How do surge arresters degrade over time?

Surge arresters contain metal oxide varistors (MOVs) — zinc oxide discs with voltage-clamping properties. Each time the arrester conducts a surge event, the MOV material experiences micro-structural changes that gradually lower its clamping voltage and increase its leakage current at normal operating voltage. In Singapore's high-lightning-frequency environment, a surge arrester can process hundreds of surge events per year. Cumulative energy absorption shifts the MOV's V-I characteristic, reducing protection margin and eventually causing thermal runaway.

What is the leakage current test for surge arresters?

Total leakage current measurement monitors the current flowing through the surge arrester at normal operating voltage (no surge applied). As MOVs degrade, their resistance at normal voltage decreases, and leakage current increases. Trending leakage current over time reveals MOV degradation before failure. Typical acceptance criteria: increase of more than 10–20% from baseline is a warning sign; a sharp step-change in leakage current indicates a significant degradation event — possibly a direct strike.

How often should surge arresters be tested in Singapore?

For medium-voltage surge arresters on Singapore's 22 kV / 6.6 kV distribution network, annual testing is the industry standard. For LV surge protection devices (Type 1 and Type 2 SPDs) in industrial and commercial buildings, testing at each periodic electrical inspection (minimum every 5 years per EMA requirements, annually for critical facilities) is recommended. After any confirmed or suspected direct lightning strike, test all surge arresters in the affected building immediately.

Can I test surge arresters without taking them offline?

For online monitoring, leakage current can be measured with a clamp meter on the earth conductor of the surge arrester — without disconnecting the arrester. However, for full diagnostic testing (third harmonic component analysis, resistive leakage current separation), specialised surge arrester analysers are needed. Some designs allow this online; others require de-energisation. For LV SPDs, most testing is done de-energised as part of the periodic inspection.

What is the difference between a Type 1, Type 2, and Type 3 surge protection device in Singapore?

Type 1 SPDs (also called Class I or B) are installed at the main distribution board where lightning current may enter the building — typically buildings with external lightning protection systems. They handle very high impulse currents (25 kA or more per wave). Type 2 SPDs (Class II or C) protect sub-distribution boards from switching transients and residual lightning surges. Type 3 SPDs (Class III or D) protect individual sensitive equipment close to the load. A complete Singapore building protection scheme typically uses all three types in coordination.

Surge Arrester Testing Singapore: Protecting Critical Infrastructure | Unitest

Singapore Is the Worst Possible Environment for Surge Arrester Complacency

Singapore is one of the most lightning-dense regions on Earth. The island averages approximately 168–180 thunderstorm days per year — compared to around 10 days in London and 35 days in Sydney. Singapore's compact geography, high humidity, and tropical convective weather systems mean intense electrical storms are a near-daily occurrence for several months of the year.

Every one of those thunderstorms produces cloud-to-ground lightning strokes with peak currents ranging from 10 kA to over 200 kA. The electromagnetic field of a nearby strike — even one that doesn't directly hit a building — induces transient overvoltages in power lines, communication cables, and earthing conductors that propagate throughout a connected electrical system. Sensitive electronic equipment — PLCs, UPS systems, medical equipment, data centre servers, telecom infrastructure — can be destroyed by transients far below the direct strike level.

Surge arresters and surge protection devices (SPDs) stand between all of that energy and your building's critical systems. But here's what most building managers don't understand: surge arrester testing in Singapore is essential maintenance, not a once-installed-and-forgotten task. Surge arresters degrade with every event they handle. In Singapore's high-lightning environment, a surge arrester on a typical commercial building may process hundreds of surge events per year — each one consuming part of its operational life.

Key Stat

Singapore's Meteorological Service data shows an average ground flash density of 15–20 flashes per km² per year in most parts of the island — among the highest recorded in Southeast Asia. For comparison, the global average is approximately 3–4 flashes per km² per year. This means Singapore's surge arresters process roughly 4–5 times more lightning events than average global installations.

How Surge Arresters Work (And How They Fail)

Modern surge arresters — both the medium-voltage type installed on power lines and substations, and the low-voltage SPDs installed in distribution boards — use zinc oxide metal oxide varistors (MOVs) as their core protective element. MOVs have a highly non-linear resistance characteristic: at normal operating voltage, their resistance is very high (essentially open circuit). When a transient overvoltage occurs, their resistance collapses almost instantaneously, allowing the surge current to flow through the arrester to earth while clamping the voltage across the protected equipment at a safe level.

This is elegant physics — but it comes with an important degradation mechanism. Each time an MOV conducts a surge event, the high current and energy absorption causes micro-structural changes in the zinc oxide crystal lattice. These changes accumulate with repeated events. Over time:

The MOV's clamping voltage decreases — it starts conducting at lower voltages, meaning it intervenes in normal voltage variations rather than only surges
Its leakage current at normal operating voltage increases — the arrester runs warm, consuming power even when not protecting against a surge
In severe cases, thermal runaway develops: the increased leakage current generates heat, which lowers the clamping voltage further, which increases current, which generates more heat — until the arrester fails, sometimes violently

A failed MV surge arrester on a 22 kV line that goes into thermal runaway does so dramatically: the porcelain housing shatters, a phase-to-earth fault occurs, and the resulting arc can cause significant equipment damage and fires. For Singapore's critical MV infrastructure — substations serving hospitals, data centres, petrochemical plants — this failure mode is an unacceptable risk.

