Chances are that you have heard about Uninterruptible Power Supplies (UPS). Maybe you have a Kensington surge protector strip that looks little more than a place to add extra AC plugs. Or perhaps you have an American Power Conversion Smart UPS that weighs more than a large Hewlett Packard Laser Jet printer. It beeps during brownouts and gets bumped under the desk when you roll your chair all the way forward. You might have heard that the UPS is running all the time, or rumor has it that it only activates when AC power gets low or disappears.

UPS systems are intended to improve the quality of AC power in order to provide uninterrupted operation of AC powered equipment. A UPS takes in normal quality AC power and provides two enhancements to do this. One, a general power quality improvement, and two, a redundant (back up) power source. Power quality defects which may be improved by the UPS include surges, noise, or sags. A UPS system provides redundant power by supplying the load with a primary power source and then providing a backup power source in case of the failure of the primary source.

The general UPS may be operated as either a standby type UPS or an on line type UPS. The main difference is which power path is chosen to be the primary power path. For standby UPS operation, the transfer switch is set to choose the filtered AC input as the primary source, and switches to the battery / inverter as the backup source in case of the failure of the primary source (AC). For on line operation, the transfer switch is set to choose the battery / inverter as the primary source, and switches to the input AC as the backup source in case of the failure of the primary source (battery / inverter). This distinction between on line and standby UPS operation is very simple, but it has some important differences in operation.

In the strictest sense, there is no such thing as an uninterruptible AC power source for computers. In an AC power system, the power fed to any load depends on the characteristics of the load. A computer is a special type of load which draws power in the form of pulses which occur every 8.3 milliseconds or 120 times per second. Therefore, there is no such thing as an uninterruptible power source for computers because computers only draw interrupted power.

In order to better understand this situation, think of the power consumed by a typical Compaq 386 computer. Normally, these measurements would be reported in terms of numbers for voltage (Volts), current (Amps), and power (Watts), but these values represent long term averages and ignore the details of how the power is delivered as a function of time. If we look at our sample configuration and study the time course of Volts, Amps, and Watts consumed by the system over a period of 1/20th of a second, we should see that power flow is interrupted (zero) for most of the time when the computer is operating on normal AC power. Since we know that the power drawn by the computer logic circuits is flowing all the time, one could correctly infer that the computer must have some internal energy storage that it feeds off of during the periods when the power flowing into the computer is zero.

All computers have an internal energy storage device, called a capacitor, which acts like a rechargeable battery. This battery operates the computer (including disk drives) during the times (120 occurrences per second) when the input power is interrupted. The capacitor has a very limited capacity and cannot run the computer for more than about 50 milliseconds without being recharged. Fortunately, the AC power line replenishes the capacitor with a pulse of power every 8.3 milliseconds or about 6 times more often than required.

Surge suppression is an important function of any power protection system. Surges are capable of causing severe damage to computer hardware. However, it must be understood that surges represent a very small fraction of the types of power disturbances that affect computer operations. Undervoltage conditions, including sags, brownouts, and blackouts are responsible for most computer malfunctions and data loss. Sags are the momentary reduction in supplied AC voltage. It lasts less than a few seconds as is the most common type of power problems. Brownouts are a reduction of line voltage for an extended period from minutes up to a few days. The complete absence of AC power, known as a blackout, is conceptually the easiest type of undervoltage problem to understand. However, it is the least frequent problem for computer operations.

In order to understand why surges represent only a small fraction of computer power problems, it is necessary to become familiar with the statistics of power disturbances and the susceptibility of computers to these disturbances. Data line surge suppression is often as critical as power line surge suppression. Surges come in many different sizes, with the largest 1000 Volt surges occurring but once or less per year, while 50 Volt surges occur hundreds of times per hour. The actual statistics at a given site may be considerably different, particularly if the site is in a high risk surge area.

Statistics, however, or even on site measurements of surges are useless without some knowledge of the susceptibility of computers to surges. Blanket statements such as "surges will corrupt data or damage computers" is like saying "if the wind blows, your house will fall down." In either case there is a threshold where the dreaded event occurs, but unqualified blanket statements are very misleading.

All computer manufacturers are aware of power surges and have designed and tested their equipment to be reliable in the face of typical surge activity. This job has been made easier as a result of the natural surge immunity inherent in the design of switching type power supplies used in virtually all modern computers. Surge suppressors do not eliminate surges, but rather reduce them in size.

The most inexpensive designs for reducing the size of surges involve the shunt or parallel system. The shunt type use a simple Metal Oxide Varistor clamping device. This type of suppressor has a clamping voltage of around 300 to 400 volts which means that 300 to 400 volts is the threshold at which surge suppression begins. Therefore, with this type of suppressor, surges of less than this threshold value are passed through directly to the user's equipment. This type of surge suppressor is really designed to protect against catastrophic equipment damage and is better suited for use with household appliances than for sensitive loads like computers. More than 90% of the surge suppressor sold today are of this type.

The best type of surge suppressor uses a series or isolation design. In this case, circuit elements are placed between the AC line and the user equipment. The circuit elements are designed so that they will pass the normal 60 Hz AC power but have greatly increased electrical resistance at high frequencies. In this fashion, such components act as a brick wall to surges instead of mere diverters. Available series type elements are not as ideal as desired, usually due to size, cost, and heat dissipation limitations, so they are combined with shunt type clamping and filter elements to achieve superior surge reduction performance.

How does this apply to you? It concerns you more if you are on a network. Networked computer systems are much more susceptible to data loss due to power failure than are single user systems. This is true because in most network operating systems, the hard disk directory is held in RAM memory and only periodically updated on the hard disk. Therefore there is a chance that the directory on the disk may not be current at the time of a power failure, which may make it impossible to recover data from the disk. So plan for a good UPS if your department is getting a network, and make sure your server is connected to a UPS if you are already on a network.