Today, in the field of power quality, there are many terms that are used to describe devices that are designed to protect equipment from the impact of voltage surges or transients on conductors connected to the equipment. Two common terms include Surge Suppression Equipment (SSE) and Surge Suppression Devices (SPD). Another common term, and the term that we will use, is Transient Voltage Surge Suppression (TVSS). All these different terms are used to describe the same basic devices.

The purpose of TVSS is to eliminate or reduce damage to such equipment as computers, telephone systems, fax machines, various manufacturing equipment, and other devices, by limiting transient (surge) voltages and currents (surges) on electrical circuits. These transients or surges may come from inside a facility, or may be injected into a facility from the outside. When machinery inside a facility such as motors, air conditioners, laser printers, etc. is switched on and off transients can result which can cause equipment malfunction, failure, or damage. If a car hits a utility pole, or if lightning strikes at or near a power line, large transients may be carried into a facility. These transients are capable of causing significant damage to equipment and electronics.

The transient causes damage to a device when the transient voltage exceeds the weakest exposed component's ability to withstand that voltage. This "withstand" voltage is quite high on some equipment (over 1000 volts on a typical motor) but is quite low for other types of equipment (a microprocessor circuit may be able to withstand only a few volts).

Transients normally flow into equipment via electrical conductors. However, it is common for equipment to have entering electrical paths other than its power leads. These paths can include telephone lines, data-com lines, measurement and control lines, DC power buses and neutral and ground lines. It is possible, and even common, for transients to travel over these paths as well.

The correct protection principal is to install TVSS devices that connect to all points of potential voltage threat and limit this voltage to a level below the equipment "withstand" voltage. For example, with power lines, voltage transients must be limited between line to neutral, line to ground, and neutral to ground. With telephone lines, which contain two wire pairs, both line to line and line to ground modes need to be protected.

TVSS devices may contain any number of discreet electronic components. Each component has strengths and weaknesses. These components include metal oxide varistors (MOVs), gas discharge tubes (GDT), avalanche diodes, reactive circuits, and capacitors..

Most commercial surge suppressors combine several of the above technologies along with noise reduction circuitry, and better surge suppressors arrange them in multiple stages to prolong life and improve response time. Heavy-duty components such as GDTs or large MOVs may form the first stage and absorb the brunt of the surge. A second stage with tighter control (more MOVs or avalanche diodes) knocks the surge voltage down even more.

TVSS protection is typically applied at several points throughout a facility. This equipment is classified in three categories. Category C type equipment is applied at the service entrance. The service entrance point installations are considered the first line of defense against externally caused power disruptions. TVSS installed at these points will help guard against lighting strikes entering a facility via power lines. Usually, heavy-duty protection is required at these points to suppress voltage transients to levels that future layers of protection can eliminate.

A second common location for TVSS application is distribution panels. This is classified as category B type equipment. Usually TVSS application at distribution panels forms a second layer of defense against externally generated transients. However, protection at this point also is very effective in suppressing the much more frequent internally generated transients.

Data and telecommunications circuits are extremely vulnerable to transients. This is especially true at building entry points. Because this equipment is often so essential and important, serious protection is essential.

Critical equipment is often fed from branch panels. TVSS protection applied at branch panels can address the ever-changing transient conditions that are often common at these locations.

Finally, TVSS is often applied at the individual circuit level. Category A type equipment is applied at these locations. Process control equipment, sensing and monitoring equipment, DC power supplies and other such equipment may all require individual protection. In addition, plug-in units are often installed at point of use locations. This point of use application provides a final layer of protection against both internally and externally generated transients.

As you can see, TVSS is a complicated subject. We have just scratched the surface. It is important to remember that all TVSS devices are not created equally. The user must choose a device based on the likelihood of damage, the value of the equipment to be protected, and the degree of protection desired. Watch for future articles dealing with specific TVSS application.