ECE 252 Introduction to Electrical Engineering

Lesson 14. Electrical Safety


Major Topics: electrical safety, effects of electricity on the body, fibrillation, electrical resistance of the body, residential wiring.

Minor Topics: skin puncture, body resistivity, lightning, GFCIs, isolation transformer, defibrillator.

Objectives: After studying this lesson you should be able to:

1. Determine the probable effect of an electric current between two points on the body.

2. Determine current level in the body.

3. Wire up a wall socket.

4. Explain how a GFCI works.

5. Answer questions on electrical safety.

Effects of Electricity on the Body

Muscles and nerves of the body conduct information by electrical and electrochemical means. For example, electrical impulses generated in the brain travel down the spinal cord, are transmitted to peripheral nerves, and then activate muscle movement. Likewise, external stimuli, for example touch or heat, generate electrical signals that travel through nerves to the brain. It is reasonable to expect, therefore, that electricity applied externally to the body will affect nerves and muscles.

Table 1 presents data on the effects of 60 Hz current on the body. This frequency is used as an example, because 60 Hz current is the most common source of current and the most common cause of accidental electrocution. It should be understood that the values given here are approximate, and that authorities and experts in the field are in disagreement over some of the values.

Current Effect
20 A Permanent brain damage
5 A Respiratory arrest
2 A Central nervous system damage
1 A Burns
80 ma Ventricular fibrillation
50 ma Asphyxia
9 ma Muscles frozen
1 ma Pain
0.2 ma Threshold of perception
0 No effect

Table 1. Effects of continuous 60 Hz current
between the two arms. Currents given are approximate.

Starting at the bottom of the table, the threshold of perception is 0.2 ma, which means that currents below this level cannot be detected. Currents near this level are not dangerous in themselves, but they may be dangerous in a contributory sense. For example, if a man standing on a tall ladder received a low-level shock, he might be startled into falling off.

Currents above 1 ma are often painful, but really dangerous currents are those above 9 ma, which can cause the muscles of the body to be frozen. This occurs because the externally applied electricity overrides the internally generated signals to the muscles, making them impossible to move. More precisely, this "let-go-current" threshold is defined as 9 ma for men (˝ of 1% of men cannot let go of a 9 ma current) and 6 ma for women. This situation is obviously extremely dangerous, and it is made worse by the fact that skin resistance decreases the longer the current is on. Therefore, if the voltage is constant, the current will increase the longer the person is connected to it.

Asphyxia occurs when the chest muscles are frozen, making it impossible for the person to breathe. The value of 50 ma given for this is somewhat fuzzy since values in the literature range from 18 ma to 60 ma. Recall that the current was defined as flowing between the two arms, so if the entrance and exit points for current are different from this, one can expect a different value for the asphyxia threshold. The chest has a larger cross-sectional area than the arm, so there will be a higher current density (amps/meter2) in the arm than in the chest. This can be seen in Figure 1, and it is the reason that 9 ma of current may cause the arm muscles to freeze but not the chest muscles. If a person is attached to a current that causes asphyxia, he will die if he is not removed from the current within about 5 minutes.

You should see an image here instead of this text.

Figure 1. Current flow in the body.

The principal cause of death by accidental electrocution is from ventricular fibrillation. The heart, like any other muscle of the body, is affected by electrical currents, but the heart has some special electrical properties: it is synchronous and self-stimulating. This means that the heart will continue to beat even when disconnected from the nervous system, and that it will beat in a rhythm which efficiently pumps blood. Ventricular fibrillation is asynchronous contraction of the heart muscle. When this occurs blood flow essentially stops. This can occur from a number of causes, most of which involve "heart failure" of various kinds, but we are concerned here with fibrillation caused by external electrical stimulation. If an electrical current passes through the heart it may "depolarize" some areas of the heart while leaving others unaffected. This can interrupt the synchrony of the heart and cause it to go into fibrillation. Fibrillation remains even after the electrical stimulation has ceased. External cardiac massage may be of some use until (and if) the heart naturally regains its synchronous rhythm, but a more effective method is to employ a defibrillator.

Most defibrillators found in hospitals consist of two large electrodes and a capacitive discharge circuit. The two electrodes are placed on the chest, and a large capacitor charged with a high DC voltage is discharged through the chest. The high current causes the heart to completely depolarize. When the current is removed, the heart should start to beat properly. To reemphasize an earlier point, it is partial depolarization (electrical excitation) that causes fibrillation; complete depolarization allows the heart to regain its synchrony.

