1-1 Introduction
Surge (lightning) arresters are the first line of defense against lightning- and switching-generated surges, safely conducting transient energy to ground and shielding primary equipment. Their condition is best evaluated with dissipation factor (tan δ) / active power–loss testing at power frequency (50/60 Hz), which can be performed on single-, double-, or multi-unit stacks using GST/UST configurations. Unlike impulse testing—which requires non-portable equipment—power-loss measurement is practical in the field and highly diagnostic. When applied at a consistent test voltage below the arrester’s rating, the test can reveal moisture ingress, salt or dirt deposits, corrosion, cracked porcelain, open shunt resistors, defective pre-ionizing elements, and gap faults. Prior to testing, any leakage-current detectors or counters should be shorted to ground and then restored before service. Because sealed, damaged arresters may hold dangerous internal pressure, strict safety clearances are mandatory during all tests.
1-2- Surge (Lightning) Arresters Overview
Surge arresters protect electrical systems by neutralizing transient discharge currents caused by lightning or switching events.
Testing Surge Arresters:
A complete test includes:
- Impulse and over-voltage testing,
- Power loss measurement at a specified test voltage using normal 50/60 Hz operating frequency.
Field Testing:
- Impulse and over-voltage tests are typically not performed in the field due to the need for large, non-portable equipment.
- Power loss measurement is a proven and effective method for evaluating arrester integrity.
- Power losses are automatically calculated and can be displayed by selecting the “Active Power P” value in CAPTAN 12 software.
The surge arrester power loss test can reveal:
- Moisture ingress,
- Salt deposits,
- Corrosion,
- Cracked porcelain,
- Open shunt resistors,
- Defective pre-ionizing elements,
- Faulty gaps.
Note: Exercise extreme caution when handling damaged surge arresters, as sealed units can develop dangerously high internal gas pressures. Ensure all personnel stand clear during testing due to the risk of violent failure.
Test Voltage
Surge arresters are constructed using semiconductor or metal oxide materials, which exhibit a non-linear volt-ampere characteristic. To ensure meaningful comparisons between different units or previous measurement results, surge arrester tests should always be performed at the same test voltage.The test voltage should be lower than the rated voltage of the arrester.
Test Procedures:
If the arrester is equipped with a leakage-current detector or discharge counter, short-circuit the device by grounding the base of the arrester directly. Remove the short-circuit before returning the arrester to service.
1-2-1- Single-Unit Arrester:
Arrester assemblies consisting of a single unit per phase can be tested using the Grounded-Specimen Test (GST) mode.
Preparation:
- De-energize and ground the line connected to the arrester and disconnect the line from the arrester.
Figure 1: Connecting CAPTAN 12 for single-unit surge arrester test (GST mode)
Table 1: Test Connections for single-unit surge arrestor
| DUT | High Voltage | MEAS GND | Test Mode |
| Stack 1 | HV terminal of arrestor | Earth of Stack 1 | GST |
1-2-2- Double-Unit Arrester Stack
For double-unit arrester stacks, de-energize and ground the connected line, then disconnect it from the arrester stack before testing.
Figure 2: Connecting CAPTAN 12 for double-unit surge arrester test
Table 2: Test Connections for double-unit surge arrestor
| DUT | High Voltage | Input A | MEAS GND | Test Mode |
| S1 | 2 | 1 | 3 | UST-A |
| S2 | 2 | 1 | 3 | GSTg-A |
1-2-3- Multi-Unit Arrester Stack
Table 3: Test Connections for multi-unit surge arrestor
| DUT | High Voltage | INPUT A | INPUT B | MEAS GND | GND | Test Mode |
| S1 | 2 | 1 | 3 | 5 | 5 | UST-A |
| S2 | 2 | 1 | 3 | 5 | 5 | UST-B |
| S3 | 4 | 3 | 5 | 5 | 5 | UST-A |
| S4 | 4 | 3 | 5 | 5 | 5 | UST-B |
Test Results Analysis
Temperature Normalization:
Typically, normalizing measurement results to a standard temperature is unnecessary, as most surge arresters exhibit minimal temperature dependence.
However, if temperature significantly influences the results, establish a temperature correction curve for each arrester design.
Surface Leakage:
Account for surface leakage during power loss measurements.
Minimize leakage by wiping the porcelain with a dry cloth. In some cases, use cleaning agents, waxes, or heat to clean the porcelain surface.
Comparison of Results:
Compare power loss values with:
- Previous measurements,
- Similar units under the same conditions.
- Prioritize manufacturer data if available.
Abnormal Losses:
Investigate any deviations (higher or lower) from the established range.
Higher than Normal Losses:
Contamination: Moisture, dirt, or dust deposits on the inside surfaces of the porcelain housing or sealed-gap housing.
Corroded gaps.
Aluminum salt deposits: Caused by moisture interacting with corona by-products.
Cracked porcelain.
Lower than Normal Losses:
- Broken shunting resistors.
- Broken pre-ionizing elements.
- Mistakes in assembly.
- Poor contact or open circuits between elements.
1-3- Conclusion
Power-frequency dissipation factor (tan δ) and active power-loss testing provide a reliable, field-ready indicator of surge arrester health across all stack configurations. Results should be compared against manufacturer data when available—and always against previous measurements and similar units under identical conditions—with attention to surface leakage control and, only if necessary, temperature effects. Higher-than-normal losses typically point to contamination, corrosion, aluminum salt deposits, or cracked porcelain, while lower-than-normal losses often indicate broken shunt resistors or pre-ionizing elements, assembly errors, or open contacts. By standardizing the test voltage, following the prescribed GST/UST connections, and enforcing strict safety practices, operators can detect developing defects early, prevent arrester rupture or violent failure, and maintain robust surge protection for the network.
