Protecting computer systems 

against power transients 

Because small systems have moved from computer rooms into officesy factories, and homes, 

users and systems designers must deal with the subtle dangers the machines encounter 

For the third time in less than three weeks, the sky 

darkened and thunder rumbled in the distance. With 

that ominous warning, the appointed "thunder 

scout" decided it was time to pull the plugs of the 

central unit and remote terminals of his CAD/CAM 

computer system. Better to shut down the operation 

than risk damage to the system, as had occurred in 

the two previous storms. 

But pulling the plugs did not help. When the 

storm was over and the system was restarted, permanent damage had been done to it-to the chagrin of both the 

operator and the service contractor. 

In this common example, the damage was caused not by powerline surges but by differences in ground potential at various terminals of the system. The oversimplified assumption that powerline surge problems could be eliminated had led the uninformed 

operator to attempt a simple prevention step. Not only did it not 

work, but it created a safety hazard: unplugging the line cords 

removed the safety ground, leaving the equipment still connected to the data lines where the surges were occurring. 

Understanding the general causes of, and remedies for, power 

transients can help users of small computer systems, especially 

stand-alones, protect their systems with do-it-yourself methods. 

More complex systems may need the attention of a specialist. Systems designers should also be aware of the way users hook up 

their systems, the potential damage that could be caused by power 

transients, and side effects of incorrectly applied measures. 

Growing concern among computer users that power-line surges may damage equipment or cause loss of data has created a 

market for surge suppressors. But clear performance standards 

in the industry are lacking, and several standards-writing groups 

are striving to develop adequate ones. To make a difficult choice 

among these devices, the consumer should learn some basic rules 

about selecting and installing a suitable surge suppressor. Even 

the best surge suppressor, if incorrectly applied, might not work 

and could cause adverse side effects. 

Transient origins 

While the term "transient" is often understood as a transient 

overvoltage, it is also more broadly interpreted as the occurrence 

of any disturbance, either on the power line or the computer system's data line. 

The most obvious source of an electrical disturbance is a lightning strike, but the lightning bolt need not hit power lines to cause 

damage. Because the electromagnetic field radiated by the lightning current couples into the conductors of power lines or data 

lines, it induces transient voltages along these conductors. Also, 

as the lightning current spreads into the ground, it produces 

differences in potential at points that are normally at "ground" 

potential. Conductors spanning some distance between their ends 

Franqois Martzlo ff 

National Institute of Standards and Technoloav 

in the area where the lightning current is spreading 

will be exposed to these differences of potential, or 

to a transient overvoltage. 

Though the direct effects of lightning can be dramatic, their relatively low rate of occurrence can lull 

one into complacency, and most of their widespread 

indirect effects can be overcome through sound protection practices. On the other hand, electrostatic 

discharges, which could be considered miniature 

lightning discharges, require only the fingertips of 

mortals rather than an Olympian fistful of lightning bolts to have 

very serious effects ["How to defeat electrostatic discharge," 

IEEE Spectrum, August 1989, pp. 36-40]. 

A less obvious but more frequent source of transients is switching sequences in the power system. Switching can be a normal, 

recurrent operation such as turning a local load on and off, or 

it could entail occasionally clearing an overload or short circuit. 

These switching transients cover a wide range of frequencies 

and amplitudes. Some have a brief duration (nanoseconds) and 

involve little energy (millijoules). While they present little risk 

of damage, their high-frequency spectrum makes them likely 

sources of interference. Others have a longer duration (microseconds or even milliseconds) and involve greater energy (up to 

hundreds of joules) with lower frequencies. They have the opposite trait of low risk of interference because of the relatively 

low frequencies, but because of the longer duration and increased 

energy, they have a higher damage risk. 

Another source of disturbance to computer systems is the occurrence of an undervoltage that could be caused by a nearby 

startup of heavy loads or by distant faults, such as lightninginduced line flashover, falling trees, or utility lines downed by 

runaway vehicles. While transient overvoltages can be easily 

suppressed-a more correct description would be "mitigateduby a simple added device that diverts the excess energy, the reduced 

energy associated with an undervoltage cannot be supplemented by a simple device. Different methods are needed for a solution of that problem. 