If fibrillation is not corrected within about 5 minutes, the victim will die. Permanent brain damage due to oxygen starvation may occur within 3 minutes. This is the reason why heart teams in hospitals are required to react with speed and efficiency to a cardiac arrest. With external heart massage, medication, and resuscitation, the times mentioned above can be extended to varying degrees.

A 60 Hz current between the two arms of 80 ma would cause ˝ of 1% of the population to go into ventricular fibrillation. More precisely,
Ifib = K/√T
where K = 100 for children and 185 for men, and T is time in seconds of exposure to the shock. This equation is fairly accurate for 8.3ms < T < 5 sec. If a shock is directly applied to the heart, such as may accidentally happen with a defective cardiac catheter, as little as 80µa can cause fibrillation.

Above currents of 1 A, skin and subcutaneous burns occur. At a current of 2 A central nervous system (CNS) damage may occur. This current level may damage the spinal cord and cause temporary or permanent loss of feeling in the area through which the current flowed.

Currents above 5 A between the arms may cause respiratory arrest. Naturally, currents flowing in the head, rather than between the arms can cause respiratory arrest at much lower currents than 5 A (approximately 100 ma). Unlike asphyxia, which is caused by the freezing of the chest muscles, respiratory arrest continues after the current is removed. The victim stops breathing because the "breathing center" in the medulla has been disrupted. Mouth-to-mouth resuscitation should be administered to a victim who is suffering from respiratory arrest until (and if) the CNS recovers from the shock. Recovery may not occur for up to ˝ hour.

Currents greater than 20 A result in permanent brain and nerve damage by causing excessive heat generation (due to I2R). The spinal cord and lower brain would be damaged the worst, possibly resulting in paralysis from the neck down.

Electrical Resistance of the Body

Although current is the direct hazard in accidental electrocution, it is voltage that causes current to flow. This is simply a statement of Ohm's Law. If we know the resistance of the body and the applied voltage, we can calculate the current that will flow.

Body resistance, excluding the skin, is approximately 500 Ω between any two points. More exact values can be calculated knowing that the resistivity, ρ, of the body is about 1 Ω-m and using the formula,
You should see an image here instead of this text.
where A is cross-sectional area of the tissue, and S is the distance the current travels through the tissue. This equation is seldom necessary, since skin resistance is usually much more than internal body resistance.

In general, the total resistance of the skin at the point of entrance and exit of the current is between 1 kΩ and 100 kΩ. The former applies if the areas are wet and large, the latter if the skin areas are dry and small. More precisely, the resistance of a l cm2 area of skin is approximately 200 kΩ ± 100 kΩ when dry, and 1% of that when wet. As mentioned previously, skin resistance drops when current flows through it for a long time.

Some calculations can be made using the above information. If a person were accidentally connected to at 120 V 60 Hz source (common residential power), typical current flow for dry skin would be,

I = 120 V/(100 kΩ + 500 Ω) = 1.2 ma

Checking this against Table 1, we see that the current is painful, but not particularly dangerous.

If the skin is wet:

I = 120 V/(1 kΩ + 500 Ω) = 80 ma

This is quite possibly a lethal current.

Effect of Voltage and Frequency

Based on considerations such as this, and actual case histories, it can be shown that if the voltage is less than about 40 volts, one is relatively safe from electrocution. This is not to say that one could not imagine a situation where 40 V could be lethal; but there are no recorded electrocutions at voltages this small.

On the other hand, if the voltage is extremely high, there may be somewhat less risk of death than for a moderately high voltage. This surprising assertion is because at very high voltages the heart may completely depolarize and therefore not fibrillate.

If the voltage or frequency is high, skin resistance becomes negligible. For V > 240 volts, "skin puncture" may occur -- a hole is burned in the skin. For f > 1000 Hz, the skin behaves like a capacitor rather than a resistor, and the reactance becomes small even though the resistance may be high. Reactance is the imaginary part of impedance and is similar to resistance in many ways. The reactance of a capacitor is inversely proportional to frequency. Therefore, as the frequency increases, the current through a capacitor will increase.

