The truth about transient voltage surge suppressors

A series of fatal fires recently have put transient voltage surge suppressors (TVSS) to the forefront of news again. It wasn’t necessarily the normal application of such devices that created hazards. When I heard a person stating he was removing all of the TVSS outlet strips from his house because they were unsafe and caused death, I thought it was time to review the facts again.

While surge suppression strips can be convenient with the multiple outlet plugs they provide, the amount of protection they offer can be misleading to the uninformed, and this protection will vary greatly, depending on the internal components. Since many electronic devices are very sensitive to voltage transients (spikes and surges), they should be protected whenever the total cost of the device being protected versus the cost of the suppression device is greater. Don’t forget the total cost includes the lost productivity when the equipment is down, not just the repair or replacement cost.

The environment where a TVSS is used considerably affects its lifetime and protection capabilities. While improperly designed surge suppressors can occasionally fail and start a fire, some fires are due to improper use. For example, connecting a suppressor capable of handling 15A to an extension cord capable of supporting only 10A can cause a fire, and the risk is greatly increased when the cord is hidden under a rug or furniture. Heat is a major contributor to reduced protector lifetimes and failure. Because the internals of most surge protectors dissipate heat, and this dissipation increases with temperature, thermal runaway and failure are possible without proper ventilation. This apparently was the case in one of the recent fatal fires.

In order for a surge suppressor to be used outdoors, it needs to be rated for such use by UL, NEMA and other appropriate specifications. Another important note is that surge protection devices are intended for use only on the load side of a main service disconnect. Because most suppressors divert the resulting surge current to the ground connection, it is essential that surge protectors be used only with properly grounded outlets and not with 3-to-2 prong adapters.

Several characteristics of a protector need to be considered to ensure proper selection. It is essential the protector complies with the second edition of the UL1449 specification and is labeled as such. This specification revision requires thermal fuse protection against protector meltdown, that the protector is safe against catastrophic overvoltage, and that the protector is safe against shocks after being damaged. Also, the protector so rated can sustain two 3,000A surges and 20 500A surges, and it protects line-neutral, line-ground and neutral-ground, which provides both normal and common mode protection.

The clamping voltage or suppressed voltage rating listed on a protector is the approximate maximum voltage that will be seen by the load when the protector is working properly. The UL lists several voltage classes, and typically, lower clamping voltages are better, as long as they are above the peak levels of normal conditions.

The response time of a protector indicates how quickly a suppressor will react to a transient, and a quicker response is better. The typically quoted number for the quality of a suppressor is the Joule rating, which indicates the energy dissipation a suppressor can take before failing—the higher the number, the better. This energy rating is calculated from surge current, surge duration and clamping voltage, but measurements vary between manufacturers. A nominal voltage, maximum continuous operating voltage and single pulse surge current rating may be specified as well.

Most surge protectors are made with metal oxide varistors (MOVs), a voltage-controlled resistor placed between two lines (e.g., the hot line and ground). At a low voltage, the resistance is high, and no current flows; as the voltage increases, the MOV begins to lower its resistance and conduct more current, limiting the voltage that reaches the load. For large transients, the resistance lowers enough to essentially provide a short circuit through the MOV, which protects the load but can damage the protector. Subsequent transients may result in reduced protection. Other suppression methods, such as a gas discharge arrestor, can survive larger power dissipation but have longer response times.

Also, a suppressor has a current-
limiting fuse between the source and load to protect the MOV and load from overcurrent, especially if the MOV fails as a short circuit. If this occurs, the load is protected, but the rest of the electrical network needs to handle the short circuit current. By placing a fuse in the protector in series with the source and load before the MOV, a short circuit will blow the fuse and protect the load. If the MOV failed as an open circuit with no other protection devices in the circuit, all subsequent surges would reach the device that is intended to be protected, without the user knowing. This is true also if the fuse is placed in series with the MOV in the path.

Without proper design and proper overcurrent protection, the inside of TVSS outlet strip can end up looking like the above picture and, in the worst case, result in a fire, sometimes with a tragic outcome. EC

BINGHAM is pursuing a master’s degree in engineering at Cornell University and can be reached at stb25@cornell.edu.