Electrical Power

Lesson 3:

Safety and Electrical Power

Computers are electronic equipment and therefore consume electricity. Although most components in a computer operate on low voltages—in the safe range between 3.3 and 12 volts DC—the main power source and the monitor use high voltages. This lesson sets out some basic high-voltage electrical safety guidelines.


After this lesson, you will be able to:

Identify high-voltage hazards

Define some common guidelines for electrical safety

Electrical Safety Is Your Responsibility

Standard wall outlets in the United States provide a nominal 120 VAC and are rated to deliver currents between 15 and 20 amps.

Under certain conditions, it is possible to receive a lethal shock from much lower voltages than these. Inside a computer, and especially inside the monitor, voltages as high as 30,000 volts can exist even after the power is turned off.

It is vital to follow basic electrical safety guidelines when servicing a computer. There is no substitute for common sense.

However, here are a few tips:

1. When in doubt about the correct way to safely service a part of a computer, don't do it. Have an experienced professional do the necessary work.

2. Always use grounded outlets and power cords.

3. Switch the power off and disconnect all equipment from its power source before removing any covers.

4. Always replace blown fuses with fuses of the correct rating and type.

5. Do not work alone—you might need help in an emergency.

6. Remove all jewelry and any wristwatch. These are conductors that can cause short circuits.

6. Have trained personnel service computer power supplies and monitors; these devices use and store potentially lethal voltages (often for days or longer).

7. Work with one hand. Using two hands can cause a direct circuit, via your heart, from one object to another.

8.In the United States, common AC wiring uses the color coding given in the following table.


Live or Hot
Green or bare copper


Color codes for AC wires and DC wires can be different.

For example, the ground wires on the P8 and P9 connector for an AT-style motherboard are black.

Lesson Summary
The following point summarizes the main elements of this lesson:

Electrical safety is your responsibility.

Be sure to follow all safety guidelines listed for the devices you work with, and do not work inside of products such as monitors or power supplies without special training.

Quiz Basic Elecricity Techonology HTML