DC currents are less dangerous than 60 Hz currents of equivalent rms value because the DC current does not change with time. The 60 Hz current does, and therefore presents greater danger of fibrillation by presenting various currents to the heart. On the other hand, very high frequencies (above about 10 kHz) are not particularly dangerous since the current changes directions so rapidly that muscle and nerve cells do not have time to depolarize. Indeed, high frequency (500 kHz) and high voltage (800 V) power is used in "electrosurgery" -- the use of electricity to cut tissue and coagulate bleeding tissue. It turns out that frequencies between 60 Hz and 400 Hz (common commercial power frequencies) are the most dangerous of all.

Here's a video that may help you appreciate the dangers of high voltage electricity: High power line worker.

Lightning

Electrical accidents account for about 1000 deaths per year in the United States. Lightning accounts for about 35 more deaths and about 300 injuries. Almost all of the lightning deaths occur between the months of May and September. This is probably because most of the thunderstorms occur in this period and also because people are more likely to be outdoors.

Click here to see a lightning simulation.

About 10% of the lightning deaths are tree related. Lightning will tend to strike the tallest object in the vicinity. Thus it would be safe to take shelter under a tree in a forest, but hazardous to do the same thing on a golf course.

Less than 1% of the lightning deaths are phone related. Lightning can travel down phone lines and enter residences. Lightning arresters are provided to prevent this. See Figure 2 for one type of lightning arrester (carbon block).

Figure 2. Lightning arresters.

Lightning arresters are designed to short high voltage to ground, but occasionally they fail.

Electricity in the Home

From the Pole to the Home

Figure 3 was prepared by Dr. Donald J. Scheer, and is used with permission. It shows the way three-phase power is converted to voltages suitable for use in the home. Three-phase power arrives as three wires strung at the top of the utility pole. The voltage between any pair of wires is 12,400 V, and the voltage from any wire to ground is 7,200 V. One of the overhead three-phase wires is tied through the primary coil of the distribution transformer to ground. The secondary of the transformer is center-tapped to provide two 120 V single-phase supplies. The two ends of the secondary of the transformer provide 240 V single-phase. This "service drop" may serve as many as 15 residential customers.

You should see an image here instead of this text.

Figure 3. Conversion of three-phase power to single-phase power.

Power Distribution in the Home

Figure 4 shows the wiring diagram for electricity entering a residence. The watt-hour meter on the outside of the house measures the amount of electrical energy that is used. From the service entrance box, the breaker panel or fuse box distributes 120 V and 240 V power for use in the residence. In older homes, fuses may be used instead of circuit breakers.

You should see an image here instead of this text.

Figure 4. House wiring block diagram.

To carry power throughout a building, conduit is probably the best installation to have. Nevertheless, few homes are equipped with conduit. It is found much more often in industrial buildings. Conduit is thin-walled metal pipe that can be bent with a special tool. It carries the black wire (120 V, "hot") and the white wire (120 V return or neutral, at ground potential). It also acts as a ground for all "boxes" and other electrical connections in the house. This ground ordinarily carries no current; that is the job of the white wire. Conduit provides good mechanical protection and fire protection for the wiring.

Next best, depending on the installation, is armored cable, commonly called "BX." This is a flexible spiral-wound metal cable that (usually) carries the black wire, the white wire and a bare ground wire. It provides fairly good protection for the wiring. It is not waterproof.

The most common (and cheapest) installation in residential use is plastic coated cable, officially NM cable, but usually called "romex," although Romex® is a registered trademark of General Cable Industries. Romex comes in 2-wire and 3-wire varieties. The 2-wire variety has no ground wire, and is not approved for new installations. NM cable provides minimal mechanical and fire protection, but it is waterproof (in the NMC version), so it is suitable for underground use. Three-wire "romex" is acceptable for residential wiring under Kentucky and Louisville wiring codes. Romex is the most common cable found in residential wiring.

Electrical Outlets

Figure 5 shows a detail view of an outlet box. The box itself may be metal or plastic. Metal boxes are themselves grounded. Note how the hot (black), return (white), and ground (green or bare) wires are connected.

You should see an image here instead of this text.

Figure 5. Typical outlet box connection details.

Figure 6 shows various kinds of wall sockets. There are many other kinds of sockets, but these four are the most common in residential wiring. When correctly wired, the 120 V black wire should be on the right and the white should be on the left. Occasionally 3 prong safety sockets are installed with the ground on top, thus making the return on the right. Most older installations have standard or polarized sockets. New installations in Kentucky must have 3 prong Type B safety sockets.

You should see an image here instead of this text.

Figure 6. Various wall sockets.

