Lightning Arrester — Maintenance, Failure Modes & LCM III Condition Monitoring

Maintenance Schedule

• Monthly: visual inspection for overheating signs at connecting points; clean insulator with dry cloth, check flash marks/chipping/surface cracks; record surge counter discharges (if provided); record leakage current value (if provided).
• Quarterly (additional): check/tighten main and earth connections; check/tighten grading rings if applicable.
• Half-Yearly (additional): measure leakage current and record, if a leakage current monitor is provided, per spec TI/SPC/PSI/LCMLA/0030.

Yearly records for each LA must include: location, rating, make, serial no., date of manufacture, date of commissioning, date of measurement, IR value, THRC value of leakage current, total leakage current, number of surges passed, and remarks.

Pre-Monsoon Check: before monsoon onset, ensure no scheduled maintenance is overdue for LA. Additionally ensure: proper cleaning of all arresters; IR value of each unit POSITIVELY checked (to avoid bursting/failure); all connections tight; leakage current monitor reading checked (if provided); surge counter reading checked (if provided).

Failure, Causes & Remedial Action — 5 Real Documented Failure Modes

From a seminar on LA maintenance (23rd-24th August), the following common failures were documented, each with a genuine root cause and a real, escalating remedial procedure.

The general remedial pattern across all 5 failure modes: (1) disconnect and unload the LA from line carefully; (2) if the LA is a single unit, check IR of a spare unit — if within permissible limit, replace the defective LA entirely; (3) if the LA has multiple units, identify and remove just the defective unit, replacing it with a healthy unit of the SAME MAKE (never mix makes); (4) check IR of the fully re-assembled LA — must be within/more than permissible limit; (5) re-erect, re-take IR value, reconnect all connections, and commission. The 'Hot Spot on Top Connector' failure has an extra first-response step before this pattern: if the connection is merely loose, simply TIGHTEN it and replace overheated nuts/bolts — only escalate to unit replacement if the connector plate is badly overheated/damaged beyond simple repair.

DOs and DON'Ts

• DO: before maintenance/inspection/replacement, take a shutdown, earth the concerned circuit, and disconnect the arrester from the line end as a safety precaution.
• DO: keep vent ports away from transformers/other equipment when erecting, to prevent damage in case of arrester failure.
• DO: always ensure the ground connection is firmly made BEFORE connecting the arrester to an energized line. If a discharge counter is used, it must be connected before the arrester connects to an energized line.
• DO: in multi-unit arresters, use SAME-MAKE units only.
• DON'T: never work on an arrester without a shutdown in the concerned circuit.
• DON'T: staff should NOT climb the arrester during maintenance/installation — this may break the porcelain.
• DON'T: never install a chipped or cracked porcelain housing.
• DON'T: never lift by the line terminal — use rope around the housing instead; never tilt during handling (upright only).
• DON'T: if abnormal sound (suggesting broken internal components) is heard from any arrester, that arrester must NOT be used.
• DON'T: never mix different makes in a multi-unit arrester; never use the arrester as a bus support.
• DON'T: the earth terminal of the arrester must NEVER be isolated — when removing a surge monitor, the disconnection must be BY-PASSED WITH AN EARTHING SHUNT.

Condition Monitoring by LCM III

In normal service, a metal oxide surge arrester (MOSA) carries a continuous but small leakage current, typically 0.1-2 mA — dominated by a CAPACITIVE component, with the RESISTIVE component only 5-25% of the capacitive one. MOSAs normally operate at a continuous voltage of 60-80% of rated voltage. Because ZnO's non-linear characteristic makes even a large increase in resistive leakage current (Ir) barely noticeable within the total leakage current, measuring the 3rd harmonic resistive component specifically — via LCM III — is the best way to obtain sensitive, reliable information about arrester health without any shutdown.

LCM III — Principle & Five Named Accessories

Principle: the non-linear resistance of ZnO blocks introduces a 3rd harmonic resistive component in the leakage current when stressed by a sinusoidal voltage — this component best indicates changes in the arrester's non-linear characteristics over time. Since harmonic content in the operating voltage itself also increases harmonic content in the leakage current, the measurement must COMPENSATE for system-voltage harmonics, or an apparent increase in resistive current could be a false signal rather than a genuinely faulty arrester.

• Leakage Current Transformer (LCT): connected across the earthing strip of the LA to measure TOTAL leakage current flowing through it.
• Dish Antenna Probe (DAP): placed at the base of the LA to pick up harmonics in the SYSTEM VOLTAGE, giving the capacitive component of leakage current.
• Telescopic Hot Stick (THS): used to place the DAP at the base of the LA (from a safe distance, live).
• Signal Conditioning & Measuring Unit (SCU): takes signals from LCT and DAP as inputs, determines the resistive component of leakage current.
• Signal Processing & Data Management Unit (SPU): data management system with self-test/signal simulation, stores per-LA information (network-wide), saves leakage current values over time, and displays leakage-current waveforms WITHOUT needing an external oscilloscope.

How to Use LCM III — 4-Step Procedure

1. Connect the LCT across the earthing strip of the LA, to measure total leakage current.
2. Place the DAP, using the THS, at the base of the LA — to pick up system-voltage harmonics, giving the capacitive current component.
3. Connect the SCU with leads from the LCT and DAP, to determine the resistive component of leakage current.
4. Connect the SCU and SPU — leakage-current waveforms of the various components can be viewed directly in the SPU without an external oscilloscope.

Measured Parameters (5 Display Fields)

LCM applications: as a portable instrument for regular condition checks; to monitor an arrester over a shorter/longer period (e.g. investigating unexpectedly high leakage current in more detail); or permanently installed for continuous registration at substations of major importance. Recommendation: the first resistive-leakage-current measurement should be taken shortly AFTER installation, as the best reference 'fingerprint' for all future comparisons. Ageing typically causes a GRADUAL increase in leakage current over time — establishing regular measurement procedures allows detecting aged arresters and removing them from service BEFORE failure.