Testing Methods: How to Know if Your Surge Arrester Is Degrading

Method 1: Total Leakage Current Measurement (Online)

The most practical continuous monitoring approach for MV surge arresters. A clamp meter placed around the earth conductor of the surge arrester measures the total current flowing through the arrester at normal operating voltage. This current has two components:

Resistive component: The true leakage through the MOV material, which increases with degradation
Capacitive component: Due to the capacitance of the arrester housing — constant with temperature, unrelated to degradation

Total leakage current trending over time is a valid diagnostic, but it conflates the two components. For a more precise diagnostic, use the third harmonic analysis method below.

For LV SPDs in distribution boards, total leakage is less easily measured online (the SPD's earth terminal is internal to the board). Assess these by measuring the board's total earth leakage increase after SPD installation, or by de-energised testing during maintenance shutdowns.

Method 2: Third Harmonic Analysis (Resistive Leakage Isolation)

The resistive component of leakage current (the degradation indicator) is in phase with the supply voltage, while the capacitive component leads voltage by 90°. Specialised surge arrester analysers separate these components using harmonic analysis — specifically, the third harmonic of the resistive current is a sensitive indicator of MOV degradation that is free of capacitive contamination.

This method requires a dedicated surge arrester analyser rather than a standard clamp meter. It is the internationally recommended method for MV surge arrester condition monitoring per IEC 60099 and is used by Singapore's power utilities and major industrial operators for their high-value arrester assets.

Method 3: De-Energised DC Leakage Test (LV SPDs)

For LV Type 1 and Type 2 SPDs, the most common testing approach during periodic inspection is a de-energised DC leakage current test. With the SPD disconnected from the supply, apply a DC voltage slightly below the SPD's clamping voltage and measure the leakage current. Compare against the manufacturer's specification and against previous measurements. A significant increase indicates MOV degradation.

This test is simple and can be performed using a quality insulation resistance tester if the correct voltage range is selected. However, be careful: many LV SPDs have integrated thermal disconnect devices that interrupt the circuit when the SPD overheats — verify that the disconnect hasn't operated before testing, and verify the test voltage doesn't exceed the SPD's rated DC withstand voltage.

Pro Tip

In Singapore's high-lightning environment, keep a surge event log for your building. Most modern SPDs have a surge event counter — a simple LED or mechanical indicator that shows how many significant surge events the SPD has absorbed. Check this counter at every inspection. An SPD that has absorbed 50+ significant events may be at end of life regardless of its leakage current reading, because the energy rating is cumulative. Some manufacturers specify a maximum rated discharge count — treat this as a hard replacement trigger, not a guideline.

When to Replace: Decision Criteria for Singapore Buildings

Surge arresters should be replaced when:

Leakage current has increased by more than 20% from the installation baseline over any 12-month period — indicates accelerating degradation
Third harmonic resistive current exceeds the manufacturer's alert threshold — the most reliable single indicator for MV arresters
Any visible physical damage: cracked housing, discolouration, burn marks, or evidence of partial discharge tracking on porcelain or polymer housings
Following a confirmed direct strike to the structure or its LPS — even if the arrester appears functional, its energy handling capacity is now reduced and the margin of protection cannot be assumed
Age exceeds 15–20 years in Singapore's high-event-frequency environment, even if leakage appears normal. MOV degradation can be non-uniform, and a uniformly high leakage reading may mask uneven degradation in one column of the arrester stack
SPD replacement indicator is active — many LV SPDs have a window indicator or LED that shows the device has absorbed a disqualifying event and must be replaced

Watch Out

A surge protection device that has operated its thermal disconnect — its internal safety mechanism that disconnects it from the supply to prevent fire — is effectively dead. It looks installed. The breaker feeding it is still on. But it is no longer protecting anything. In Singapore buildings, this is more common than facilities teams realise — SPDs silently die in events that no one records, and the next lightning season passes with no protection in place. Check the SPD indicator windows at every inspection. A grey or red indicator window means replace immediately.

Protecting Singapore's Critical Infrastructure: The Full System View

For hospitals, data centres, MRT-related facilities, water treatment plants, and other critical Singapore infrastructure, surge protection is a system discipline, not a point solution. A comprehensive approach includes:

External lightning protection system (LPS) to BS EN 62305 / SS555, with annual inspection and earth resistance testing
Type 1 SPDs at all building service entrances, coordinated with the LPS earth
Type 2 SPDs at sub-distribution boards
Type 3 SPDs at sensitive equipment panels
SPD coordination — voltage protection levels and energy distribution between SPD types must be engineered; randomly installed SPDs from different manufacturers at different locations may not coordinate and can leave gaps in protection or cause the downstream SPD to absorb energy the upstream SPD should have handled

For surge arrester testing instruments and expert advice on protecting Singapore's critical infrastructure, browse our electrical testers and the Fluke industrial range for high-accuracy current measurement for leakage testing. Our SAC-SINGLAS accredited calibration laboratory can calibrate clamp meters and other instruments used in surge arrester condition monitoring programmes. For complex MV surge arrester testing programmes, speak to our technical team at the contact page.

Surge Arrester Testing: Protecting Singapore's Critical Infrastructure