Classic Computer Magazine Archive CREATIVE COMPUTING VOL. 10, NO. 5 / MAY 1984 / PAGE 44

Protect your computer from the power line monsters. David H. Ahl.

Protect Your Computer From The Power Line Monsters

The title of this article may sound somewhat silly. After all, you plug your computer into the power line; why would you want to protect it from the power line? Obviously you wouldn't-- you want the voltage from the power company--but you don't want all those other things that hitch a free ride on the line such as surges, spikes, sags, and noise.

As people invest more and more money in their personal computers, they become more concerned about power line protection--and with good reason. One lighting bolt can wipe out an entire computer system, while much smaller disturbances can create disk read/write errors, blow out modems, erase portions of memory, or worse.

All the magazines, including Creative Computing, carry pages of ads for surge filters, power line conditioners, and uninterruptible power systems. Which one is for you? How can you evaluate these units? Do you need one at all? Read on.

Power Line Monsters

There are five major sources of trouble on the power line: blackouts, brownouts, long-duration transients, short-duration transients, and high frequency interference. You many not have all of them where you work or live, but chances are you are subject to one or two.

For example, Creative Computing is located in an area of light industry with an electrified railroad track within 15 feet of the rear of the building. Blackouts and brownouts are rare, but transients and noise (high frequency interference) are common. On the other hand, I live in a more rural area and can count on at least one blackout practically every week. Some are as short as a few seconds, but many last for several hours. In addition, my lights dim whenever an air conditioner or shop tool starts in any house on the road.

A blackout is a total interruption of power. Short blackouts (a few seconds) generally result from the utility company cutting in a different generator or making a repair to a line, whereas longer blackouts are due to trees pulling down lines, lightning striking a transformer, and the like. No computer can run through a blackout, short or long, unless it is attached to an uninterruptible power supply that kicks in the instant it senses a decline in power line voltage.

A brownout is a planned reduction in voltage by the utility company during a period of peak power consumption. They occur most frequently from 5:00 to 8:00 p.m. during the summer months when people return home and all turn on their air conditioners at once. During a brownout, the nominal voltage (120 v) may be reduced to as low as 102 volts, although 110 to 115 is more common. In general, such reductions should not affect equipment adversely since the normal operating range of most electronic equipment is 105 to 130 v. On the other hand, during a brownout, your system will be much more susceptible to transients.

We have loosely defined a long-duration transient as one lasting 100 microseconds or longer. It can be either a sag (drop in voltage) or a surge (increase in voltage). Probably the most common is a sag due to a heavy load coming on line, for example, an air conditioner compressor or shop tool. Electric motors require much more current when starting than when running; you have probably noticed a brief dimming of lights when a motor starts. Since the entire power network essentially is compensating for a sag, it probably will not drag the voltage down enough to affect your computer adversely unless, as we mentioned earlier, it occurs during a brownout. Since most power line protection equipment cannot handle this situation, during a brownout your best bet is to turn off your computer and read a book until voltage is back up.

A long-duration surge is most commonly caused by the utility company changing the markeup of the local power grid. In other words, a generating plant may be cut which causes a momentary increase. Or a large load may be shut down. In either case, the network quickly compensates and brings the voltage back to normal. Most power line protection equipment compensates for surges along with the next problem, spikes.

A short-duration transient, usually a spike, is one of the most common problems, and potentially one of the most damaging. There are many causes of spikes: lightning, short circuits, turning on and off large inductive loads, and major changes in network loading. Lightnining is by far the most destructive short-duration transient. A lightning strike typically lasts two microseconds or less, but the amount of energy transmitted is astronomical. The only real protection against lightning is a well-grounded lightning rod--lightning arresters on TV antennas run a poor second--but even a lightning rod grounding a direct stirke is likely to be carrying so much voltage that flashover strikes will jump to other grounded (and non-grounded) conductors.

While direct strikes may be rare, strikes in your general geographic area can be transmitted on power lines as high voltage spikes of 1000 volts or more. Best bet in a lightning storm: turn off your computer and unplug it.

