Metal-Oxide Surge Arresters: Working Principle, Selection Parameters and Installation Guide for Distribution Systems

Why Modern Distribution Protection Relies on Zinc-Oxide Arresters

Almost all surge protection applied to medium- and high-voltage equipment today uses zinc-oxide (ZnO) metal-oxide surge arresters without series spark gaps. Compared with the older silicon-carbide gapped design, the gapless metal-oxide arrester responds faster, has a flatter protection characteristic, and absorbs far more energy in a smaller envelope.

The reason lies in the material's extreme non-linearity. Under normal system voltage the arrester draws only microamperes, so it behaves almost like an open circuit and has negligible effect on the network. When a surge arrives, the same element becomes a low-impedance path and diverts thousands of amperes to earth, clamping the voltage across the protected equipment to a defined residual level. As soon as the surge passes, the arrester returns to its high-impedance state.

The Parameters That Determine Correct Selection

Arrester selection is a balance between two opposing goals: the arrester must survive continuous and temporary overvoltages, yet it must clamp low enough to protect the insulation of the equipment it shields. Choosing a higher-rated arrester improves survival but reduces the protection margin.

Rated voltage (Ur) and continuous operating voltage (Uc / MCOV)

The rated voltage is the maximum power-frequency voltage the arrester can withstand for a defined short duration without thermal instability. The continuous operating voltage (called MCOV or Uc in the North American tradition) is the maximum power-frequency voltage that may be applied indefinitely.

Two standards frameworks govern this: IEC 60099-4 (metal-oxide surge arresters without gaps for AC systems) and IEEE C62.11. They describe the same physics with different terminology, and a specification written only against one of them, without the class-specific part and the actual application data, is ambiguous.

How to pick the continuous operating voltage

• Solidly grounded systems: the continuous operating voltage should be at least the maximum line-to-ground voltage. On a 12.47 kV multigrounded system this works out to approximately 7.65 kV.
• Ungrounded or impedance-grounded systems: the continuous operating voltage should be at least 90 percent of the maximum phase-to-phase voltage, because a single line-to-ground fault raises the healthy-phase voltage to full line value for the duration of the fault.
• Temporary overvoltage check: the arrester's TOV capability curve must exceed the magnitude and duration of any expected temporary overvoltage from load rejection, back-feed, or ferroresonance.

Residual voltage and protection margin

The residual voltage is what appears across the arrester terminals during a discharge. It must sit comfortably below the basic lightning impulse level (BIL) of the protected equipment. Typical practice is to require a protection margin of at least 20 percent between the arrester's residual voltage at the nominal discharge current and the equipment's impulse withstand level.

Nominal discharge current and line discharge class

Distribution arresters are commonly rated at 5 kA or 10 kA nominal discharge current. The line discharge class (1 through 5) quantifies the energy the arrester can absorb from a long-duration switching surge without damage. Distribution-class units are appropriate for transformers and cable terminations on distribution feeders; station-class units with higher energy capability are specified where switching surges are severe.

Pressure relief rating

This is not a current-carrying rating. It is a short-circuit rating that confirms the arrester will fail in a controlled manner rather than explosively if its internal element is punctured, so that a fault in the arrester does not endanger nearby personnel or equipment.

Reference Data for Common Distribution Ratings

Note that a rated voltage of 10 kV is frequently used on nominal 10 kV systems; the surplus above the theoretical minimum provides the TOV margin needed for an ungrounded or resonant-grounded network.

Installation Practices That Decide Whether Protection Works

An arrester with impeccable ratings can still fail to protect if it is installed badly. The dominant factor is lead length. The inductive voltage drop along the connecting leads adds directly to the residual voltage seen by the equipment, so the total lead length between the protected equipment, the arrester, and the earthing system should be as short and as straight as practically possible. Where long leads are unavoidable, consider a second arrester installed close to the protected equipment terminals.

Other installation points worth enforcing:

• Route the earth lead directly to the station earth grid or the transformer tank earth, not to a convenient nearby structure.
• Avoid sharp bends and loops, which raise the impulse impedance of the lead.
• Maintain the specified creepage distance; polymer-housed distribution arresters are normally designed for a minimum specific creepage of about 31 mm/kV, suitable for heavy pollution.
• Observe the minimum clearance between the arrester and adjacent equipment. Too close disturbs the voltage distribution across the arrester; too far exposes the equipment to higher overvoltage.
• Mount polymer arresters vertically unless the manufacturer explicitly permits otherwise, and support the unit so that wind and short-circuit forces are not carried by the line connection alone.
• Fit an impulse counter or a leakage-current monitor where the operator needs evidence of surge activity or of progressive degradation.

Commissioning, Maintenance and Diagnosis

Arresters are passive devices, but they should not be forgotten after energisation.

• Before energisation: measure insulation resistance, verify the earth connection by continuity test, and confirm the arrester rating against the drawing — incorrect rating is a frequent commissioning defect because the units look identical externally.
• Routine inspection: look for cracked or eroded polymer sheds, damaged seals at the end fittings, and signs of moisture ingress. Contamination should be washed off at intervals appropriate to the site pollution level.
• Leakage current measurement: a rising resistive leakage current under the same applied voltage indicates ageing of the metal-oxide element. Trending the value over years is more informative than a single reading against a threshold.
• Counter readings: an unexpectedly high count suggests the network is experiencing repeated switching or lightning activity that deserves separate investigation.
• Replacement criteria: any arrester that has absorbed a direct lightning strike or has operated repeatedly should be tested, and any unit showing mechanical damage should be replaced promptly rather than left in service.

Frequently Asked Questions

Why not simply choose the highest-rated arrester available? Because a higher rating raises the residual voltage, which reduces the protection margin for the insulation you are trying to protect. Select the lowest rating that will survive the system's real overvoltage conditions.

Can one arrester protect a whole substation? One well-placed arrester can protect equipment in its immediate vicinity. Long cable runs and separate structures need their own arresters, since the travelling wave will regenerate between them.

Do arresters need a spark gap? No. Modern metal-oxide arresters are gapless by design; the zinc-oxide element provides both the insulation under normal voltage and the conduction path under surge.

How often should an arrester be replaced? There is no fixed interval. Replace on condition: visible damage, failed insulation test, or measured leakage current that has risen significantly from the original commissioning value.