Electrical Safety

In electronics we must be concerned with the protection of out equipment from damage and ourselves from electrical shock or worse. Let’s discuss protecting ourselves first, since if we get killed we won’t be able to finish whatever project we are working on.

All of the circuits you can come into contact with inside a computer are harmless to human beings because they operate at 12 volts or less. The sole exception to this is inside the power supply where the connections to house current are made, and before it is reduced to the low voltage DC supplied to the computer circuits. Anyone foolish enough to take apart the power supply without disconnecting it from the power cord may as well go for the Darwin Award for eliminating stupidity from the gene pool.

Human skin has a surface resistance to electricity of about 22,000 ohms. Because of this you would need to contact about 25 volts before you would even start to feel anything. The component of electricity that can hurt you is the current. Interestingly, small currents, well under an ampere are the most dangerous. A shock of an ampere or more will cause the heart to clamp (stop) and often it will start beating again on its own as soon as the current is eliminated. A shock in the 20 to several hundred milliamp range can cause the heart to fibrillate (quiver) and in that condition it is much more difficult to restart.

Many of you have been told that charged capacitors are dangerous. Only large capacity ones at fairly high voltages, as stated above, can overcome skin resistance. No such capacitors exist inside a computer. Further, safety agencies such as UL require that any capacitors that can supply a dangerous shock must have a bleeder resistance across them that will quickly discharge the capacitor to a safe level once supply power is disconnected.

The only place you need to be worried about shock is inside a CRT based monitor where the anode and focus supplies are measured in the thousands of volts but at a very low current. The picture tube (CRT) can retain a shock danger for a long time, even after being shut off.

Static Electricity

Although the computer offers you little danger, you can be electrically dangerous to your computer! Some materials will generate an electrical charge when friction occurs between them. The simple act of a person walking on a nylon carpet can quickly build up a static charge on the surface of the persons body. Because the charge has no path to ground, it can reach many thousands of volts, though at an extremely low amperage. If that person then touches an electrical circuit the static can discharge to ground through the circuit. Certain types of electronic components are extremely sensitive to static discharge and can be destroyed as a result. To prevent such damage it is important to touch a ground point, such as the metal computer case (before it is unplugged) prior to touching any circuit component in or outside of the computer, This procedure will eliminate any static charge harmlessly.

This discussion could easily go on to book length, but we have covered the basics sufficiently to remove some of the mystery of electricity, and allow for intelligent decisions in powering our computers while protecting them, and ourselves, from damage. There are a lot of myths constantly repeated by well meaning people due to a lack of understanding technology. Electronics is a fascinating subject that is touching our lives far more than ever before.

 

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