Surge Protective Devices (SPD): How They Work, Types, and Selection Guide

A transient overvoltage (surge), caused by lightning strikes or switching operations, can severely damage sensitive electronic equipment. A Surge Protective Device (SPD), also called a surge arrester or surge protector, normally stays in a high-impedance state during normal operation. When the line voltage exceeds its clamping threshold—typically within nanoseconds—the SPD switches to low impedance and diverts the surge current safely to ground, clamping the residual voltage to a level the downstream equipment can tolerate.

Per IEC 61643-11, SPDs are classified into three types for cascaded protection:

Type 1​ (spark gap / gas discharge tube, tested with 10/350 µs waveform) is installed at the main distribution board to discharge direct lightning energy.

Type 2​ (metal oxide varistor, tested with 8/20 µs waveform) is installed at sub-distribution boards to handle induced surges and switching overvoltages.

Type 3​ (fine protection, often with TVS diodes) is mounted close to sensitive terminal equipment to suppress residual surges from Type 2 devices.

Type 1 and Type 2 SPDs should be coordinated with a backup protective device such as a Type D miniature circuit breaker.

When selecting an SPD, check these key specifications:

Uc (Maximum Continuous Operating Voltage):​ must exceed the system's maximum operating voltage (e.g., Uc ≥ 275 V for a 230 V AC system).

Up (Voltage Protection Level):​ lower is better; typically ≤ 1.5 kV for protecting sensitive electronics.

In (Nominal Discharge Current) / Iimp (Impulse Current):​ choose according to the SPD type and local lightning exposure.

Response time:​ should be < 25 ns for power SPDs.

Prefer models with a visual status indicator to show end-of-life or fault conditions.

Proper earthing and short connection leads (< 0.5 m) are essential for effective protection. A coordinated Type 1 + Type 2 + Type 3 scheme offers the best safeguard for modern electrical installations.