Surge Arrester Selection Guide: How to Choose the Right Arrester for Your System

Selecting the wrong surge arrester can lead to catastrophic equipment failure during the first major overvoltage event.

1. Identifying Your System’s Protection Requirements

Before evaluating any arrester model, you must clearly define the electrical and environmental conditions your system presents. The arrester must withstand continuous operating voltage, temporary overvoltages, and switching surges while providing adequate protective margin for downstream insulation.

System Voltage and Configuration

Determine your system’s maximum continuous operating voltage (MCOV). For solidly grounded systems, the phase-to-ground voltage applies; for ungrounded or impedance-grounded systems, the full phase-to-phase voltage may appear across the arrester during a fault. Failing to account for grounding configuration is one of the most common selection errors.

Overvoltage Environment

Characterize the overvoltage threats your system faces:

• Lightning intensity: Ground flash density (NG) determines expected lightning surge frequency and magnitude
• Switching surges: Long transmission lines and capacitor banks generate switching overvoltages that can exceed 2.0 pu
• Temporary overvoltages (TOV): Fault conditions, ferro-resonance, and load rejection can sustain overvoltages for cycles to seconds

Insulation Coordination Target

Identify the withstand levels of the equipment you need to protect — transformers, cables, GIS. The arrester’s residual voltage (protective level) must provide sufficient margin below the equipment’s Basic Insulation Level (BIL) or Lightning Impulse Withstand Voltage (LIWV).

2. Key Selection Parameters and How to Evaluate Them

Once you understand your system’s requirements, you must evaluate each critical arrester parameter against those requirements. Each parameter carries specific implications for safety and performance.

MCOV (Maximum Continuous Operating Voltage)

MCOV is the maximum power-frequency voltage that can be applied continuously across the arrester. It must be at least equal to the highest sustained system voltage the arrester will see. For a 138 kV system with a maximum voltage of 145 kV, the phase-to-ground MCOV requirement for a grounded system is approximately 84 kV.

Rated Voltage (Ur)

The rated voltage establishes the arrester’s TOV capability — the temporary overvoltage it can withstand for a defined duration (typically 1 second or 10 seconds). Rated voltage is always higher than MCOV, usually by a factor of 1.25 to 1.44. If your system experiences TOV during ground faults, ensure the arrester’s rated voltage provides sufficient margin.

Energy Class (Line Discharge Class)

Energy class defines how much energy the arrester can absorb from switching surges without degrading. IEC classifies arresters into Line Discharge Classes 1 through 5, while IEEE uses similar energy-withstand ratings in kJ/kV of MCOV. Higher transmission voltages and longer lines demand higher energy classes.

Protective Level (Residual Voltage)

The residual voltage under lightning impulse (8/20 μs) and switching impulse (30/75 μs) determines how well the arrester protects downstream equipment. Lower residual voltage provides better protection but typically requires more MOV blocks, increasing cost and energy absorption capability.

Protective Margin Calculation

The protective margin must satisfy both lightning and switching impulse requirements:

• Lightning protective margin: BIL / Residual voltage (lightning) ≥ 1.20 (minimum 20% margin)
• Switching protective margin: BSL / Residual voltage (switching) ≥ 1.15 (minimum 15% margin)

Housing Material

Silicone rubber housings offer superior pollution performance, hydrophobicity transfer, and lighter weight. Porcelain housings provide proven long-term mechanical strength and resistance to animal damage. For polluted or coastal environments, silicone is strongly preferred.

3. Step-by-Step Selection Workflow and Practical Tips

With your requirements defined and parameters understood, follow this proven workflow to reach a reliable selection decision.

Step 1: Determine System Maximum Voltage

Obtain the highest operating voltage (Um) from your utility or system design. Convert to phase-to-ground for grounded systems: Um / √3. For ungrounded systems, use the full phase-to-phase voltage.

Step 2: Calculate Required MCOV

MCOV required = Maximum phase-to-ground voltage × 1.05 safety factor (some utilities apply 1.10). Round up to the nearest standard MCOV rating.

Step 3: Evaluate TOV Requirements

Determine the magnitude and duration of temporary overvoltages during faults. Use your system’s fault study results. Cross-reference with the manufacturer’s TOV capability curve at the applicable duration.

Step 4: Select Energy Class

Based on system voltage, line length, and switching surge energy, select the appropriate line discharge class. For most 69–230 kV systems, Class 3 is typical. For 345 kV and above with long lines, Class 4 or 5 is required.

Step 5: Verify Protective Margins

Calculate both lightning and switching protective margins using the arrester’s published residual voltages. If margins fall below 20% (lightning) or 15% (switching), consider an arrester with lower residual voltage or evaluate whether insulation coordination can be adjusted.

Step 6: Choose Housing and Mounting

Select housing material based on pollution severity (silicone for pollution class III+), and choose mounting configuration (pedestal, suspended, or base-mounted) based on substation layout and available space.

Step 7: Final Verification

Cross-check all parameters simultaneously: MCOV, TOV, energy class, protective margins, and housing. Ensure no single parameter is compromised to satisfy another. Request the manufacturer’s type test reports for IEC 60099-4 or IEEE C62.11 compliance.

4. IEC vs IEEE: Selection Methodologies Compared

The two dominant international standards — IEC 60099-4 and IEEE C62.11 — approach arrester selection from different philosophical starting points. Understanding these differences prevents confusion when interpreting manufacturer data or working on international projects.

On international projects, always verify which standard the client specifies and confirm the manufacturer provides type test certificates for that standard. Many manufacturers test to both standards, but the test parameters and pass criteria differ — an arrester meeting IEC requirements may not automatically satisfy IEEE, and vice versa.

5. Summary

• Pain point addressed: Incorrect arrester selection causes equipment damage, system outages, and safety hazards during overvoltage events
• Requirements first: Always define system voltage, grounding configuration, TOV environment, and insulation coordination targets before selecting any arrester
• Parameter-by-parameter evaluation: MCOV, rated voltage, energy class, and protective level must each be verified — no single parameter can compensate for another
• Workflow ensures completeness: The 7-step selection workflow prevents omissions, especially the critical protective margin verification
• Standard awareness matters: IEC and IEEE use different starting points and classification systems; confirm the applicable standard before procurement