What Tests and Maintenance Are Required for Surge Arresters?

Maintenance is required because surge arresters operate passively and silently in normal operation, so their deterioration is not always noticed until the moment they are needed. These devices protect transformers, cables, circuit breakers and other switchgear equipment by limiting transient overvoltages caused by lightning and switching, and a unit that looks problem-free from the outside may still be unhealthy internally. Cracks, moisture ingress, loose connections, aging or deterioration of the internal MOV blocks can all quietly reduce protection performance. Regular testing and maintenance therefore answer a question routine observation cannot: whether protection actually continues. Because the arrester's success also depends on installation quality and the grounding arrangement, maintenance evaluates the connection points and grounding path together with the device body itself.

Depending on the application, surge arrester maintenance includes visual inspection, surface cleaning, connection and grounding path verification, surge counter and disconnector checks, thermal inspection while energized, leakage current tracking, watts-loss assessment and, in some cases, insulation resistance testing. In some fields, reference voltage, V-I characteristic or manufacturer-specific diagnostic methods can also be used to evaluate the internal MOV blocks more sensitively, although these may not be routine for every facility. The right program is not identical everywhere: distribution type, station type, duty criticality, environmental conditions and operating history should be considered together when deciding which methods to apply. Whatever the mix, results gain most of their meaning through comparison with similar arresters in the same facility and through trend tracking across successive maintenance visits.

The first step is safety: before working on a surge arrester, the related section is taken out of service with the correct operating sequence, absence of voltage is verified and safe working conditions are established according to the field procedure. Then visual inspection begins, searching the body for cracks, breaks, deformation, swelling, puncture marks, surface contamination, carbon traces, signs of water ingress, corrosion, terminal looseness and wear on connecting conductors. This applies to both silicone and porcelain bodies, since deterioration visible on the outer surface is often the first sign of deeper problems. Outdoor units deserve extra attention because UV exposure, pollution, salt, industrial atmosphere and moisture affect performance in the long term. The connection points and grounding path are evaluated together with the arrester itself.

Thermal inspection shows whether a surge arrester is running hotter than similar devices around it, which is one of the most valuable field indications of internal trouble. Using a thermal camera or suitable infrared measurement while the equipment is energized, the temperatures of comparable arresters are checked against each other. A unit behaving significantly hotter than others in the same phase group or equipment set may indicate increased internal losses and the start of deterioration. Interpretation relies on comparison rather than absolutes: the difference against partner devices and the change over time are more meaningful than a single temperature reading. Practical conditions matter as well, since comparisons made when solar effect is low give healthier results and avoid mistaking sun-heated surfaces for genuine internal heating.

Leakage current measurement is important because the health of an MOV surge arrester changes first in the resistive component of the current flowing through it. As aging, moisture ingress or internal deterioration progresses, this resistive component grows, and since it is the part that causes heating, the deterioration feeds on itself over time. Tracking leakage current therefore catches the problem at an early stage, before it turns into visible damage or failure. This is also why watts-loss assessment, which measures the power loss caused by the small currents passing through the arrester, stands out in advanced condition monitoring, especially in more critical and high-voltage applications. For reliable results, the measurement setup must prevent external surface leakage caused by contamination from distorting the readings.

Not always. In MOV surge arresters, a large part of the total AC current may be capacitive, so a simple reading of total current often does not provide a healthy interpretation of the device's condition. What is truly meaningful is the resistive component or a third-harmonic-based evaluation, because it is the resistive current that changes as aging, moisture ingress or internal deterioration develops, and it is the resistive current that causes heating. This is why advanced condition monitoring applications focus on resistive leakage current or watts-loss tracking rather than simple total readings. Measurement discipline matters too: a dirty body, damp surface or incorrect connection method may lead to incorrect interpretations, so these tests should be performed by experienced teams with a suitable test setup.

The grounding connection is critical because a surge arrester works by directing impulse current to earth, and the quality of that path determines how well the protected equipment survives the event. Both the phase connection and the grounding connection should be short, solid and low impedance. A loose bolt, an oxidized contact surface, a broken conductor strand or an unnecessarily long connecting conductor can increase the residual voltage level during an impulse, which means the transformer or switchgear being protected sees a higher voltage than intended. For this reason, the connection path at both ends of the surge arrester deserves the same seriousness as the body itself during maintenance, and even mechanically small looseness found around terminal bolts or the grounding line should be corrected.

The surge counter value is not a definitive health indicator on its own, but it provides important operating information about how many impulse events the arrester has experienced in the field. It becomes genuinely useful when read together with other findings such as thermal behavior, leakage current measurements and visual condition. In structures that include a disconnector, maintenance must also verify that the disconnecting element has not accidentally operated or mechanically deteriorated, because otherwise the arrester may appear to be in service while it has silently been disconnected and protection has actually been lost. Checking the counter and disconnector at every maintenance visit, and archiving the counter history, turns these simple auxiliary devices into a valuable part of the overall condition picture.

Insulation resistance testing provides a practical, fast screening check on surge arresters, used especially on the low- and medium-voltage side. Its main value is distinguishing clearly faulty units, since failed arresters often show short-circuit or low-resistance behavior that this test picks up quickly. It is not a test that tells everything by itself: the method must be selected carefully because surface leakage current may distort the result, and the reading gains value mainly with a pass-fail logic when interpreted together with other findings such as visual inspection, thermal comparison and leakage current tracking. In a well-built maintenance program it therefore serves as one screening layer among several, helping decide which arresters deserve closer diagnostic attention rather than delivering a final verdict on its own.

Record keeping is important because surge arrester problems are usually understood not from a single measurement but from changes over time. Visual inspection results, cleaned areas, tightened connections, thermal images, counter records, leakage current measurements, watts-loss results and replacement recommendations should all be archived regularly. Comparison is the heart of arrester diagnostics: temperatures, leakage values, counter history and visual condition of similar devices in the same facility are evaluated together, and in three-phase structures clear deviations between phases are strong signs requiring detailed inspection. Such comparisons only work when past data exists. Without trend tracking, surge arrester deterioration is often noticed only after a fault occurs, which defeats the entire purpose of maintaining equipment whose job is to prevent exactly those faults.