Over the years, the need to learn more about the characteristics of these transients has led to various projects aimed at 

monitoring power-line disturbances. One result of these projects-which we performed by isolated researchers, sometimes 

with equipment designed by the researchers rather than commercial equipment-is that their reports are based on different assumptions and definitions of disturbances. Comparing results 

can thus become difficult and confusing [see table, overleaf]. 

Leaving the problems of monitoring transients and designing 

protective devices to the specialists, an informed user can take 

several steps toward buttressing the reliability and integrity of the 

system. The first step is to distinguish between mere temporary 

upset and permanent damage, each of which has a different impact on the user, depending on the relative importance of the 

operation. For a commercial setup, disrupting the operation can 

be more expensive than repairing the damage so that protecting 

IEFE SPFCTRUM APRIL 1990 0018-9235/90/0400-0037$1 00 c 1990 IEEE 3 7 
data integrity ranks high. For an engineer working at his home 

computer, however, damage protection may be more important 

than some data loss. In this case, limiting the protection expense 

to avoiding damage and accepting interruptions may be preferred. 

Vulnerable stand-alones 

Small computer "systems" can be categorized as stand-alone 

systems or distributed systems. A stand-alone system is typically a one-operator setup consisting of a desktop computer coupled with a printer, or any microcomputer not linked to a network. Distributed systems range from a simple stand-alone 

augmented by a telephone or other network link to multiplestation systems or process control systems with remote sensors 

and actuators. 

Found in offices, laboratories, and homes, stand-alone systems 

can be disrupted or damaged by two possible causes. First, transients with low amplitudes (less than 1000 volts) are buffered by 

the computer's power supply but might still couple into circuits 

and cause glitches. Transients of high amplitudes (over 1000 V) 

may at worst damage the power input components and are likely to cause glitches at best. Second, power interruptions (sags or 

outages) can cause a momentary shutdown. 

Transient damage protection for these systems is simple to 

achieve and is probably built in to some degree. However, until 

the day arrives when equipment has its transient capability stated on the nameplate (which may be sooner than expected because 

the Europeans are increasingly concerned with electromagnetic 

Common troublesome scenarios 

compatibility issues), the user has no way to know the extent of 

that protection. The European approach, motivated by a 1989 

Directive on Electromagnetic Compatibility promulgated by the 

European Community Council, requires that equipment must 

operate satisfactorily in a specified environment without introducing intolerable disturbances into that environment. Thus, this ability is likely to be stated explicitly, in addition to the usual voltage, frequency, and current ratings now required. 

So far, the approach has been one of purchasing additional 

peace of mind by inserting a separate surge suppressor (also called 

spike protector and transient voltage suppressor) on the power 

cord. Prices for these devices range over an order of magnitude, 

and claims of performance may include the fastest response (an 

irrelevant issue) and the lowest clamping voltage (a reliability risk 

because the transient protector may fail under abnormal power 

fluctuations). 

Though its basic technology does not change rapidly, details 

of the rating and packaging of a surge suppressor are driven by 

market competition. Ideally, its rating should reflect three basic 

requirements: the nominal line voltage, the surge current capability, and the clamping voltage during the surge. All of these 

should be stated by the maker of the device with due consideration to the user's needs for protection and long-term reliability. 

At this time, there is only one performance standard in the 

United States for transient voltage surge suppressors, UL 1449, 

which was developed by the Underwriters Laboratories. This standard specifies primarily the safety aspects of the product, but 

does contain some performance specifications. The UL 

in same outlet 

in different outlets 

Power-line anddataline interfaces (such as a facsimile or an- swering machine) 

Distributed system 

with remote terminals (such as three 

PCs connected to a printer or three 

dumb terminals 

linked to a central 

processing unit) 

Systems in separate 

buildings 

Line transient 

Ground potential 

differences 

Differences in ground 

potential during sur- ges, even with individual line protection 

Line transients in 

individual cords; 

operation of built-in 

suppression raises 

"ground" potential 

Line transients in 

individual cords; 

ground potential 

differences 

industry consensus standards 

on the severity of the environSpike suppressor ment. In the UL test, a specified surge current is applied to 

Local ground the device and the maximum 

window resulting voltage is measured; 

this is then indicated on the 

product. 

Product literature for some 

Local ground devices, however, makes claims 

window of response time in nanoseconds-even picoseconds-a 

feature that is not important in 

a power system. Nanosecond 

pulses do not propagate very far 

in power systems, and measuring a picosecond response time 

in support of the claim would 

be a technical challenge. LikeLocal ground win- wise, emphasis on achieving the 

at each termi- lowest clamping voltage only nal; an optical-fiber 

link is an alternative demonstrates imbalance in the 

design goals: the object of a 

surge suppressor is to lessen the 

surge level from the thousands 

of volts that can occur occasionally; it is not to shave off 

the last tens of volts from the 

special (a protection level in a "lower is 

specialist's task) be- better" bid for ranking in the 

cause of problems purchaser's choice. An excesdue to ground grid sively low clamping voltage indesign, National troduces the risk of premature 

Electrical Code is- aging, even failure, of the device 

sues, when the power line goes and so on; an optical-fiber link is an al- through repeated momentary 

ternative overvoltages, or "swells. " 

The second type of disturIEEE SPECTRUM APRIL 1990 
Types of disturbances and effective protection equipment 

Undervoltage Overvoltage 

Momentarv overvoltaae 

(swell) Sag --------- 

Spikes, impulses, surges 

Outage 

Electrical noise, harmonic distortion 

Undervoltage or overvoltage are conditions of abnormally low or high voltages last~ng more than a few seconds 

and caused by circuit overioads, poor 

voltage regulation, and intent~onal 

reductions by the utility (brownout) 

Voltage sags (voltage decreases outside normal tolerance lasting less than 

a few seconds) that are often caused 

when heavy loads are started, by 

lightning, and by power system faults. 

swells are brief voltage increases often 

caused by sudden load decreases or 

turn-off of heavy equipment 

Spikes (~mpulses, switching surges. 

or lightning surges) are microsecondto millisecond-long voltage increases, 

ranging in amplitude from 200 to 6000 

volts and caused by lightning, switching of heavy loads, and short circults 

or power system faults 

An outage is the complete loss of 

power for several milliseconds to 

several hours and may be caused by 

power system faults, accidents involv- 

~ng power lines, transformer failures. 

and generator failures. Some sensitive 

equipment may be disrupted by outages as short as 15 ms 

Electrical noise is a distortion of the 

normal sinewave power and can be 

caused by radar and radio transmitters, fluorescent lights, power electronic control circuits, arcing utility 

and industrial equipment, loads with 

sol~d-state rectifiers, and switchlng 

power supplies typically used in computer systems 

bance, a sag or outage, cannot be corrected by a surge suppressor. 

The computer operation is interrupted when the sag or outage exceeds the capability of the internal dc supply to power the logic 

and memory circuits. Most computers have a built-in capability 

to maintain operation for a short time when this power is lost, but 

that supply is drained out if the interruption is long enough. If 

the computer is using a disk drive when the sag occurs, a shutdown is likely; in an office using several identical machines, some 

ride through a disturbance while others, especially those reading 

from a disk, shut down and have to be restarted. Protection against 

such sags and outages requires an uninterruptible power supply 

(UPS), which is now readily available. In fact, the volume of UPS 

production as well as competition has brought prices down so low 

that purchasing one becomes a viable solution and, for users dependent on the continuity of their operation, a must. 

Unexpected pro blerns 

A power outage or sag on distributed systems has the same effect as for stand-alone systems. A more subtle ~roblem, however. 

has crept in for some sophisticated systems thai includeautomat: 

ic restart, or rebooting, after a power interruption. Anecdotes have 

All equipment is affected, although most equipment is 

designed to tolerate 120 volts 

? 10 Dercent 

Sags affect power-downsensing crrcuitry on computers and large controllers 

and can cause equipment to 

shut down; swells can damage equipment (including 

splke suppressors) that have 

insufficient tolerance 

Electronic loads can be destroyed and transformer or 

motor insulation broken down 

All equipment is affected 

Electrical norse drsturbs 

microprocessor-based equipment, such as microcomputers and programmable 

controllers; harmonic distortion causes motor loads, sucl 

as compressors, pumps, and 

disk drives, to overheat 

Voltage regulator, line conditioner, or uninterruptible 

power supply (UPS) 

Voltage regulator, ferroresonant transformer, line 

conditioner, or UPS 

Spike suppressor (also called 

surge suppressor). or some 

line conditioners 

UPS or standby power supply 

Filter, isolation transformer, 

UPS, or some line conditioners 

circulated of damage caused by repeated sags during the automatic 

rebooting sequence, typically occurring because of multiple lightning strokes or during fault clearing with automatic reclosing by 

the utility system. 

