So many things can cause spikes and surges, but a solid understanding and proper selection of TVSSs can help prevent damage to your equipment.

What is a transient voltage surge suppressor (TVSS)? It's nothing more than a very fast switch that protects your equipment from AC power line surges. The most common type of power conditioning equipment, the TVSS has been around for many years in its current form. Although they don't save energy, the number of TVSSs sold as energy-saving devices has dramatically increased in recent years. This trend may reflect greater consumer interest in energy conservation, but the fact remains: The only function served by a TVSS is to protect sensitive equipment from surges.

What does a TVSS do? It guards against power system transients, commonly known as spikes or surges. Various things cause transients. The most common, lightning, can cause surges in excess of 50,000V. Even with a TVSS, a lightning strike (also called an impulsive transient) will more than likely damage your equipment. Nothing can guarantee total protection from a direct lightning hit, but one type of TVSS (lightning arrester), installed at the main service entrance, provides the best protection.

Transients cause electronic equipment to fail because of the high energy usually associated with them. For example, the energy contained within a lightning strike is at a level that can destroy or degrade integrated circuits in microprocessor-based equipment. Everyday occurrences also cause transients. Every time a large piece of electrical equipment turns off, a brief surge, called a switching transient, results. Although these don't have the energy of a lightning transient, they occur more frequently and are often responsible for the majority of electronic equipment degradation.

With the exception of a direct lightning hit, proper use of TVSSs help protect your sensitive electronic equipment. To find out if your equipment has experienced a transient, look for these symptoms: damaged electronic equipment, such as computers, fax machines, televisions, and communication equipment; increased service calls on electronic equipment; and unexplained system shutdowns or errors.

How does a TVSS work? Since a TVSS is a voltage-sensitive switch, it constantly monitors the AC voltage input and output waveforms. Nothing happens under normal conditions, but when the voltage difference between the power source and the protected line gets high enough, the switch closes and diverts the transient away from the electronic equipment. We often call this switching voltage the clamping voltage of the TVSS. There are three types of TVSSs used in the market today: clamps, crowbars, and hybrids.

Clamps divert the surge to a grounding reference. The voltage waveform appears to cut off where the surge exceeds the rating of the TVSS. The majority of TVSSs purchased today are "clamps." They are quick to respond; however, they can't handle large amounts of surge current.

Crowbars are slow to respond to a surge, but they can handle enormous amounts of surge current. Called crowbars because they exhibit a negative resistance, they have the same effect as "shorting out" two car battery terminals with a crowbar. Unfortunately, relying solely on a crowbar can be hazardous to the electrical system because it's actually shorting the protected AC power conductors to ground.

Hybrids combine the fast response of the clamp with the energy capabilities of the crowbar. You do this with a sophisticated process of internally constructing the two with fuses and current-limiting impedances. This unit is very expensive and requires careful planning to make sure it won't harm equipment powered from AC power lines. But, it's the best of both worlds as far as surge protection goes.

How to select, install, and use a TVSS. There are two basic types of TVSSs: plug-in and hard-wired. You install plug-in TVSS units by plugging them into wall outlets to protect specific appliances or electronic devices. You use hard-wired TVSS units as a primary defense for sensitive circuits or even an entire building.

You should install plug-in TVSS equipment between the equipment that needs protection and its power source. Proper building wiring and grounding is essential if your plug-in TVSS is to do its job. For example, if there's no ground present at the circuit, the TVSS cannot divert the surge to a grounding reference. As a result, it remains idle. Never plug large motor loads into the same TVSS protecting the electronic equipment. These loads include fans, copiers, some laser printers, and window air conditioners. Locate the TVSS where you can easily access it and monitor the failure mode indicator (if equipped).

When installing hard-wired units, make sure you keep all conductor connections (including the ground) from the power panel to the TVSS as short and straight as possible.

Here's what to look for in a TVSS:

  • UL listing as "TVSS" or "Transient Voltage Surge Suppressor" on the UL label. Note: Some TVSS "look-alikes" are actually "current taps." These devices have no protective value and will have the word "TAP" near the UL label. Avoid them for use as a TVSS.

  • UL Clamping Voltage of 400V (0.4kV) or less for plug-in TVSS, or commensurate with your service voltage on hard-wired TVSSs.

  • Operation indicator.

  • Manufacturers warranty for at least one year, and schematics describing components in hard-wired TVSSs.