Key Considerations for Varistor Selection in Surge Protection Design

In SPD design, the Metal Oxide Varistor (MOV) serves as the main voltage-clamping component. Proper selection and application of the MOV are crucial to ensure system safety and stability.

Given the significant variations in power grid quality, lightning frequency and strength, and the surge withstand capability of end devices across regions, surge protection designs must be adapted to local conditions rather than using a one-size-fits-all approach.

All protection designs should be verified under real or simulated operating conditions to ensure reliable performance.

Selection of Varistor Voltage

The varistor voltage (U1mA) is typically calculated as:

U1mA = K × Uac

K is a coefficient related to power quality, usually ranging from 2 to 3;

Use smaller values in areas with stable power (e.g., cities) and higher values in regions with poor power quality (e.g., rural or mountainous areas).

Uac is the effective AC voltage (e.g., 220–240V).

For 220V~240V power systems, MOVs with varistor voltage ratings between 470V~620V are recommended.

Choosing a higher varistor voltage reduces false triggering and extends lifespan, though the clamping voltage may be slightly higher.

Calculation of Nominal Discharge Current

The nominal discharge current (In) of the MOV should withstand the surge current that may occur in actual operation.

Selection should account for the largest surge events likely to occur annually.

Refer to the MOV’s surge lifetime curve under 10 pulse events.

It is recommended to use approximately 30% of the maximum surge current (In ≈ 0.3 × IP).

Use of Parallel MOVs

When a single MOV cannot meet the discharge current requirement, multiple MOVs can be connected in parallel to increase surge capacity.

Even when current requirements are met, MOVs may be paralleled to lower the clamping voltage.

Key points for parallel configuration:

Devices must have matched parameters to ensure even current sharing;

Recommended matching tolerance: ΔU1mA ≤ 3V, Δα ≤ 3.

Summary

Proper selection and parallel configuration of MOVs are the foundation of a reliable surge protection system.

Design and validation should be tailored to local power conditions, lightning patterns, and equipment requirements.