Surge protective device

[edit] Introduction

A surge protective device (SPD) is a device intended to limit transient overvoltages and divert associated surge currents in an electrical installation. SPDs are used to reduce the risk of damage to electrical and electronic equipment caused by transient disturbances such as those associated with lightning, electrical switching and faults in power systems.

Transient overvoltages can affect power supplies, control systems, information technology equipment, communications systems and other electronic equipment. An SPD provides a controlled path for surge current so that the voltage appearing across protected equipment is limited to a level appropriate for the installation and equipment.

SPDs form part of an overall electrical protection strategy. Their effectiveness depends on appropriate selection, installation, coordination with other protective devices and, where relevant, the provision of lightning protection and equipotential bonding.

[edit] How surge protective devices work

Under normal operating conditions, an SPD presents a high impedance and has little effect on the electrical circuit. When a transient overvoltage occurs, the SPD changes its electrical characteristics and provides a low-impedance path for the associated surge current. This limits the voltage that can appear across the protected circuit. Once the transient has passed, the device returns to its normal high-impedance state.

Common technologies used in SPDs include metal oxide varistors (MOVs), gas discharge tubes (GDTs) and semiconductor devices such as transient voltage suppressor diodes. Different technologies have different electrical characteristics and may be combined within a single SPD.

An SPD does not normally absorb all of the energy of a surge. Instead, it diverts surge current and limits the resulting voltage. The connection between the SPD and the electrical installation is therefore important, particularly because the inductance of connecting conductors can produce additional voltage during a rapidly changing surge current.

[edit] Types of surge protective device

SPDs can be classified according to their intended location and application. Devices may be installed at the origin of an electrical installation, within distribution boards or close to sensitive equipment. Different types are used for different levels and characteristics of surge exposure.

SPDs are also classified according to the type of system and the nature of the surges they are intended to handle. In installations incorporating a lightning protection system, the SPD arrangement may need to provide protection against partial lightning currents as well as switching transients.

Surge protection can also be provided on circuits other than mains power supplies. SPDs are available for telecommunications, data, signalling, control and other electrical circuits where transient overvoltages could cause damage or disruption.

[edit] Selection and installation

The selection of an SPD should take account of the characteristics of the electrical installation, the expected sources of transient overvoltage and the equipment requiring protection. Important parameters include the system voltage, frequency, earthing arrangement, maximum continuous operating voltage, nominal discharge current, impulse current capability where applicable, voltage protection level and short-circuit withstand characteristics.

The location of the SPD is also significant. Installing protection at the origin of an installation can reduce the propagation of incoming transients, while additional SPDs may be required closer to sensitive equipment. Where several SPDs are installed, they should be appropriately coordinated so that their operating characteristics are compatible.

The wiring arrangement and conductor lengths between an SPD and the equipment being protected should be considered during installation. Excessive conductor length can increase the residual voltage presented to the equipment during a surge.

SPDs should be installed in accordance with the requirements of the applicable electrical installation standard and the manufacturer's installation instructions. Their installation should not compromise other protective measures within the electrical system.

[edit] Applications and limitations

SPDs are used in residential, commercial, industrial and infrastructure installations. Applications can include protection of computers and electronic equipment, building management systems, fire and security systems, telecommunications equipment, industrial control systems and other equipment that may be susceptible to transient overvoltages.

Protection may be particularly important where an installation contains expensive or safety-critical electronic equipment, where interruption of operation could have significant consequences, or where the installation is exposed to a relatively high risk of lightning or electrical disturbances.

An SPD does not prevent lightning strikes and cannot guarantee that equipment will remain undamaged following a direct lightning strike or severe surge. Effective protection against lightning may require a coordinated system incorporating external lightning protection, earthing, equipotential bonding and SPDs. The appropriate measures depend on the characteristics and risk profile of the building and its electrical and electronic systems.

SPDs can also deteriorate or fail following repeated or particularly severe surge events. Many devices incorporate an indication of their operational status and some are designed to disconnect from the installation when their protective components reach the end of their service life. Inspection and replacement should be carried out in accordance with the applicable requirements and the characteristics of the installed device.

[edit] Surge protective devices and power strips

A power strip or extension lead primarily provides additional socket outlets and does not necessarily provide protection against transient overvoltages. A product described as a surge protector incorporates additional protective components designed to limit transient overvoltages.

The presence of an SPD does not, however, make an electrical installation inherently safe from all forms of electrical fault. SPDs are one element of an electrical protection system and do not replace fuses, circuit-breakers, residual current devices or other protective measures.

[edit] Protection against lightning

Lightning can produce substantial transient overvoltages in electrical and electronic systems. These can be caused by a direct strike or by electromagnetic and conductive effects associated with a nearby strike. A lightning protection system provides a controlled path for lightning current and helps protect a structure from physical damage, while SPDs can reduce the risk of damage to electrical and electronic systems from associated transient overvoltages.

Where a building has an external lightning protection system, the design of surge protection should be coordinated with that system. Equipotential bonding and suitable earthing arrangements are also important in controlling potential differences during a lightning event.

[edit] Maintenance and inspection

SPDs should be inspected as part of the maintenance of the electrical installation. The inspection should include checking the condition and status indication of the device, where provided, and identifying signs of overheating, physical damage or failure.

The required maintenance interval depends on the type of installation, the environment, the risk of lightning and surges, and the requirements of the relevant standards. An SPD that has operated beyond its specified capability or reached the end of its service life may require replacement.

[edit] Related articles on Designing Buildings

• Power surge
• Lightning protection system
• Lightning conductor
• Earthing
• Electrical safety
• Electrical wiring
• Conductor
• 18th Edition Wiring Regulations
• Concept services design

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