This article gives information on how to install Surge Protective Devices (SPDs) in order to protect against transients overvoltages generated by powerline disturbances and natural phenomena such as lightning striking near exposed conductors, according to Clause 2.7.3.

SPDs are not intended to protect against long-duration overvoltage conditions, power outages, or brownouts. The installation of SPDs may be required under one or more of the following conditions:

• High Lightning Risk: Locations that have frequent lightning activity.
• Frequent Power Disturbances: Areas that are industrial zones and whose power supply is not stable.
• Remote Power Supply: Areas that are situated at the end of a long overhead powerline.
• Exposed Locations: Situations where there are installations on elevated terrains such as hills.
• Distant Sites: These include outer suburban or rural areas that have a distance between installations.
• Sensitive Equipment: Locations that have sensitive devices such as home offices, home theaters, or computer networks.

Installation of SPDs

Location

• SPDs are located where the main electrical supply originates or at the main switchboard.
• SPDs are installed at remote switchboards and must be coordinated with a primary SPD, in a manner specified by the switchboard manufacturers.
• In some situations, such as sensitive electronic equipment at hand, supplemental localized protection may be needed, which applies to the equipment itself.
• Specialized sites such as telecommunications hubs, remote telemetry stations, and industrial process control facilities require special consideration. See AS/NZS 1768 for further information.

Installation

• Installation: Installed after the main switch but before any Residual Current Device (RCD).
• Protection: Protected by a separate fuse or circuit breaker.
• Connection at main switchboard: Between phase and neutral at the main switchboard.
• Connection at Remote Switchboards: For switchboards without MEN connections, connect from each phase to neutral and from neutral to earth.
• Labelling: SPDs shall be marked clearly and indelibly in accordance with Clause 2.10.5.1.
• Status Indication: If SPDs are alarmed to provide status indication, they shall fail safely.

Figure F1 shows the connection of a primary SPD for a single-phase system.

Figure F2 shows an example of a secondary SPD installation for a single-phase system.

Selection of Surge Protection Devices (SPDs)

Surge Ratings for Domestic Supplies

• Surge Rating: Imax = 40 kA per phase (8/20 μs impulse)
• Maximum Operating Voltage: 275 V AC to 320 V AC

Installation Categories

Overcurrent Protective Devices for SPDs

Protection Requirements

Current Rating Guidelines

• Follow Manufacturer Recommendations: It must not exceed the SPD’s maximum backup fuse rating as specified by the manufacturer.
• Be Less Than the Upstream Device Rating: The protective device rating should always be lower than the immediate upstream device.

• 32 A for a 40 kA SPD
• 63 A for a 100 kA SPD

RCD Considerations

• An SPD, connected downstream of an RCD (Residual Current Device), shall have the RCD’s breaking capacity be at least 3 kA.
• S-type RCDs compliant with AS/NZS 61008.1 and AS/NZS 61009.1 are acceptable.

Conductor to Connect SPD

Minimum Cross-Sectional Area for the Connecting Conductor

Length

Neutral Conductor Connection

Surge Protective Earthing and Bonding

Overvoltage Protection

• Prevent injury to persons.
• Prevent damage to equipment.

Figure F3 show an example of how a transient develops a hazardous voltage difference in a building

A transient overvoltage on the powerline can cause a large Surge current to flow to earth via the MEN link in the electrical switchboard. It is usually tens of thousands of volts that temporarily rise the local earth potential. Every object connected to the local earth—earthed appliances like a refrigerator, metallic pipes, or concrete floor slabs—will also go up in potential.

To make it less expensive to install overvoltage protection in residential buildings, all conductive services should be routed into the structure as close as possible to the main earth bar. For optimum performance, the conductor connecting the telecommunications primary protection SPD to the main earth bar should not exceed 1.5 meters in length.

It is highly recommended that metallic services such as gas and water pipes, and conductive structures like concrete floors and metallic walls, be bonded. Primary protection for telecommunications lines is usually provided by a gas discharge tube type SPD. In residential environments, it should be installed in a wall box near the electrical switchboard and connected directly to the main earth bar. Figure F4 shows the connection within the protector housing.

There is specific concern for the earthing of conductive telecommunication apparatus. Currently, standards require equipment to have a hardwired earth connection for the types listed. The SPDs on telecommunications services and cable screens as well as the antennas and CATV system need to be bonded via the telecommunications earth to the main earth bar.

Some electric installation practices may limit accessibility to the earth electrode as well as to the earthing conductor. Installing an additional bonding bar or terminal that should be placed outside of the principal switchboard would ensure that an appropriate short telecommunications bonding conductor exists. This can often be installed on an outside wall adjacent to the residential’s meter box or switchboard. Installing such a bar at the time of a new switchboard installation will only assist in the telecommunications service installers’ efforts.

This approach is detailed in AS/CA S009 and is illustrated below in Figure F4.

Figure F4 shows a preffered method of bonding the primary telecommunications protector to the main earth

Surge Protective Devices are very important in the protection of electrical systems against transient overvoltages resulting from powerline disturbances and natural phenomena such as lightning. Proper installation and coordination with applicable standards, such as AS/NZS 1768 and AS/CA S009, ensure effective protection and minimize risks to sensitive equipment and infrastructure.

The other important considerations include an appropriate SPD selected based upon the environment and exposure levels, correct installation locations within the main switchboard, and then following the manufacturer’s rating for surge and overcurrent protection. Safe earthing and bonding practice add other safety features to telecommunications and other applications.

With this information on SPD installation requirements and best practices, homeowners, businesses, and specialized facilities can cut down on transient overvoltage risks associated with systems and protect valuable electronic equipment.