A much more common cause of spikes is the turning off of a large inductive load such as a motor or transformer. The residual energy within the magentic field of the device is transmitted over the power line and, depending upon the proximity and size of the device, could be as much as 5000 volts. Until the transformer core or motor winding is fully discharged (far less than one second), this transient will continue in an osciallatory manner. Smaller spikes, but bothersome nevertheless, are produced by turning on and off hair dryers, blenders, vacuum cleaners, and other small appliances. And general hash is created by small motors, fluorescent lamps, and the like.

High frequency noise is in a frequency range much higher than electrical current (60 Hz), usually from upper sound frequencies (7000 Hz) to 50 MHz or more. Since computers do many things in these frequency ranges (mpu cycle rates of 1 to 8 MHz, disk transfer rates of 50 to 500 KHz), excess noise can prove most annoying.

This noise is frequently called RFI (radio frequency interference) or EMI (electro-magnetic interference), but no matter what the name, the effect is the same: unwanted glitches. The noise can be transmitted through free space and picked up by a power cord acting as an antenna, or it can be carried directly on the line.

Power Line Protection

Manufacturers make a bewildering array of power line protection devices ranging from uninterruptible power supplies costing as much as $1000 (and more) to plug-in single line filters for under $5.00. For even less money, you can build your own device.

Steve Ciarcia discusses power line pollution and how to build an inexpensive transient suppressor in an excellent article in Byte, December 1983. However, assuming yhou would rather buy a commercial one, what should you know to make a good decision?

We obtained eight devices typical of the many available. We did not get an uninterruptible power supply, so perhaps we should discuss it first. This is the ultimate device, and indeed the only one that can protect against blackouts and major sags. Most of them use a battery which is kept charged during normal operation. However, if the voltage dips below a predetermined level, the battery cuts in and keeps the computer running. For how long? Not forever. Some units give you an audible warning and then provide power for 20 minutes or so until you can shut down the system in an orderly manner. Others provide power for a few hours.

If you have a hard disk system and are in an area subject to frequent blackouts, you might want to follow the lead of one of my neighbors who has the ultimate system: an uninterruptible power supply and a gasoline powered motor generator set that can be turned on for any outage lasting longer than a few minutes.

What To Look For

When you go shopping for a power line conditioner, you will learn one thing very quickly: there are no universally-used standards of measurements in the industry. Unlike computer which can be measured in terms of kilobytes of memory, 8 or 16 bits, disk capacity, and so on, there are few such measures for power line conditioners.

As we mentioned above, there are five potential problems on the power line. The first, blackouts, can be cured only with an uninterruptible power supply. Ditto, sags, particularly during a brownout. And you can't really expect a power line conditioner to take the place of a lightning rod. What's left?

Surge Protection

Surge protection is most often provided by means of a device called a metal oxide varistor (MOV). When the voltage is lower than its conduction threshold, it is inert. However, the instant the voltage exceeds this threshold, it becomes a conductor and shunts the excess voltage to ground. This device is the primary mode of protection offered by most commercial surge protectors. Inexpensive units have only one MOV; however, for complete protection, you should select a unit with three MOVs, one across the line and one from each side to ground. Some manufacturers refer to this as common mode and differential mode protection.

Protection provided by MOV devices is most effective on spikes. It provides some protection against long-duration surges, but none at all against voltage sags.

Kalglo provides complete specifications on all of their units in comprehensible language. For example, the unit we tested, the DPC, has a 5 nanosecond response time, handles a pulse of 4500 amperes, and the clamping starts at 150 volts. Electronic Protection Devices provides a different set of specs (for the Lemon): forward surge of 1440 amperes for 1/120 sec., pulse power dissipation of 10,800 watts for one ms, clamping response 5 nanoseconds. Other manufacturers say similar things in different terms. We look forward to the day when we can easily compare apples with apples.

In shopping for a surge protection device, you should look for one with a fast clamping time (times from one picosecond to fifty nanoseconds are typical). The clamping voltage should be reasonably low, preferably under 180 volts, and the pulse current handling (amperes) and power dissipation (watts or joules) both should be as high as possible.