The polarized socket is intended to allow certain types of plugs (with one prong larger than the other) to be inserted in only one way, so that the correct side of the equipment will be grounded. Some television sets have such plugs. Three prong sockets will accept obsolete, polarized, or three prong plugs, and these are considered the safest and best sockets.

Procedure for Replacing a Receptacle

  1. Turn off power to the socket either at the breaker box or at the wall (if the socket is controlled by a wall switch).
  2. Use a multimeter to test  to be sure the power is off.
  3. Remove the face plate.
  4. Remove the screws connecting the receptacle to the box.
  5. Remove the old receptacle.
  6. Connect the black wire to the brass screw of the new receptacle.
  7. Connect the white wire to the chrome screw.
  8. Connect the green or bare wire to the green screw.
  9. Screw the new receptacle into the box.
  10. Replace the face plate.
  11. Use the multimeter to do a continuity check to assure that there is zero resistance between the return line and the ground line.
  12. Turn the power on.
  13. Use the multimeter to check the voltage between the hot side and the return side of the receptacle.

Shielding and Grounding

Three-prong plugs are the safest because the third wire is provided to ground the chassis of the equipment. Figure 7 shows a properly grounded appliance. If an internal fault should occur that accidentally connects the 120 V "hot" side to the chassis, the fuse will blow or the circuit breaker will open. The user will not be hurt. Figure 8 shows an ungrounded appliance connected to the power source with a 2 prong plug. An internal fault may cause 120 V to be connected directly to the chassis. This is very dangerous and potentially lethal.

You should see an image here instead of this text.

Figure 7. Properly grounded appliance.

You should see an image here instead of this text.

Figure 8. Ungrounded Appliance.

When attempting to connect 3-prong plugs to 2-prong sockets, a dangerous device known as an adapter or "cheater" is often used. This is shown in Figure 9. If this device is used at all, it should be used properly, that is by connecting the green wire under the screw on the outlet. Before making this connection, use a multimeter to check that the screw is at ground potential.

You should see an image here instead of this text.

Figure 9. 3 prong to 2 prong adapter.

Another method of making safe electrical equipment is by "double insulation." In such equipment, the case (chassis) is usually nonconducting and the internal parts are insulated. Double insulation has a good safety record, but it is still dangerous when wet.

Ground Fault Circuit Interrupter

Most electrical accidents occur by interposing a person as a path to ground (not to return or neutral) for 120 V. A device has been invented that can sense this occurrence and correct it. It is the ground fault circuit interrupter (GFCI) shown in Figure 10. This is sometimes just called a "ground fault interrupter" (GFI). The current flowing in the black and white wires is the same under ordinary circumstances, thus the flux in the toroid is zero. If a ground fault occurs, the current in the black wire will exceed the current in the white wire, flux will be developed in the toroid and a voltage in the coil. This will activate the control circuit, which will open the relay shutting off the power and saving the victim from electrocution. Ground fault trip current is approximately 5 ma. GFCIs are now required on all outdoor electrical installations and in all wet locations, such as kitchens and bathrooms. When installed in indoor locations, GFCIs are sometimes daisy-chained with ordinary receptacles. This is done to save money. If properly wired, all sockets on the daisy chain have ground fault protection.

If provided for all electrical installations, GFCIs would eliminate 81% of electrocutions and many electrically caused fires. Double insulation, if universally used, would eliminate 57% of electrocutions.

You should see an image here instead of this text.

Figure 10. Ground fault circuit interrupter.

Isolation Transformer

Another method of protection from shock is the isolation transformer shown in Figure 11. If either side of the output is touched (but not both simultaneously) there is no danger because there is no complete circuit to ground.

You should see an image here instead of this text.

Figure 11. Isolation transformer.

Arc Fault Circuit Interrupter

An arc fault circuit interrupter (AFCI) is intended to prevent fires by detecting electrical arcs. It can detect parallel arcs (arcs between two wires) and serial arcs (arcs that occur when a wire is broken or a terminal is loose), while rejecting incidental arcs (arcs that occur, for example, when pulling out a plug). AFCIs are now required for almost all rooms in new residential construction. They can be built into receptacles or circuit breakers.

When the AFCI detects an arc, it disconnects the circuit. AFCI outlets can be daisy-chained similarly to GFCI outlets. Lightning strikes and voltage spikes can sometimes trigger AFCIs in error.

Safety Hints