How to Select Surge Arresters for Solar PV and BESS Transformer Protection: A Field Engineer's Guide

Surge arrester selection gets treated as a checkbox item on most solar and BESS project equipment lists. The specification says "surge arrester, 10kV, IEC 60099-4 compliant" and procurement finds the lowest price that fits the description. The result, in a meaningful proportion of cases, is an arrester that is technically compliant but undersized for the actual surge energy environment of the installation — a gap that only becomes visible after a lightning event causes damage that adequate protection would have prevented.

This guide covers the selection parameters that actually determine whether a surge arrester will perform as intended in a PV or BESS application, starting with the ones most commonly underspecified.

Continuous Operating Voltage: The Parameter That Gets Ignored Most Often

The continuous operating voltage (Uc) of a surge arrester must exceed the maximum continuous operating voltage that will appear across the arrester terminals under any normal operating condition, including temporary overvoltages from load rejection, earth faults, or resonance conditions.

For a 10kV system, the nominal phase-to-earth voltage is 5.77kV. A straightforward calculation suggests an arrester with Uc of 6kV should be adequate. In practice, temporary overvoltages on distribution systems — particularly on isolated neutral or resonant earthed systems common in Southeast Asian and Australian distribution networks — may reach elevated levels during single-phase earth faults; the magnitude depends on the neutral earthing arrangement, protection clearing time and the project study.

An arrester sized only for nominal voltage that experiences a sustained temporary overvoltage above its Uc rating will begin to conduct thermally, heat up, and potentially fail in a mode that looks like lightning damage rather than thermal failure from overvoltage. Arrester failures of this type are chronically misattributed in the field.

For a 10 kV system, values such as 8.4 kV or 10.2 kV may be used as design starting points in some earthing arrangements, but the final Uc selection must follow the system temporary-overvoltage study and the arrester manufacturer's application data. Check the earthing arrangement of the system you are connecting to before specifying Uc — this single parameter explains more arrester field failures than any other specification error.

Energy Handling Capacity: Why the Datasheet Number Is Not the Whole Story

IEC 60099-4 defines arrester energy handling capability in terms of line discharge class, from Class 1 (lowest energy handling) through Class 5 (highest). For distribution transformer protection, line discharge Class 2 or Class 3 is commonly evaluated; the final class should follow transformer rating, fault level, line exposure and the manufacturer's energy-coordination study.

The line discharge class rating assumes a specific test waveform and a specific number of test shots. Real lightning environments do not follow test waveforms, and a location with high lightning incidence — common in tropical regions of Southeast Asia and in parts of Australia — means the arrester will accumulate energy from multiple events over its service life.

Composite polymer housed arresters — now the standard for outdoor distribution applications, replacing the older porcelain housed designs — handle multiple surge events better than their energy class rating alone suggests, because the polymer housing allows better heat dissipation between events than the sealed porcelain housing. For utility-scale PV installations in high-lightning-incidence areas, composite polymer housing is often preferred for its weight, pollution-performance and installation advantages; the project specification should confirm the required housing and tracking-resistance class.

Placement: Where the Arrester Goes Determines What It Protects

An arrester installed at the transformer terminals protects the transformer. An arrester installed at the cable entry point of a building protects the cable entry equipment. An arrester installed 50 meters from the transformer, connected by an unshielded lead, provides substantially less protection to the transformer than its datasheet rating suggests — the inductance of the connecting lead generates a voltage drop during surge current flow that adds to the voltage appearing across the transformer terminals.

For transformer protection, the arrester should be mounted as close to the transformer terminals as physically practical, with the shortest possible connecting leads. The lead length matters: each metre of connecting lead adds inductive voltage during a fast surge; the actual value depends on lead geometry, surge current rise time and installation layout, so the protection study should minimise and calculate lead inductance rather than rely on a fixed rule. For a transformer with a lightning impulse withstand voltage of 75kV, an arrester that limits voltage to 30kV at its terminals but is connected with 5 meters of lead can deliver 35kV or more to the transformer terminals — still within withstand limits, but with less margin than the protection level calculation assumed.

DC-Side Protection in PV Systems: A Different Calculation

PV array DC collection systems require surge protection devices (SPDs) at combiner boxes and at inverter DC inputs, but the specification approach differs from AC-side arrester selection.

DC systems do not benefit from current zero crossings to extinguish arc following a surge event. A DC SPD must be capable of extinguishing the follow current — the power-frequency current that flows through the SPD after the surge has passed — under DC conditions. Not all SPDs designed for AC applications will extinguish follow current reliably under DC voltage. The DC voltage rating of a DC-side SPD must exceed the maximum DC open-circuit voltage of the connected string under cold, high-irradiance conditions — based on the maximum cold-temperature open-circuit voltage of the connected strings and the safety margin required by the applicable PV standard and device manufacturer.

IEC 61643-11 governs SPD selection for DC applications in PV systems and should be the reference standard for DC-side protection specification, not IEC 60099-4 which covers AC surge arresters.

What to Ask Your Supplier Before Ordering

The following questions will quickly distinguish suppliers who understand their products from those who are primarily competing on price:

What is the Uc rating and how was the system earthing arrangement factored into the selection? What is the line discharge class and what energy handling assumption underlies it? Is the housing material composite polymer or porcelain, and what is the tracking and erosion resistance rating of the polymer? What is the partial discharge level at 1.05 times Uc, and is there a test certificate available? For DC-side SPDs: what is the maximum DC voltage rating and what is the follow current interruption capability?

Suppliers who can answer these questions with specific numbers and supporting documentation are selling products they understand. Those who respond with "our products meet IEC standards" and a price list are selling compliance paperwork, not engineering solutions.