Surge Arresters and Insulation Coordination: IEC 60099-4 / IEC 60071 Guide

How to coordinate metal-oxide surge arresters with system grounding, temporary overvoltage, residual voltage, equipment withstand and lead inductance.

1. What the Arrester Must Coordinate

IEC 60099-4:2014 covers gapless metal-oxide surge arresters for AC circuits above 1 kV. IEC 60071-1:2019 provides the insulation-coordination procedure for selecting rated withstand voltages associated with the highest voltage for equipment. The arrester is therefore one part of a system study, not a substitute for the equipment BIL or power-frequency withstand rating.

2. Core Selection Parameters

ParameterMeaning in the specification
UcContinuous operating voltage the arrester can withstand continuously
UrArrester rated voltage associated with temporary-overvoltage capability
UresResidual voltage at the specified discharge current and wave shape
Energy / line-discharge dutyAbility to absorb switching and lightning energy without thermal runaway
Housing / creepagePollution, altitude, UV, salt and mechanical environment suitability

For a solidly grounded three-phase system, the continuous phase-to-earth voltage starts from the system highest voltage:

Uph-e = (Um) / (√(3))

The final Uc and Ur choice must include grounding factor, fault-clearing time and temporary overvoltages; it must not be selected from nominal line voltage alone.

3. Layout and Lead-Inductance Rule

Place the arrester as close as practicable to the protected transformer, cable termination or GIS bushing. Keep phase and earth leads short, straight and separated from sensitive control wiring. The protection margin compares the equipment withstand level with the arrester residual voltage plus the voltage rise created by lead inductance during a steep surge.

4. Verification Checklist

  • Define lightning impulse, switching impulse and power-frequency withstand for the protected equipment.
  • Model direct strokes, incoming-line surges, transformer terminals, cable transitions and GIS interfaces.
  • Check earth-grid impedance, discharge counter, leakage-current monitoring and thermal stability.
  • Verify altitude, pollution, creepage, housing class and mechanical cantilever against the actual site.

*Reference: IEC 60099-4:2014; IEC 60071-1:2019; IEC 60071-2:2023.*