CertiGuide to A+ (A+ 4 Real)  9  Chapter 11: Power Supplies, Surge Protection and Uninterruptible Power Supplies

Testing an ATX Power Supply 1 Metal Oxide Varistor (MOV)

With the energy crisis in California and the recent massive blackout in New England, it appears as though power disruptions have become a fact of life. We have become dependent upon a constant supply of electricity to power our lights, appliances and electronics. An interruption in the supply of electricity can range from a minor inconvenience, to the destruction of sensitive electronics.

The Air Force's 45^th Weather Squadron, based at Cape Canaveral, Florida, states that lightning strikes somewhere on the surface of the earth about 100 times every second¹³⁵. It is estimated that at any given moment nearly 2,000 thunderstorms are in progress over the earth's surface. There are at least 100,000 thunderstorms a year across the United States. The U.S. has 20 million cloud-to-ground lightning flashes each year! Lightning causes 5 billion dollars of economic impact each year in this country alone.

Lightning can send a power surge along any conductive line and can destroy everything in its path. Lightning current averages 30,000 amps and lightning voltage is 300 million volts. The most likely scenario is not a direct lightning strike, but rather, a nearby strike. If the surge is powerful enough, it travels instantaneously through electrical wiring and telephone lines with the electrical force equivalent to a tsunami.

The electric supply that we have come to depend upon to power our world, is filled with variances and fluctuations. These fluctuations do not affect standard appliances, like lamps and toasters, but can destroy sensitive electronic equipment, such as computers, modems, and printers. Power disturbances and surges can come from many sources. The electric utility companies are not solely to blame. Even in properly wired buildings, many everyday appliances, like microwave ovens, refrigerators, vacuum cleaners, and hair dryers, can cause data-damaging interference.

Surges often go unnoticed, usually lasting only 1/120th of a second. Utility power systems are often pushed beyond their capacity, resulting in unstable, unreliable power for consumers. Overburdened power grids can generate powerful surges as they switch between sources or generate surges when power is momentarily disrupted-- two examples the neighbor starting an electric motor, or the office on the floor below blowing a fuse.

A good surge protector should have these minimum specifications.

The Underwriters Laboratories listing of (UL) TVSS 1449¹³⁶ clearly noted on the package or device. Some outlet strips look like surge protectors, and the advertising on the box shows the strip connected to a computer. The strips show a UL label, however, they are actually classified as an extension cord by UL. Use only UL TVSS 1449 listed devices as surge protectors. UL TVSS 1449 surge protectors provide protection on all three electrical wires: Hot, Neutral, and Ground.

A good quality surge protector will have two indicator lights on it. One shows whether the electrical circuit the protector is connected to is properly grounded and polarized; the second light shows whether the protection circuitry is still functioning. Most surge protectors still deliver power after the protection circuitry has been destroyed. Surge protectors eventually fail, as a result of repeated “hits” by high-energy surges.

Testing an ATX Power Supply 1 Metal Oxide Varistor (MOV)