Noise Suppression

The most common noise suppression technique is to put one or more capacitors across the line. Noise at different frequencies is most effectively attenuated by capacitors of different values rather than one middle-of-the-road value. The Tripp Lite Isobar, for example, has both a capacitor for VHF attenuation and one for high frequency attenuation in parallel across each pair of outlets.

In addition to capacitors across the line, some units also have a filter consisting of one or more capacitors and a torodial choke. Like capacitors alone, these filters are tuned to particular frequency ranges, and there may be more than one stage of filters in a unit.

Specifications on noise suppression are also a mixed bag. The Isobar spec sheet, for example, lists 6 db suppression at 500 KHz, 12 db at 1 MHz, and 30 db above 5 MHz. For their commercial grade Isolator line, Electronic Specialists lists 55 db attenuation from 100 KHz to 200 MHz. Ohm Electronics is more vague and specifies noise filtering of "more than 20 db, 2 to 100 MHz' for their Scooter XP4 Guard-It. When you have apples, oranges, and pears, it is tough to make compariksons.

Electrical and RF noise tend to be localized. If noise is a severe problem for you, when shopping for a noise suppressor, you should look for one with maximum attenuation (50 db or more) over a wide frequency spectrum. If you are in a "normal' environment, a suppressor with more modest protection will probably be sufficient.

Brownouts, Surges, and Sags

As mentioned above, the MOV spike protector will provide some protection against long-duration surges, but not against sags and brownouts. For this type of protection, you need a constant voltage transformer. This is found in only the larger, more expensive units.

The typical constant voltage transformer accepts input voltage over a range of 90 to 140 volts and, depending upon the output tap selected, cna put out 110, 115, 120, or 125 volts. In commercial units, usually the 120 volt tap is selected and, after filtering, the final output is about 117 volts. Although constant voltage transformers are available separately, most typically they are part of a total package such as the Kleen Line Conditioner from Electronic Specialists. Such a unit provides all of the protection and suppression features discussed above.

When shopping for a constant voltage power supply, the main thing to look for is a unit with sufficient load handling for your computer and peripherals. While most computers alone use less than 20 watts, disk drives add another 20 watts, a color monitor as much as 120 watts, and a daisy wheel printer as much as 200 watts. Add a few more boards and peripherals, and you will be over 500 watts in no time.

Circuit Breaker

Although most computers are fused, occasionally an internal problem develops in a non-fused component. In such a case, you would like current to the device to be cut off as soon as possible, both to prevent further damage and eliminate the possibility of a fire. To provide this protection, many power line protection devices, and even low-cost power strips, incorporate a circuit breaker.

Frankly, there isn't much difference between breakers--most have a current rating equal to the rating of the unit, say 15 amperes, and are able to interrupt up to 1000 amperes of fault current, i.e., a direct short circuit.

Some units use a fuse rather than a breaker--a fuse is slightly faster acting --but the amount of protection provided is roughly the same. If the unit you select uses a fuse, be sure to buy some extras to have on hand. There is nothing more maddening than to blow a fuse on a Saturday night and be down the rest of the weekend for want of a 15-cent replacement.

Extra Features

Beyond the basic features, many power line protection devices have one or more extra features. The most common--and most useful--is isolation of each receptacle on the unit. Since digital computing equipment is one of the major sources of electrical noise, it is desirable to isolate components, particularly computers and printers.

Another extra feature is the external switches. Most common is a switch for the entire unit, but some have switches for each bank of receptacles, each pair, or even each individual receptacle.

Indicator lights are another extra feature. Again, the most common is one indicator to show when the unit is on. Some units have a status light to show when everything is operating correctly and/or to indicate a trouble condition.

Not exactly a feature, but something that differs among the various units, is the method of mounting. Some units are designed simply to sit on the floor or a table. Others have brackets for floor, wall, or under table mounting. Several of the units are designed to plug directly into a dual wall receptacle and provide up to six outlets.

You also should look at the arrangement of the outlet receptacles before you buy. If your computer (or a peripheral) has a power transformer that plugs directly into a receptacle, the transformer may cover as many as four receptacles on some power protection devices. On the other hand, if all of your components terminate in standard plugs, this is of little concern, except, of course, you must have a receptacle for each plug.

