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Electrostatic Discharge (ESD)

You must take many precautions when disassembling a computer. The electronic circuits located on the motherboard and adapters are subject to ESD. ESD (electrostatic discharge) is a difference of potential between two items that causes static electricity. Static electricity can damage electronic equipment without the technician’s knowledge. The average person requires a static discharge of 3,000 volts before he or she feels it. An electronic component can be damaged with as little as 30 volts. Some electronic components may not be damaged the first time static electricity occurs. However, the effects of static electricity can be cumulative, weakening or eventually destroying a component. An ESD event is not recoverable—nothing can be done about the damage it induces. Electronic chips and memory modules are most susceptible to ESD strikes.

Atmospheric conditions affect static electricity. When humidity is low, the potential for ESD is greater than at any other time; however, too much humidity is bad for electronics. Keep humidity between 45 and 55 percent to reduce the threat of ESD.

A technician can prevent ESD by using a variety of methods. The most common tactic is to use an antistatic wrist strap. One end encircles the technician’s wrist. At the other end, an alligator clip attaches to the computer. The clip attaches to a grounding post or a metal part such as the power supply. The electronic symbol for ground follows:


An antistatic wrist strap allows the technician and the computer to be at the same voltage potential. As long as the technician and the computer or electronic part are at the same potential, static electricity does not occur. An exercise at the end of the chapter demonstrates how to attach an antistatic wrist strap and how to perform maintenance on it. Technicians should use an ESD wrist strap whenever possible.

A resistor inside an antistatic wrist strap protects the technician in case something accidentally touches the ground to which the strap attaches while he or she is working inside a computer. This resistor cannot protect the technician against the possible voltages inside a monitor. See Figure 4.1 for an illustration of an antistatic wrist strap. Figure 4.2 shows a good location for attaching an antistatic wrist strap.

Figure 4.1

Figure 4.1. Antistatic wrist strap

Figure 4.2

Figure 4.2. Attaching an antistatic wrist strap

Antistatic bags are good for storing spare adapters and motherboards when the parts are not in use. However, antistatic bags lose their effectiveness after a few years. Antistatic mats are available to place underneath a computer being repaired; such a mat may have a snap for connecting the antistatic wrist strap. Antistatic heel straps are also available.

If an antistatic wrist strap is not available, you can still reduce the chance of ESD damage. After removing the computer case, stay attached to an unpainted metal computer part. One such part is the power supply. If you are right-handed, place your bare left arm on the power supply. Remove the computer parts one by one, always keeping your left elbow (or some other bare part of your arm) connected to the power supply. If you are left-handed, place your right arm on the power supply. By placing your elbow on the power supply, both hands are free to remove computer parts. This method is an effective way of keeping the technician and the computer at the same voltage potential, thus reducing the chance of ESD damage. It is not as safe as using an antistatic wrist strap. Also, removing the power cable from the back of the computer is a good idea. A power supply provides a small amount of power to the motherboard even when the computer is powered off. Always unplug the computer and use an antistatic wrist strap when removing or replacing parts inside a computer!

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