Image credit: ABB Low Voltage | Legrand | GE Digital Energy

Surge protective devices (SPDs) protect against electrical surges and spikes in power distribution, communications systems, and other heavy industrial applications.

SPDs, like all surge suppressors, use some type of electrical technology to divert transient overcurrents safely to ground. For more detailed information on a surge protective device's basic operation, technology type, and voltage transients in general, please see GlobalSpec's Surge Suppressors Selection Guide.

A transient voltage spike on a single phase AC system. Image credit: IDX, Inc.

Surge protective devices were once known as transient voltage surge suppressors (TVSS), which is now an obsolete term. The term SPD is typically applied to non-plugin style industrial strength surge suppression devices which are capable of withstanding major voltage transients, as opposed to the lighter-duty plugin style suppressors found in residential use. For example, true surge protective devices may be installed on a television or radio antenna to protect a system from damage stemming from lightning strikes, or in a high voltage power distribution or process control system.

Multiple SPDs mounted within a power distribution board. Image credit: Direct Industry

Specifications

Signal Protection

A surge protective device may be compatible with one of several different types of signal line, including dataline and alternating current (AC) power.

Dataline

Dataline SPDs are used to protect equipment used in communications systems and DC power applications. The signals used in these systems are typically lower voltage than AC power systems, but dataline SPDs must still be rugged enough to protect against high voltage lightning strikes and other high voltage surges. These products are often configurable to protect multiple signal lines; for example, the SPD diagrammed below uses gas tube technology to protect eight individual communication lines.

A dataline SPD schematic. Image credit: L-com

AC Power

AC power surge protective devices are designed to divert overcurrents on AC mains power lines, and may be classified as single-phase or three-phase products depending upon the type of AC power used.

Single-phase power refers to alternating current systems where voltages fluctuate in unison. Single-phase AC is cheap to install and operate, but is not powerful enough to supply large motors and other industrial equipment. By contrast, three-phase power uses three different signals which peak at varying times. Using three separate signals eliminates the fluctuations inherent in a single-phase system. When used to power large motors, three-phase power results in less vibration and simpler equipment design.

The images below illustrate the waveforms of single-phase (left) and three-phase AC.

Image credit: The Science Forum | Power Products

Connectors

SPDs may use one of several types of connectors to interface with an electrical or data system.

• Electrical outlets have standard three-prong plugs and receptacles.
• Registered jack (RJ) connectors are widely used for networking. RJ11, RJ13, and RJ14 connectors are typically used for telecommunication lines, while RJ41, RJ45, and RJ48 jacks are used for data transfer.
• Screw terminals are secured by screwing a bare wire onto a contact. While screw terminals represent one of the oldest electrical connection designs, they are still widely used for making a rugged physical connection.

• DB connectors are ruggedized D-subminiature plugs classified by the number of pins and sockets used. For example, DB-9 connectors use nine pins, while DB-25 devices use 25 pins.

• Coaxial connectors include a wide range of radio frequency (RF) connectors, including N-type, BNC, and SMA.

Type

Surge protective devices are often classified by type, which in turn is based on the device's capability and durability. SPD type is defined by IEC 61643-11.

• Type 1 devices are the most rugged available and are able to protect systems from direct lightning strikes. They are recommended for use in commercial and industrial buildings and are characterized by a 10/350 μs current wave.
• Type 2 SPDs are installed in electrical switchboards, are suitable for low voltage protection, and are characterized by a 8/20 μs current wave.
• Type 3 devices have lower discharge capacity than Type 1 or 2 products. They are mandated as supplementary protective devices to be used in conjunction with Type 2 SPDs. 

References

Wiki-EIG - The Surge Protective Device (SPD)