Introduction:
Surge arresters are crucial devices used in electrical systems to protect equipment such as transformers, cables, and switchgear from transient overvoltages caused by lightning and switching surges. However, surge arrester failure remains a significant reliability concern in substations and transmission networks. Understanding the causes of surge arrester failure, identifying its internal mechanisms, and implementing effective condition-based maintenance can help engineers prevent outages and extend the service life of critical equipment. This article explores the main lightning arrester failure reasons, their detection methods, and preventive strategies to minimize surge protection device failure rates.

Q1. What is the essential function of a surge arrester and how should a terminal failure event be interpreted?

A:

  1. A surge arrester protects high-voltage equipment from transient overvoltages by safely diverting excess current to ground.
  2. It operates using metal oxide varistors (MOVs), typically zinc oxide, which remain non-conductive under normal voltage but conduct instantly when a surge occurs.
  3. Failure analysis should distinguish between a normal protective sacrifice after years of service and a premature failure caused by design, manufacturing, or system issues.

Q2. What is the leading cause of surge arrester failure, and how does it occur internally?

A:

  1. Moisture ingress is the most frequent cause of surge arrester failure worldwide and directly affects insulation and long-term reliability, especially at normal operating voltage.
  2. Water penetrates through aged seals, cracked diaphragms, or housing defects, often drawn in by temperature-induced pressure changes.
  3. The resulting condensation corrodes MOV disks and conductors, raising resistive leakage current and initiating thermal runaway that leads to internal breakdown.

Q3. How do Temporary Overvoltages (TOVs) or excessive electrical stresses cause arrester breakdown?

A:

  1. Temporary Overvoltages occur due to system events such as ground faults, ferroresonance, or unbalanced loads, impacting the system voltage.
  2. Sustained voltages above the arrester’s rating cause MOV blocks to conduct continuously, generating heat that exceeds the unit’s cooling capacity.
  3. This results in MOV cracking, melted connectors, or flashover traces that signal dielectric failure and complete arrester fault under high voltage conditions.

Q4. What proactive maintenance techniques help prevent thermal breakdown?

A:

  1. Adopt a condition-based maintenance (CBM) strategy that relies on real-time diagnostics rather than fixed replacement intervals.
  2. Perform regular infrared thermography to detect hotspots that reveal abnormal heating or partial discharge activity.
  3. Monitor the third harmonic resistive current (THRC) and replace arresters when the current consistently exceeds 350 µA, as this indicates progressive degradation.

Q5. What operational data and internal clues are critical during forensic failure analysis?

A:

  1. Record system parameters before dismantling, including voltage level, transformer neutral configuration, and available fault current in relation to the normal operating voltage.
  2. Inspect internal parts for corrosion, oxidation, and hardened seals, which confirm moisture ingress or long-term ageing under high voltage conditions.
  3. Compare test results such as insulation resistance, watts loss, reference voltage, and voltage across the arrester against baseline data to locate weaknesses.

Q6. Why is examining a companion arrester valuable during investigation?

A:

  1. A companion arrester—an identical, non-failed unit located nearby—provides a diagnostic reference for comparison.
  2. It often preserves early signs of degradation such as seal deterioration or increased leakage that might not be visible in the failed unit.
  3. Studying both helps determine whether the lightning arrester failure was an isolated event or part of a broader systemic issue.

Q7. How does MOV disk ageing or degradation lead to failure?

A:

  1. MOV ageing occurs due to prolonged electrical stress and heat, which alter the varistor’s crystalline structure and affect its performance at operating voltage.
  2. Over time, power losses increase even under normal operating voltage, producing internal heat that accelerates degradation.
  3. When heating surpasses the arrester’s dissipation capacity, thermal runaway begins, leading to cracking or puncturing of MOV disks.

Q8. What other failure modes, apart from moisture ingress and TOVs, require immediate attention?

A:

  1. External flashover along the arrester housing caused by contamination or wildlife contact can damage insulation and seals.
  2. Internal partial discharges arising from manufacturing defects or trapped air reduce dielectric strength and promote failure.
  3. Severe lightning or switching surges beyond the arrester’s energy rating can physically fracture MOV disks and puncture insulation.

Q9. What defines an open-circuit failure mode, and why is it dangerous?

A:

  1. An open-circuit failure occurs when a severe surge destroys the arrester’s conductive path, leaving it unable to conduct fault current.
  2. This hidden failure gives no external indication yet leaves the system completely unprotected against subsequent overvoltages that exceed the applied voltage.
  3. Such arresters must be replaced immediately, and system studies performed to verify proper arrester selection and coordination.

Q10. Which technical standards should be applied during pre-commissioning arrester tests?

A:

  1. During installation, dry-air purging must achieve a dew point below –50 °C and insulation resistance above 1000 MΩ at 1 kV.
  2. A vacuum drop test should confirm tightness with leakage rates under 20 m³·Pa/min before energization.
  3. Capacitance and tan δ (≤ 0.007) tests, together with THRC below 30 µA, confirm dielectric quality and readiness for service under normal operating conditions.

Summary

  1. Surge arrester failure is primarily caused by moisture ingress, long-term MOV ageing, or excessive overvoltage stress.
  2. Implementing condition-based maintenance using thermography and THRC monitoring detects issues before catastrophic failure occurs.
  3. Forensic evaluation of both failed and companion arresters provides insights for preventing future lightning arrester failures across the network, particularly under high voltage conditions.