Electronic Equipment Protection

The sudden power increases, called surges, that momentarily disrupt a building's steady power flow can destroy many of our electrical appliances and other de-

vices. Electrical power can jump from its normal 120V up to 400V or 500V. These electrical surges are invisible and give no warning, zipping right through the main electrical panel so fast that the circuit breakers and fuses don't notice them. Fortunately, most such surges are small and don't cause much damage. Except for the massive surge caused by a direct lightning strike, which is extremely rare, surges in the past were a minor curiosity rather than a problem.

The invention of sensitive electronic devices with microprocessors changed that. Microprocessors are found in computers, stereos, TV and VCR controls, garage door openers, telephones, and an increasing number of other common devices. Fax machines and printers are sensitive to surges. Home appliances with microprocessor controllers include ranges, dishwashers, ovens, microwaves, and clothes washers. Intercoms and security systems, plug-in radios, answering machines, smoke alarms, programmable thermostats, dimmers, and motion detectors may all have microprocessors.

Microprocessors use much less voltage than our electrical systems. Each microprocessor's built-in power supply converts 120V electricity to about 5V. Microprocessors like nice, steady current. Small changes in power, even a split-second surge, can scramble the electrical signals. A surge that slips past the power supply can destroy delicate chips and burn out circuits.

If your electrical system takes a direct hit, your electronic equipment will be destroyed, but the chances of this happening are low. It's more likely that lightning will induce a surge in your building's electrical system. A lightning strike generates a brief but very powerful magnetic field in the surrounding atmosphere. Electrical wiring from the utility pole and throughout the building acts like an antenna and picks up an electrical charge from the magnetic field as it briefly forms and collapses. The lightning does not even have to hit nearby power lines to cause damage. It could generate a charge from some distance away, and any power lines between your house and the lightning strike will conduct the surge.

Surprisingly, some of the most troublesome electrical surges come from inside the building itself. Large electric motors, like the ones in a refrigerator or air conditioner, generate a surge every time they switch on. Lightweight electric motors, like the one in a vacuum cleaner, also cause surges. The surge can run through any wire in the system in any direction and out to any device on the branch circuits. Not all of these surges are harmful, but they occur regularly as motorized appliances cycle on and off.

All computer installations, even the smallest home office, need to be protected from line transients with a surge suppressor. The multitap plug-in strips with built-in surge suppressors are inadequate unless they meet specifications for surge current, clamping voltage, and surge-energy suitable for the particular installation. Major data processing installations require additional types of treatment including voltage regulators, electrical noise isolation, filtering, and suppression, and surge suppressors.

Sensitive equipment should be isolated on separate electrical feeders. It is helpful to separate sensitive equipment physically to avoid problems from switching, arcing, and rectifying equipment. Fluorescent, mercury, sodium, and metal-halide discharge type lighting, especially with electronic ballasts, can cause interference. Separating the equipment-grounding pole of the electrical receptacle from the wiring system ground is a good idea where electronic dimmers, ballasts, and switching devices are present. These specially grounded receptacles have orange faceplates or an orange triangle on the faceplate.

There is a wide range of plug-in devices, called surge suppressors, surge protectors, or transient voltage surge suppressors (TVSS), with higher quality surge suppressors costing more. They range from cord-connected multioutlet strips to large three-phase units located at the building's service entrance. All are designed to limit a surge in voltage to a level that the protected equipment can withstand without damage. This is done by placing one or more devices in the path of the incoming voltage transient to obstruct the amount of current allowed through, or by placing devices that have a lower impedance (resistance) across an incoming power line in parallel with the protected load, so that the higher transient voltage bypasses the current coming into the building. Hybrid units combine both of these methods.

In general, look for these three things in a suppres- K sor: a quick response time, low clamping voltage, and high energy-handling capacity. Suppression devices have to react quickly to sudden rises in voltage to prevent damage from the initial burst. Look for devices with quick response times, 10 nanoseconds (that's 10 bil-lionths of a second) or less. That information should be stamped on the device itself or on its package.