Other differences are largely cosmetic: color of case, type of outlets and switches, and length of cord. As mentioned earlier, prices range from $5 to over $1000, although, as with most things, you get what you pay for.

We used a variety of different devices for several months and found that they functioned as advertised. On the other hand, we did not have the facilities to subject them to an exhaustive set of tests, but simply used them in our normal working environment.

Presented below is a short description of each of the devices we used.

There are many hundreds of protection devices available, from which you should be able to find one that meets your needs and your budget. At the end is a list of manufacturers that you can contact for more information.

Electronic Protection Devices

Electronic Protection Devices (EPD) has a line of colorful devices known as the Lemon, Orange, Peach, and Lime. If the colors offend you, they are also available in plain vanilla nlight gray).

We used the Lemon ($59.95) which is an AC surge protector. The Lemon plugs into a double wall outlet and provides six receptacles. It has a forward surge rating of 1440 amps for 1/120 second, peak pulse power dissipation of 10,800 watts, clamping response time of five nanoseconds, and three MOVs (across the line and line to ground). Operational rating is 15 amps, and it has two indicators to show normal operation or circuit failure.

EPD makes an extensive line ranging from the Kiwi, a single outlet device ($19.95) to the Grizzly, an uninterruptable power system available in 200, 500, and 1000 watt configurations. An interesting new device in the EPD line is the Ground Hog, a static dissipative mat designed to be placed under a computer and hooked to ground.

Surge Purge

Like the Lemon, the Surge Purge from Citel America, Inc. plugs into a grounded duplex outlet, although the outlet cover plate is left on. The unit provides two pairs of outlets with, unfortunately, rather tight spacing. A transformer plugged into one will block the adjacent outlet. A red LED is always on indicating that power is applied to the unit.

As its name suggests, Surge Purge provides protection against surges. Response time is under one microsecond and it grounds out spikes exceeding 220 volts. In case of a gross surge, the unit switches off power after 15 seconds whereas a spike is "extinguished' in less than one-half cycle (120th of a second) and power is not interrupted.


From Networx, we used the Wire Tree, a four-outlet filtered power source. This provides surge protection and noise filtering. It uses one MOV for surge protection and a capacitor/toroid pi filter for noise filtering. Noise attenuation is 20 db from 600 Khz upward, and a minimum of 50 db from 2.8 MHz to 40 MHz.

The Wire Tree is a compact unit with a nine-foot cord, lighted rocker power switch, and 8 amp fuse. One of the four receptacles allows extra sapce for plugging in a transofrmer.

Scooter Guard-It

The Scooter Guard-It from Ohm Electronics is a surge protector/noise filter. It has three MOVs for both common and differential mode protection. Clamping occurs at 225 volts, and peak current is 6000 amps. Noise filtering, provided by the use of two toroids and capacitors, is more than 20 db from 2 to 100 MHz.

The unit has four outlets, rocker power switch, power indicator light, and 15-amp circuit breaker. It can be mounted in any position and has a six-foot cord.

Tripp-Lite Isobar

Tripp Manufacturing has been making special lighting equipment since 1922, and today offers a comprehensive line of electronic power equipment such as precision regulated power supplies, line stabilizers, uninterruptible power supplies, and line filters.

The Isobar is termed a line filter; we tested the model with four double outlets ($97.95). Two- and four-outlet versions are also available. The eight-outlet unit has three MOVs for surge protection; four pi filter banks (one for each pair of outlets) with a torroidal choke and two capacitors in each for filtering. Four pairs of capacitors provide noise suppression in the high frequency and VHF ranges. Clamping starts to occur at 140 volts and response time is five nanoseconds.

The unit is housed in a heavy aluminum case which can be mounted in any position. It has a 15-foot cord, lighted rocker switch and 15-amp circuit breaker. Load handling is 1875 watts.

The Max from Panamax

The Panamax Voltage Surge Suppressors, nicknamed "The Max' provide surge suppression and optional noise filtering. At 5 picoseconds, clamping time is one of the faster available; clamping level is 200 volts. Maximum spike current is 5000 amps and energy dissipation is 1 million watts. Noise filtering is provided in both the common and differential mode. Frequencies from 10 KHz to 100 MHz are attentuated from 12 to 70 db.

