What Is Surge Protective Device (SPD)?

1. Overview

Surge Protective Device, also known as a surge protector, is an essential component used in lightning and overvoltage protection for electronic equipment. It was previously referred to as a “lightning arrester” or “overvoltage protector.” The international abbreviation for Surge Protective Device is SPD.

The primary function of an SPD is to limit transient overvoltages and divert surge currents. When sudden overvoltage enters a power line or signal line due to external interference—such as lightning—an SPD restricts the voltage within a safe range or discharges excessive energy to the ground. This prevents the protected equipment or system from being damaged.

Selecting an SPD is central to preventing potential risks. The level of protection required depends on the user’s safety awareness and the technical standards of the application.

If a residential distribution box already includes a Class B SPD, and the risk of lightning in the area is low, users may choose not to install an additional SPD in the indoor distribution box. Instead, they may use surge-protected power strips specifically for household appliances.

SPDs are typically installed either in parallel or series with the equipment they protect. When an overvoltage occurs, the SPD conducts instantly, diverting the surge and limiting the voltage to safe levels. This protects devices in the circuit from electrical damage.

SPDs are suitable for AC 50/60 Hz power systems with nominal voltages from 220V to 380V. They protect against indirect lightning, direct lightning, and other forms of transient overvoltage. They are widely used in residential buildings, commercial facilities, and industrial environments.

2. Purpose

Surge, sometimes called “voltage spike,” is a sudden and momentary overvoltage that far exceeds normal operating voltage. It is a rapid, intense pulse that lasts only a few microseconds.

When external interference causes an electrical or communication line to generate a sudden spike voltage or current, the SPD activates immediately, shunting the surge and preventing it from damaging downstream devices.

Common causes of surges include:

- Lightning strikes
- Short circuits
- Power switching
- Startup of heavy machinery or large motors

A product equipped with surge protection components can absorb sudden, high-energy pulses and prevent connected devices from being damaged.

3. Basic Characteristics of SPDs

(1) High surge-current handling capacity, low residual voltage, and fast response.
(2) Advanced arc-suppression technology to eliminate fire risks.
(3) International electrical standards require SPDs to include safety disconnection mechanisms.
(4) International electrical standards also require SPDs to include working status indicator lights.
(5) Robust structure and reliable performance.

4. Working Principle

Different types of SPDs have different structures depending on their application. However, all SPDs contain at least one type of nonlinear voltage-limiting component.

Common components used inside SPDs include:

- Spark gaps
- Gas discharge tubes (GDT)
- Metal oxide varistors (MOV)
- Transient voltage suppression diodes (TVS)
- Inductors and coils

These components work together to provide fast, effective surge suppression and reliable overvoltage protection.