Protection
Surge arresters and surge protective devices limit transient overvoltages from lightning and switching before they can break down insulation. Field testing combines inspection, connection and ground checks, insulation measurements and, on station-class arresters, a watts-loss comparison, to show the device is intact and ready to act.
Surge arresters and surge protective devices, usually shortened to SPDs, sit between an energized conductor and ground and stay almost silent in normal service. When a transient overvoltage arrives, from lightning or from a switching operation, they conduct, clamp the voltage to a level the protected equipment can survive, and return to their high-resistance state. Testing covers medium-voltage and high-voltage arresters on transformers, buses, lines and cable terminations, the arresters and surge capacitors fitted to rotating machines, and the low-voltage SPDs installed in switchgear and switchboards. The work consists of visual and mechanical inspection, checks of the bolted connections and the ground path, insulation measurements on the arrester element where the design permits, and a watts-loss or leakage comparison on station-class units. Because an arrester is a protective device that is rarely seen working, the tests are how its condition is known at all.
An arrester that has failed protects nothing, and often nobody knows. Moisture ingress through a damaged seal, a cracked housing, contamination, internal element degradation after repeated surges and a loose or corroded ground path can all leave a device unable to clamp the next transient. The next lightning or switching surge then reaches the transformer, motor or switchgear it was supposed to shield, and the cost is an insulation failure and an outage. A degraded arrester can also fail in service, going into thermal runaway, which in the worst case ruptures the housing and causes a fault of its own. Low-voltage SPDs wear out by absorbing surges and often carry an indicator or a status contact, but the indicator itself needs to be confirmed. Periodic testing finds these problems before a surge does.
A metal-oxide arrester block is a nonlinear resistor. At normal operating voltage it carries only a very small leakage current, mostly capacitive with a small resistive part. When voltage rises beyond a knee in its characteristic, the resistance collapses and the block conducts the surge to ground. Watts-loss is a measure of the resistive component of the leakage current. As a block ages, absorbs moisture or suffers partial damage, that resistive part grows even if total current barely moves, so a power loss reading is more sensitive to deterioration than a simple current reading. Insulation resistance reveals contamination or a conductive path across a housing, though it says little about the internal element at low test voltage. Connection resistance and ground path continuity matter because the surge current must travel through them quickly, and any impedance in the path raises the voltage the protected equipment sees. Low-voltage SPDs use similar nonlinear elements, with disconnect or fuse provisions, so their checks focus on condition indicators, wiring and grounding.
The arrester is de-energized, isolated and grounded under the owner's safety plan, and the line-side and ground-side connections are made accessible. The technician inspects the housing for cracks, chips, contamination, burn marks and corrosion, and the mounting and grounding hardware for tightness and damage. The nameplate data, class and rating are recorded and compared with the design. Bolted connections are measured with a low-resistance ohmmeter at the line terminal and the ground terminal, and similar connections are compared. The ground path is confirmed continuous to the station or equipment ground. Insulation resistance is measured from terminal to ground at a level suited to the arrester class, with the arrester isolated from the line, and the readings are compared among phases. On station-class units, an ac test set applies voltage across each phase unit and measures the watts-loss, and the readings are compared with those of the other phases and with earlier results, with attention to the temperature and surface conditions at the time. Where the equipment includes a leakage-current monitor or counter, its reading is recorded. For low-voltage SPDs, the status indicator, fuses, wiring and grounding are checked, and the manufacturer's instructions govern any electrical test, because test voltages can damage the device.
The record covers the arrester or SPD identification, class and rating, the visual condition found, the connection and ground path measurements, the insulation readings by phase, the watts-loss or leakage results where taken, any counter or indicator reading, the temperature and weather conditions, the test equipment and calibration status, and the as-found and as-left condition of the connections.
Results are judged by comparison, since no single number describes a healthy arrester. Connection resistance is compared among like connections on the same installation, and a joint that reads noticeably higher than its neighbors is suspect. Insulation resistance and watts-loss are compared among the phases, and a unit that stands apart from its siblings is the one to investigate. Trends against earlier tests matter more than any one reading, and the manufacturer's data and any published tables govern absolute limits, including how readings are corrected for temperature and surface conditions. A visibly damaged housing, a failed indicator or an open ground path is a finding regardless of the electrical numbers. The tests are listed in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance; for the actual values and criteria, refer to your own licensed copies of both and to the manufacturer's instructions.
The method depends on the arrester type and voltage. Station-class and intermediate-class arresters on substations and large transformers justify watts-loss testing, while distribution-class units are often judged by inspection, ground checks and insulation readings alone, and are frequently replaced instead of tested. Arresters on rotating machines are small and sit close to the winding terminals, so they are tested alongside the machine with the surge capacitors, and the insulation test of the machine is arranged so the arrester does not distort it. In switchgear, arresters on the incoming line or on cable terminations are isolated before the bus is tested. Low-voltage SPDs are checked by indicators, fuses and wiring, not by electrical stress. Acceptance testing establishes baseline values for later trending, whereas maintenance testing leans on the trend and on inspection after severe storms or after a fault near the equipment.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
No single sign settles it. Cracks, burn marks or a bulged housing are obvious findings, but an internal failure may show only as a higher watts-loss or leakage reading compared with the other phases or with earlier results. A tripped disconnect or a failed indicator on an SPD also points to a failed unit.
It isolates the resistive component of the leakage current, which grows as moisture, contamination or element damage develops. That makes it more sensitive to deterioration than a simple current or insulation reading, and it is easy to compare phase against phase.
Not without checking the manufacturer's instructions first. The test voltage can exceed the clamping level of the device and damage it or degrade it. SPDs are usually disconnected from the circuit before any insulation test on the surrounding equipment, and are checked by indicator, wiring and grounding.
The surge current must travel through the ground lead and its connections, and any resistance or inductance in that path adds voltage that the protected equipment sees. A loose or corroded ground can defeat an arrester that is perfectly healthy, so the path is inspected and measured.
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