In the case of surges, as soon as a simple stand-alone system 

is augmented by peripherals, additional remote terminals, networking, and sensors that require a data link, the threat that the system will be affected increases. Even what may appear as a standalone system, such as a simple desktop pair of a PC linked with 

a printer, might be at risk if the two units are plugged into different power receptacles fed by separate branch circuits from the 

breakers. 

In addition to the risk of interference or damage from surges 

on the power line, the data-line input and output ports are also 

vulnerable. Several mechanisms can inject interfering or damaging transients into the data lines of distributed systems. First, a 

problem could result because data lines act as antennas that can 

collect energy from electromagnetic fields and feed it, as noise or 

surges, to the data port's input or output, the driver or the receiver of the computer, or its peripherals. 

The problem's severity increases with the length of the data link. 

Martzloff-Protecting computers against transients 
Within the same room, the risk of damage is minimal. But as the 

communication link reaches farther out, the risk increases that a 

surge would not only interfere with a system but could damage 

it. Though there may be an unknown (to the user) built-in protection or inherent capability of the data port components to withstand these surges, little is known about the occurrence of surges 

on data lines, compared to that on ac power lines, which makes 

the task of designing protection difficult. 

For users of computer systems in the same room or the same 

corner of a building, the built-in capability probably suffices. For 

systems with longer reach, the ultimate protection is an opticalfiber link with no metallic jacket, which provides immunity against 

noise collection as well as possible surge damage. For these complex systems, however, the do-it-yourself approach should be 

replaced by one that has been designed by a specialist. 

Another mechanism that could cause trouble is the difference 

in the potential of objects at nominal "ground" potential occurring during surge events. Most data links operate with the signal 

reference conductor (shield or one wire of a group) connected to 

the chassis of the equipment. This chassis is in turn connected to 

the grounding conductor of the power cord supplying the equipment, a requirement of the National Electrical Code. Thus, if lightning or power system faults inject a high current in the site's ground 

conductors, the potential of the "grounded" points at the two ends 

of the data link differs. This potential difference causes a current 

to flow into the data link, possibly exceeding the capability of the 

input or output components. 

The user can stay with conductors for the data link or convert 

(or initially design) it to an optical-fiber link, an approach that 

is becoming increasingly popular as hardware costs fall with economies of scale. However, if the conversion electronics at the ends 

of the fiber link are disturbed by electrical noise, that noise will 

be faithfully transmitted, not blocked. 

If a conductive data link is to remain, the remedy is to insert 

protective devices that are complementary for the power line and 

data line. These devices typically operate by limiting the overvoltage or attenuating the higher frequencies by filtering, which works 

effectively on the power line but not on the data link. Here, filtering is not possible because it would affect the signals; limiting the 

overvoltages will eliminate that damage risk, but might still let 

through a spurious signal. Thus, data integrity may be more difficult to achieve unless the software includes inherent immunity 

or fault tolerance. 

Side effects 

Avoiding damage with protective devices may then seem to require only the insertion of a power-line surge suppressor at the 

wall receptacle and a data-line surge suppressor at the input to the 

computer. This apparent simplicity, however, is deceptive because 

the very operation of this device, if incorrectly installed, can have 

a side effect that would put the data link components at risk, a 

mechanism that is only beginning to be fully recognized. 

Still another mechanism can be demonstrated by a scenario that 

can occur in any building with power and telephone service. The 

incoming telephone line is provided with surge suppressors (carbon blocks or gas tubes) that divert surges to the nearest grounded conductor, generally a nearby water pipe. The manufacturer 

of the computer or modem used for the computer-telephone-line 

linkup may have provided a protective device within the equipment. Alternately, the surge-conscious user may have inserted a 

protective device in the power cord. But should a surge occur on 

either the data line or the power line, the corresponding protective device will dutifully divert that surge to the nearest ground. 

Since the "nearest ground" may not be the same for the connection of the two suppressors, the surge current in the ground connection raises the potential of one side with respect to the other, 

placing the data input at risk. 

The solution is a miniature setup of the "ground window" concept developed by telephone companies in protecting their central station switches: all cables entering a room or a complete floor 

in a building are routed through a single "window" where grounding conductors, shields, and ground connections of protective 

devices are bonded together. In this manner, there cannot be any 

potential difference between the various ground reference points 

within the room or floor. 

Some surge suppressor manufacturers have adapted that concept to a portable version of the ground window, a device consisting of a suppressor for the power line and one for the data line, 

but packaged in a single box most likely sharing the same ground 

connection. This local ground window is now found in computer 

or hobby stores and is easily recognizable because it features both 

a power connection (male plug for connection to a wall receptacle and female receptacles for powering the loads) and a pair of 

data link connectors (input and output). Depending on what is 

needed, these connectors can be a standard telephone jack, a multipin RS232, or a cable television coaxial connector. The device 

is then inserted near the computer, with the power cord and data 

link routed through its connectors. 

Another protection scheme is always available: disconnect the 

system when not in use! In fact, some of the consumer guidance 

folders inserted by the utilities with their monthly bills mention 

that approach. That option may not be practical for commercial 

operations, where some link could be left connected, creating the 

risk of ungrounded equipment. Thus, if applied, every link to the 

outside world must be disconnected. 

To probe further 

A good source of information on the basics of lightning is the 

book Understandiing Lightning by Martin A. Uman, available from 

Academic Press, New York, 1971. Solutions to noise problems are 

given a general treatment in the second edition of Noise Reduction Techniques in Electronic Systems by Henry W. Ott, available 

from John Wiley & Sons, New York, 1989. Fundamentals of surge 

protection techniques are treated in Protection of Electronic Circuits from Overvoltages by Ronald B. Standler, also available from 

John Wiley & Sons, 1989. Another useful reference is Uninterruptible Power Supplies by David C. Griffith, published by Marcel Dekker, New York, 1989. 

Guidance on the nature and severity of transients (not specifications for protective devices) is given in the IEEE Guide for Surge 

Voltages in Low Voltage AC Power Circuits, American National 

Standards Institute, 032.41-1980, available from the IEEE Service 

Center, 445 Hoes Lane, Box 1331, Piscataway, N.J.; 800-678-IEEE. 

A paper by Francois Martzloff and Thomas Gruzs titled "Power 

Quality Site Surveys: Facts, Fiction and Fallacies" in the November 1988 IEEE Industry Applications Society Tmnsactions presents 

a review of recording, analyzing, and reporting transient disturbances on power lines. Another paper, "Coupling, Propagation, 

and Side Effects of Surges in an Industrial Wiring System," by 

Martzloff in the same Transactions is in press. The journal is available from the IEEE Service Center. 

About the author 

Francois Martzloff (F), an electronics engineer at the National 

Institute of Standards and Technology (NIST) in Gaithersburg, 

Md., is responsible for a program of power quality and surge protection. He serves as chairman of the IEEE Working Group on 

Surge Characterization and as Secretary of a Working Group on 

~lectroma~netic Compatibility (on characterization ofthe electromagnetic - environment) of the International Electrotechnical Commission, contributing to the development of related standards. Before joining NIST, he worked in corporate R&D for General 

Electric Co. in Schenectady, N.Y., working on transient measurements, surge protection of electronics, and application of varistors. He holds a B.S. from the Ecole Speciale de MCcanique et 

d'Electricit6, Paris, an M.S.E.E. from Georgia Institute of Technology, Atlanta, and an M.S. in industrial administration from 

Union College in Schenectady. The concepts in this article are 

based on his research work and troubleshooting experience, as well 

as years of discussion with IEEE colleagues. + 

40 IEEE SPECTRUM APRIL 1990