lightning current from being safely discharged, rendering the arrester useless. Monitoring devices and discharge counters may become stuck, and a jammed pointer can give a false impression of normal operation when the device has already failed. Loose grading rings disturb the electric field distribution, aggravate partial discharge, and affect the performance of the entire assembly.
Before carrying out maintenance, the arrester's condition must be accurately assessed. Measure insulation resistance with a 2500 V megohmmeter and compare the values with factory data or historical records; a significant drop indicates a problem. The DC 1 mA reference voltage test and leakage current measurement at 0.75 times that voltage are key indicators for judging varistor aging: the reference voltage must not fall below the specified value, and the leakage current should normally not exceed 50 μA. If the leakage current increases markedly and the resistive current rises at the same time, it is highly likely that the varistor discs are damp or aged. In-service current monitoring, performed with a portable live-line tester or an online monitoring device, focuses on the total current and the resistive current component; an abnormal rise in resistive current is an important fault signal. Infrared thermography is also very effective - compare the temperature differences between sections and phases of the same arrester group; an unusually high temperature spot strongly suggests an internal defect. If internal discharge or a poor seal is suspected, an ultrasonic partial discharge detector can assist with diagnosis.
Maintenance work must be carried out in strict sequence. First, reliably isolate the relevant circuit, complete the work permit, use a voltage detector to confirm absence of voltage, and then immediately install grounding leads; all three phases must be grounded and discharged. If overvoltage protective devices are installed inside switchgear cabinets, residual charges must also be released. Next, disconnect the high-voltage and low-voltage leads and mark the phase positions. Carefully inspect the porcelain housing or composite housing for cracks, burn marks, and flashover traces. Wipe the external insulation surfaces with anhydrous ethanol or a dedicated cleaning agent; heavily contaminated areas should be thoroughly cleaned. For composite-housed arresters, avoid solvents that may damage the silicone rubber. For older gapped arresters that can be dismantled, open the end cover in a clean and dry environment, inspect the sealing rings for aging, the desiccant for discoloration, and the spring compression. If the varistor discs show pitting, burn marks, or loss of zinc coating, replace the entire set with matching discs. Modern metal-oxide arresters are mostly fully sealed and cannot be opened; if a problem is found, replace the entire unit section. Replace aged seals and desiccant. If the composite housing sheds are severely damaged and the core rod is exposed, do not attempt to patch them by wrapping; the entire unit must be replaced. Corroded or deformed grading rings should be reshaped or replaced. All conductive contact surfaces should be polished with fine emery paper and coated with a thin layer of electrical contact compound to prevent electrochemical corrosion. The base must be kept clean and dry, and its insulation resistance must meet the required standards. Discharge counters or monitoring devices must be tested for operational reliability; replace any with large current meter deviations, as a failed monitor means loss of condition visibility. Check the grounding system, verify that the down conductors are free of corrosion breaks and that bolts are tight, measure the grounding resistance to ensure it falls within acceptable limits, and confirm the centralized grounding device has good continuity. During reassembly, restore the arrester by phase sequence, tighten bolts evenly, and reconnect the high-voltage and low-voltage leads. After installation is complete, perform insulation resistance and DC tests; only after confirming the values are acceptable should you remove the grounding leads. Before removing them, double-check that all connections are correct and no tools have been left behind.
Safety precautions must be emphasized. Even after the power is disconnected, arresters can retain residual charge; they must be fully discharged and kept grounded until the work is finished. Substation arresters are often installed at height - use safety harnesses and pass tools via ropes to prevent dropped objects from injuring people. If a sealed structure must be opened, choose a dry, clear day and minimize the operating time to prevent moisture absorption by the varistor discs and insulation components. When replacing varistor discs or an entire arrester section, ensure that the rated voltage, nominal discharge current, and energy absorption capability match the original design, so as not to compromise protection coordination. During assembly, take care to prevent foreign objects such as bolts and washers from falling into the arrester, as this could cause a short circuit. When performing DC high-voltage tests, operators should stand on insulating mats, the test equipment must be reliably grounded, and the arrester should be immediately discharged and grounded after the test.
After maintenance is completed, a comprehensive test report should be prepared and the data compared with historical records. Within one week of energization, a detailed infrared temperature measurement and a continuous current retest should be arranged to confirm that temperature rise and resistive current are normal. All maintenance records must be entered into the equipment ledger to support condition-based maintenance. Today, arrester maintenance is transitioning from time-based maintenance to condition-based maintenance. Online monitoring systems collect total current, resistive current, and operation counts in real time, and by analyzing trends together with ambient temperature and humidity, they allow a more accurate assessment of equipment health, so that outages are scheduled only when necessary, thus improving supply reliability while reducing costs. Even so, periodic on-site verification and off-line testing remain indispensable, as online monitoring systems themselves can suffer from false alarms or missed detections. An arrester may seem simple, but it must operate reliably in that split second when an overvoltage strikes. Only with high-quality daily maintenance will it perform its protective role when it matters .