Suppressors react to the voltage level. The clamping voltage sets the maximum voltage that the suppressor will allow through. When the voltage rises, the suppressor kicks in and diverts all voltage above the set level. Look for surge protection that clamps at 300V or less.

The longer the surge lasts, the more energy it carries, and the more energy the suppressor must divert or absorb. A suppressor will burn out immediately if the surge exceeds its energy-absorbing capability. Utility companies in many parts of the country will install a suppressor designed to absorb most lightning-induced surges for about $160. It's mounted at the electric meter or main panel and reacts relatively slowly, but it can absorb the intense energy induced by most nearby lightning strikes. You can choose this type and still install faster responding protectors to handle the residual charge that gets in, as well as lighter surges produced from inside the home.

Two other features are useful in a surge suppressor. Firstly, the suppressor should have three-line protection. That means that the device should protect all three wires—the hot, neutral, and ground—since surges can travel through any one of them. Secondly, make sure that the suppressor has some sort of indicator so you know if it is no longer working. Most suppressors can take only so many hits before they start to wear out and need replacement.

The right amount of protection involves balancing expense against efficacy. Good surge protection for even a home computer is expensive. The cost of the protection must be weighed against the cost of the equipment itself to see if it is worth protecting devices such as TVs and VCRs, as well as less expensive equipment. For a building in a region with frequent thunderstorms, such as Florida, it might be worthwhile to protect less expensive equipment too. It is worthwhile to buy good suppressors for all expensive and essential equipment. Some units suitable for small offices include a surge suppressor, line voltage conditioner, and backup battery for under $200 (Fig. 29-6). Unplugging equipment that isn't in use is a no-cost protection against surges.

Uninterruptible power supplies (UPS) are designed for computer and data processing facilities that can't tolerate power outages over 8 to 50 milliseconds (thousandths of a second) without serious risk of data loss. A UPS is an arrangement of normal and backup power supplies that transfers a facility's critical load from normal to backup mode in so short a time that no computer malfunction results. The minimum required by computer industry guidelines for computer equipment tolerance is 8.3 milliseconds. Standby power usually runs for five to ten minutes, which is enough time to shut down the equipment manually or automatically. Some industrial processes can't tolerate any shutdown, and the standby power for them is designed to run as long as needed. The selection of appropriate UPS systems can be highly complex.

Users of electronic equipment sometimes report mysterious glitches, frozen screens, locked keyboards, and other computer malfunctions that seem to come from nowhere. Many of these problems can result from irregularities in the power feeding the computer. Com-

Figure 29-6 Power supply protection.

puters code and store information on the basis of very small changes in voltage, and any deviation from standard voltage can cause them to malfunction.

Random high-frequency voltages superimposed on the power supply voltage in the form of radio frequency interference (radio noise) and other irregularities can cause data errors in data processing equipment like computers and their peripherals. Slow voltage fluctuations can result in overheating, data loss, and premature equipment failures. Large, rapid voltage fluctuations, known as spikes and transients, can cause equipment burnout and system collapse. Electrical noise comes from electronic equipment power supplies, lighting dimmers, solid-state motor controls, and power line carrier systems (which we discuss later on). Arc welding, switching transients, and local magnetic fields can also cause problems. Noise problems respond to electrical isolation, filtering, and noise suppression.

Facilities attempt to avoid these problems by providing clean power for their electronic equipment. Clean power can be defined as any power that is within a range of 5 percent above or 10 percent below the standard 120V, and free from electrical noise generated by other machines using the circuit. A typical wall socket may supply electrical power more than 10 percent below 120V, and momentary deviations occur regularly due to outages, spikes, and electrical noise.

Many large companies offer computer grade power, which is desirable for all new office spaces and full building renovations. These power conditioning systems convert utility-supplied electric power, with its surges, spikes, radio frequency noise, and voltage fluctuations, to a pure, noiseless, accurately voltage-regulated sinusoidal waveform. Power conditioning is not the same as a UPS, which maintains power during utility failure, but both may be included in a single equipment package.

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment