It would be difficult, at best, to find someone in today's high-tech electronics world who hasn't been affected by electrical overstress. The small geometry and high density of circuit components in this electronic age is susceptible to transient overvoltage of a few volts. High-density microcircuits routinely operate at three or five volts and have low tolerance for transient overvoltage.

Published studies have demonstrated that annual costs of electrical disturbances exceed $30 billion in the USA alone. Data processing downtime attributed to power quality has increased from 27% in 1980 to almost 50% at present.

Almost every user of electronic equipment has observed that equipment either fail outright, go "off-line", go to "reset", or experience shortened life. Computers, from industrial control to personal computers frequently lose their way, act strange or require soft re-start if not exhibiting outright "hard failure".

Since there is no "wear-out" phenomena in solid state devices, equipment essentially should never fail. Studies show that 75 to 90% of all electronic failure is due to overvoltage stress alone. The balance of failures is usually heat related due to faulty designs.

An agreement on common terms to describe electrical overstress has largely eluded the industry; however, the Institute of Electrical and Electronic Engineers (IEEE) and the American National Standards Institute (ANSI) refer to "Surge Voltages" and "Switching Transients" in their discussions of recommended practice for protection of electronic equipment in an AC power and data line environment.

Sources of transients range from natural phenomena to power disturbances to normal operation of "noisy" electrical equipment.

Switching Transients - Whenever the flow of current is interrupted, transient overvoltages are created. As the magnetic field of an inductor collapses, stored energy is released causing a voltage rise that attempts to maintain the current flow. Solenoids, relays, transformers, inductors, motors and so forth, are all devices that release energy when turned off. Contacts opening, fuses clearing, circuit breakers tripping etc. are other examples of current interruption which generate transient voltages. Power utility line faults, load shedding equipment activity and capacitor bank switching are also frequent sources of transient overvoltage. High frequency switching power supplies, found in almost all new equipment, are a powerful transient generator and have caused regulatory agencies to issue standards to control conducted emissions into AC lines.

ESD Transients - Electro-static discharge phenomenon is generated from the friction of two dissimilar materials. This triboelectric effect is observed when electric charges of opposite polarity build up between two surfaces, and then the surfaces are separated. The human body can store as high as 15,000 volts as one walks across a nylon carpet. Generally, only transients of over 3,000 volts would be noticed as the discharge into electronic equipment or another surface occurs. Lower voltage discharges would continue to occur but go unnoticed.

Nuclear Electromagnetic Pulse - Similar to a lightning generated EMP, but with a much faster risetime. The electromagnetic pulse is generated when nuclear ordinance is detonated. This threat has caused the military to "radiation harden" most military weaponry and tactical equipment.

Lightning Transients - (The most awesome and most damaging source of all transients). Lightning transients are most often discussed by industry transient suppression "experts" because they represent the greatest threat and source of transients known. If protective apparatus were designed to withstand the effects of lightning in a harsh industrial environment, then certainly all other threats of lower magnitude would be eliminated as well.

Lightning occurs when friction within clouds raises the charge potential to a level sufficient to ionize air and provide a conductive path either from cloud to cloud or cloud to ground.

Studies show that the current during the first stroke can exceed 200,000 Amps. Average strikes produce over 20,000 Amps during the first return stroke. (See Figure 1)

Figure 1

Electromagnetic waves are formed by the discharge and radiate at 90 degrees from the path of the strike. So we see electromagnetic waves propagating through air directly away from the discharge. We also observe ground (Earth) currents propagating away from the point of ground strike. Direct strikes on power cables inject high currents into primary circuits producing voltage transients by flowing through ground resistance or through the impedance of the primary circuit. (See Figure 2)

Figure 2

Thus, we observe lightning-generated transients flowing into primary circuits through electromagnetic coupling or direct injection. In addition, we observe ground currents flowing into equipment through the equipment ground from nearby ground strikes. These ground currents can cause transient voltage differences of high magnitude across the various ground points within a structure or from structure to structure.

Results of Transient Overvoltage

"Walking Wounded" (Latent failure) - Probably the most disturbing of the effects of transients is that they often go unnoticed. As we have shown, overvoltage transients exist in our everyday world. Unless we have outright equipment failure or very frequent operating disturbances, most users assume they are "safe". As equipment is bombarded with destructive transients, its life is shortened. We may see no effects or observe any strange behavior, and, therefore, not become concerned about the degradation occurring to our equipment.

Equipment may survive a damaging transient by showing small or no upset, only to fail in six months or so as metallization creepage eventually shorts out the "punch through" hole in the micro circuit junction.

Soft Failure - This most common failure of all types, which at best leads to shortened equipment life, and at worst shows latent catastrophic failure. Transient generated soft failures include going "off-line", reset, run error, communication error, measurement or reading errors, lock-up, lost or corrupted files, latch or lock-up, output errors and so forth.

Hard Failure - This failure is easily observed and generally causes concern. The result may be a charred mass of molten electronics, a component with its lid blown off, a cracked or burned component, a vaporized circuit board trace or wire, but sometimes leaves no visible effects. The equipment is just out of service. (See Figure 3)

Figure 3