What is a Surge Protection Device (SPD)?

A surge protection device (SPD) is a protective device designed to limit transient voltage by diverting or restricting surge current and capable of repeating these functions as prescribed. SPDs were previously known as Transient Voltage Surge Suppressors (TVSS) or Secondary Surge Arrestors (SSA). Secondary Surge Arrestors is a legacy term (often used by utilities) and most commonly refers to a device not certified to ANSI/UL 1449. In 2009, following the adoption of ANSI/UL 1449 (Version 3), the term Transient Voltage Surge Suppressor (TVSS) was replaced by Surge Protection Device (SPD).

Surge protection (SPD) is a cost-effective solution to prevent downtime, improve system reliability, data, and eliminate equipment damage due to transients and surges for both power and signal lines. It is suitable for any facility or load (1000 volts or less). Typical SPD applications include:

Electrical distribution cabinets, control cabinets, programmable controllers, electronic motor controllers, equipment monitoring, lighting circuits, medical equipment, loads, backup power, UPS equipment, HVAC
Communication circuits, telephone or fax lines, cable TV, security systems, alarm signaling, entertainment centers or stereo systems, or household appliances.

According to the National Electrical Code (NEC) and ANSI/UL 1449 standards, SPDs are designated as follows:

Type 1 SPDs: Permanently connected devices approved by UL for installation anywhere between the transformer load and the main switchgear. Type 1 SPDs can also be installed anywhere on the load side of the switchgear and are permitted to be installed anywhere on the low-voltage electrical system without the need for dedicated fuses or circuit breakers. The primary purpose of Type 1 SPDs is to protect the electrical system against external surges caused by lightning strikes on the power lines or by switching between high and medium voltage loads.

Type 2 SPDs: Permanently connected devices approved by UL for installation on the load side of the main switchgear. Type 2 SPDs may or may not require dedicated fuses or circuit breakers. Type 2 SPDs are primarily intended to protect sensitive electronic equipment and microprocessors against residual lightning energy, motor-generated surges, and surges generated within the facility.

Type 3 SPDs: These are devices installed at a conductor length of 10 meters or more from a Type 2 SPD. These devices are typically connected by wires, plugged directly into the socket type installed at the load being protected. Type 3 SPDs are primarily intended to protect sensitive electronic equipment and microprocessors against surges generated within the facility.

Type 4 SPDs: These are components and assemblies that are not UL-certified. They may bear UL-certified markings. Examples include MOVs and gas tubes.

The difference between Type 1 and Type 2 SPDs is: Type 1 SPDs do not require the installation of dedicated fuses or circuit breakers and can be installed before or after the main disconnect switch of the main panel. Type 2 SPDs require a means of disconnection such as a circuit breaker.