Laboratory manual for Electronics: Safety

Objectives:

• Discuss basic safety rules.

• Describe the effects of electric current on the body.

The purpose of this laboratory manual is to provide students of electricity with hands-on experience with electric circuits. Electricity is an extremely powerful force and should never be treated in a careless manner. This manual assumes a laboratory equipped with a 208/120 volt three-phase power system. Many of the experiments in this manual involve the use of full line voltage (208 or 120 volts). It is extremely important that you practice safety at all times. Please read and memorize the following safety rules.

• Never work on an energized circuit if it's possible to disconnect the power. When possible use a three-step check to make certain that the power is turned off. The three-step check is as follows:

1. Test the meter on a known live circuit to make sure the meter is operating.

2. Test the circuit that's to be de-energized with the meter.

3. Test the meter on the known live circuit again to make certain that the meter is still operating.

• Install a warning tag at the point of disconnection to warn people not to restore power to the circuit.

General Safety Rules

Think:

Of all the rules concerning safety, this one is probably the most important. No amount of safe guarding or "idiot proofing" a piece of equipment can protect a person as well as the person's taking time to think before acting. Many technicians have been killed by supposedly "dead" circuits. Don't depend on circuit breakers, fuses, or someone else to open a circuit.

Test it yourself before you touch it. If you are working on high-voltage equipment, use insulated gloves and meter probes designed to be used on the voltage being tested. Your life is your own, so think before you touch something that can take it away.

Avoid Horseplay

Jokes and horseplay have a time and place, but the time or place isn't when someone is working on an electric circuit or a piece of moving machinery. Don't be the cause of some one's being injured or killed and don't let someone else be the cause of your being injured or killed.

Do Not Work Alone

This is especially valid when working in a hazardous location or on a live circuit. Have someone with you to turn off the power or give artificial respiration and /or cardiopulmonary resuscitation (CPR). One of the effects of severe electrical shock is that it causes breathing difficulties and can cause the heart to go into fibrillation.

Work with One Hand When Possible

The worst case of electrical shock is when the current path is from one hand to the other.

This causes the current to pass directly through the heart. A person can survive a severe shock between the hand and one foot that would otherwise cause death if the current path was from one hand to the other. Working with one hand can sometimes be an unsafe practice by itself. The best procedure is to turn off the power. If it's not possible to disconnect the power, wear insulated gloves when handling "hot" circuits. Also wear shoes that have insulated soles and use rubber mats to cover energized conductors and components when possible.

Learn First Aid

Anyone working on electrical equipment should make an effort to learn first aid. This is especially true for anyone who must work with voltages above 50 volts. A knowledge of first aid, especially CPR, may save your life or someone else's.

Effects of Electric Current on the Body

Most people have heard that it's not the voltage that kills but the current. Although this is a true statement, don't be misled into thinking voltage cannot harm you. Voltage is the force that pushes the current through the circuit. Voltage can be compared to the pressure that pushes water through a pipe. The more pressure available, the greater the volume of water flowing through a pipe. Students often ask how much current will flow through the body at a particular voltage. There is no easy answer to this question. The amount of current that can flow at a particular voltage is determined by the resistance of the current path.

Different people have different resistances. A body will have less resistance on a hot day when sweating because salt water is a very good conductor. What a person ate and drank for lunch can have an effect on a body's resistance. The length of the current path can affect the resistance. Is the current path between two hands or from one hand to one foot? All of these factors affect body resistance.

The chart in ill SF-1 illustrates the effects of different amounts of current on the body.

This chart is general; electricity affects most people in this way. Some people may have less tolerance to electricity and others may have a greater tolerance.

A current of 2 to 3 milliamperes will generally cause a slight tingling sensation. The tingling sensation will increase as current increases and becomes very noticeable at about 10 milliamperes. The tingling sensation is very painful at about 20 milliamperes. Currents between 20 and 30 milliamperes generally cause a person to seize the line and not be able to let go of the circuit. Currents between 30 and 40 milliamperes cause muscular paralysis, and currents between 40 and 60 milliamperes cause breathing difficulty. By the time the current increases to about 100 milliamperes breathing is extremely difficult. Currents from 100 to 200 milliamperes generally cause death because the heart goes into fibrillation.

Fibrillation is a condition in which the heart begins to "quiver" and the pumping action stops.

Currents above 200 milliamperes generally cause the heart to squeeze shut. When the current is removed, the heart will generally return to a normal pumping action. This is the principle of operation of a defibrillator. It is often said that 120 volts is the most dangerous voltage to work with. The reason is that 120 volts generally causes a current flow between 100 and 200 milliamperes through the bodies of most people. Large amounts of current can cause severe electrical burns. Electrical burns are generally very serious because the burn occurs on the inside of the body. The exterior of the body may not look seriously burned, but the inside may be severely burned.

Effects of electric current on the body:

• 0.002-0.003 amp Sensation (a slight tingling)
• 0.004-0.010 amp Moderate sensation
• 0.010-0.020 amp Very painful
• 0.020-0.030 amp Unable to let go of the circuit
• 0.030-0.040 amp Muscular paralysis
• 0.040-0.060 amp Breathing difficulty
• 0.060-0.100 amp Extreme breathing difficulty
• 0.100-0.200 amp Death (fibrillation of the heart)

QUIZ:

1. What is the most important rule of electrical safety?

2. Why should a person work with only one hand when possible?

3. What range of electric current generally causes death?

4. What is fibrillation of the heart?

5. What is the principle of operation of a defibrillator?