Surge Protection Devices (SPDs)

Surge protection devices (SPDs)

pdf icon Surge Protection Devices (SPDs)

Insurance industry results indicate lightning is responsible for an estimated $5 billion in damages to property every year. Power surges from a variety of sources can do similar damage.

While lightning and power surges cannot be eliminated, some things can be done to protect against the damage caused by them. 

When lightning strikes a building there will be physical damage and a high likelihood of fire. Often there will also be damage to the building’s electrical system and its electronic equipment. Financial losses could include repair or replacement of the system or equipment, lost productivity, and other losses (loss of valuable data, etc.).

A direct hit or even near-miss on a nearby power line can result in surge currents being induced onto the utility lines. These surges will enter the electrical system via the buildings electrical service conductors. The surge currents travel along the power line searching for a ground.

Once the surge current enters a buildings electrical system, there is a good chance that damage will be done to the electrical system or equipment within that building. Surge protection devices are one way to protect against the damage caused by lightning and power surges.

What does a surge protection device do?

Surge protection devices (SPDs) are designed to limit the damage caused by surges on the devices they protect by bypassing or discharging the surge current to a safe level.

Surge protection device types

There are four types of SPDs.

Type 1 Surge Protective Devices: Surge arresters

A Type 1 SPD is called a surge arrester. It is designed to be installed on the line side of the service entrance main and be 1000 volts or less. These devices are typically installed by the local utility company.

Type 2 Surge Protective Devices: Transient voltage surge suppressors

A Type 2 SPD is called a transient voltage surge suppressor (TVSS). A TVSS must be installed on the load side of the service entrance main and be 600 volts or less.

Type 2 devices should be installed by a licensed electrician according to the manufacturer’s guidelines.

Type 3 Surge Protective Devices: Plug-in surge protectors (power strips)

A Type 3 SPD is a secondary layer of protection. It is cord-connected, direct plug-in, receptacle type that is used to protect a particular piece or pieces of equipment (computers, printers, flat-screen monitors) plugged into one SPD receptacle.

These must be installed a minimum conductor length of 30 feet from the electrical service panel.

A Type 3 (plug-in) device should be used as a secondary means of protection for electronic equipment. A Type 1 or 2 device should still be installed as the primary means of protection.

Type 4 Surge Protective Devices: Surge protection modules 

A Type 4 SPD is actually installed on the electrical component as protection.

Surge protective device certification

SPDs should be UL 1449 listed or recognized by another Nationally Recognized Testing Laboratory (NRTL).

Devices that are not UL-listed or approved by a NRTL should not be installed as they may create a potential fire hazard.

How to know a surge protection device is working

The SPDs should have a visual indicator, such as a light, that will indicate that the device is operational.

Some devices are a one-time use only. Once they bypass or discharge a surge, they are no longer operational and must be replaced.

Other heavier-duty devices will reset themselves after a surge.


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This bulletin has been prepared as an underwriting reference for members of Grinnell Mutual Reinsurance Company and does not signify approval or disapproval by the Company of any product or device. Please do not copy or reproduce any portion of this bulletin without the written permission of Grinnell Mutual Reinsurance Company. The information included in this publication and program was obtained from sources believed to be reliable, however Grinnell Mutual makes no guarantee of results and assumes no liability in connection with its use. It is the user’s responsibility to comply with any applicable regulations or laws. Information obtained from or via Grinnell Mutual should not be used as the basis for legal advice, but should be confirmed with alternative sources.