ESD
The primary electrical-power concern when working with computers is ESD (electrostatic discharge). This subject was covered fully in Tutorial 13, "The Basics of Electrical Energy." Remember that while ESD can destroy sensitive computer parts even when the discharge is imperceptible and harmless to humans. If proper ESD tools are not available, touching the case (specifically, the power supply) while working on the computer or its components will provide some protection. However, this will only work if the power supply is plugged into a properly grounded electrical outlet. For a review of power supplies and how to work with them, see Tutorial 5 "Supplying Power to a Computer."
Grounds
When used to refer to electronic equipment, the term "ground" can be confusing. Generally speaking, a ground is any point from which electrical measurements can be made. In most cases, a ground means earth ground. With early electrical systems, the earth was used as a path for electrical current to return to its source. This is why telegraphs required only one wire (the earth ground serves as the other conductor). In most instances, the frame of the computer is at ground potential or earth ground, as long as the power cord is installed and connected to a properly grounded system. Some electronic equipment uses a special path or conductor for its ground. This is known as signal ground and is not the same as earth ground.
Electronic equipment is both susceptible to and a source of electromagnetic interference (EMI). A properly grounded computer will both prevent the transmission of EMI and protect itself from other sources of EMI. Unchecked, EMI will distort images on a video display, as well as corrupt communications equipment and data on floppy disks.
High Voltages
For the most part, a computer uses 5 and 12 volts DC. However, two devices use much higher voltages: power supplies and monitors. With these two exceptions, there are generally no electrical hazards inside a computer.
Power Supplies
The power supply uses 120 volts AC. This voltage is found inside the power-supply case. In most cases, there is no need to open the power-supply case and work on the power supply. The cost of a new power supply is low enough that it is generally easier to replace than repair. However, should you decide to open the case, be careful. Remember, the power switch on most computers (usually located on the front of the computer) also uses 110 volts AC to turn the power supply on or off. If you are working on a computer and leave it plugged in to provide proper grounding, this could present a hazard.
Monitors
Monitors use very high voltages (30,000 volts) to drive the CRT. Remember that monitors are dangerous even when unplugged. They can store this high voltage and discharge it if you touch the wrong parts. Working inside the monitor case should be left to a properly trained technician with the necessary tools.
Power Safety Guidelines
The following are some general guidelines to follow when working around computers:
Never wear jewelry or other metal objects when working on a computer. These items pose an electrical threat that can cause shorts, which will destroy components.
To avoid spills, never use liquids around electrical equipment.
Do not defeat the safety feature of the three-prong power plugs by using two-prong adapters.
Replace any worn or damaged power cords immediately.
Never allow anything to rest on a power cord.
Avoid using extension cords. These can become tripping hazards. Also, they may not be rated to carry the current requirements of the system.
Keep all electrical covers intact.
Make sure all vents are clear and have ample free-air space to allow heat to escape.
Some peripheral devices such as laser printers and scanners use high voltages. Before removing any covers or working on any of these devices, be sure to read the manufacturers' manuals carefully.