The Max is available with one, two, four, and six outlets. We used the four-outlet version which has a lighted rocker power switch, indicator light that shows correct operation, fuse, and six-foot power cord.

Panamax also makes an interesting device, the UVRA, an under voltage reset alarm that sounds an alarm and cuts off power when the voltage drops below a safe operating level. Another device, the PowerMax, provides battery backup power for 15 minutes.

Kalglo Spike-Spiker

As a well-established manufacturer of AC power controls, high wattage dimmers, motor speed controls, load limiting devices, and alarm systems, it was only natural that Kalglo Electronics should enter the protection field. Their line includes four "power consoles' and six units that plug directly into an outlet (no cord). We tried the Deluxe Power Console Plus (DPC+).

The DPC+ is surge suppressor and noise filtering device. It has eight individually switched outlets divided into two banks of four outlets each. The unit also has a main on-off toggle switch, 15-amp fuse, and two indicator lights, one each for the common and differential mode surge protection circuits.

Voltage spikes are protected in six progressive stages on both common and differential modes with an initial clamping level of 131 volts and response time of one picosecond. The DPC+ can handle a pulse surge of 18,200 amperes. Noise filtering is provided by using inductive/capacitive series-parallel low-pass networks in five stages on both common and differential modes.

The unit is designed to be used on a table top since that gives the user access to the individual switches; we found this feature quite convenient. It has a seven-foot grounded cord. Other power consoles in the Kalglo line have four to eight outlets and varying levels of protection.

Kleen Line Conditioner

Electronic Specialists, Inc. is one of the poineers is making devices for the protection of high technology products. Their 40-page catalog lists a broad line of devices such as power line filters, spike suppressors, isolators, interrupters, regulators, and wire-in devices.

We tried one of the top-of-the-line devices, the Kleen Line Conditioner. This is a regulator/filter/suppressor and is one of the only devices that compensates for brownouts and voltage sags. Input voltage can range from 90 to 140, but output voltage is always 117 volts plus or minus four percent. Furthermore, the output waveform has no more than 3% harmonic distortion.

Protection against surges is provided; clamping voltage is 200; spike current can be 6500 amps with peak pulse power an astounding 4,300,000 watts.

Two sets of noise filters are employed: a balanced pi network for the differential mode with 50 db attenuation in the frquency range of 70 KHz to 200 MHz and an L/C network for the common mode with 30 db attenuation from 100 KHz to 200 MHz.

Four Kleen Line models, which handle from 250 to 2000 watts, are available. As might be expected with constant voltage transofrmers inside, they are big and heavy. We had the 250 watt unit which measures 14 X 8 X 7 and weighs 31 pounds. It has a six-foot cord and two receptacles.

Table: Manufacturers Of Power Line Conditioning Equipment

Photo: The Lemon from EPD plugs into a duplex wall outlet and provides surge protection for six receptacles.

Photo: The active components of The Lemon are hideen under a small PC board.

Photo: Citel Surge Purge plugs into a duplex wall outlet.

Photo: The Wire Tree from Networx has one MOV for surge protection and capacitor/toroid network for RFI/EMI filtering.

Photo: Scooter XP-4 Guard-it Control Center is a four-outlet power strip with surge protection and RFI/EMI filtering.

Photo: Tripp Lite Isobar strip has eight receptacles, switch, and indicator light.

Photo: Inside the Tripp Lite Isobar are three MOV surg protectors (red), circuit breaker (white, left), and a pair of filter capacitors (blue and beige) and toroidal choke for each pair of receptacles.

Photo: The Max has a power switch, indicator light, fuse, and four noise and surge protected outlets.

Photo: Constant voltage transformer in the Electronic Specialists Kleen Line Conditioner is a massive unit that compensates for power dips. The unit also provides surge protection and RFI/EMI filtering.

Photo: Kalglo DPC+ Spike Spiker provides surge protection and RFI/EMI filtering. Each of the eight receptacles is switched and two indicator lights indicate correct operation.

Photo: Kleen Line conditioner from Electronic Specialists.