UNDERSTANDING ELECTRICITY AND ELECTRICAL DANGERS
ELECTRICITY AND ELECTRIC SHOCK BY C. BRUCE EDWARDS
Electricity is the study of electrical charge. The study of electricity is analogous to that of hydraulics–the study of water flowing in pipes. Whereas water quantity is measured in buckets or gallons, the quantity of electrical charge is the coulomb. Whereas water flows in hollow conduits such as pipes and hoses, electric charge flows in solid conduits called conductors or wires. Water flow rate is measured in terms of the number of gallons passing a given point along a pipe or hose in one minute–i.e., gallons per minute (gpm). Electrical flow rate–electric current–is measured in terms of the number of coulombs of charge passing a given point along a wire in one second. For convenience, an electrical flow rate of one coulomb per second is referred to as one ampere.
ENERGY AND VOLTAGE, AC AND DC
For water and charge to do work, they must be given energy. Whereas a pump provides energy to water, increasing its pressure head, devices such as electrical generators and batteries impart energy to electric charge. This energy is measured in terms of voltage–the number of joules of energy imparted to each coulomb of charge. A 120-volt generator imparts 120 joules of energy to each coulomb of charge. In an alternating-current (AC) generator, a piece of rotating machinery, that charge reverses direction repeatedly as the rotor turns. The frequency of reversals of current generated by the North American electrical power grid system is 60 reversals or cycles per second (60 hertz).
A 12-volt battery imparts 12 joules to each coulomb, but the current always flows in the same direction. It flows out of the positive battery terminal, follows a closed path through a load such as a light bulb, then returns to the negative terminal, each coulomb of charge having given its 12 joules of energy to the load. At the battery`s negative terminal, each coulomb of charge has its energy raised by 12 joules by chemical action as it is returned to the positive terminal. This is called direct current (DC).
Hydraulic and electrical energy can be converted to useful forms of energy when water and charge flow from a point of high head or voltage to a lower head or voltage. Not all hydraulic or electrical energy can be converted to useful forms. An example would be that involved in turning the shaft of a motor or turbine, in which case the energy conversion process is not perfectly efficient. Some or all of the energy is converted to thermal energy (heat).
When a human body is in the flow path of electric charge, virtually all of the electrical energy is converted to thermal energy–the heat that causes the burns that can arise from electric shock.
Just as water flows in a loop from a source such as a lake, which has a pressure head of zero (atmospheric pressure), and has its pressure head raised by a pump, then loses head through a hose and nozzle, and then returns to atmospheric pressure where the head again is zero (Figures 1a and 1b), so charge flows in a closed loop called a circuit (Figure 1c), where the voltage is referenced to electrical ground–a “grounded” system.
Current will flow through any path between “live” parts of the circuit and ground. Therefore, a person touching “ground” and a live object can receive an electrical shock. Touching only live or ground should not result in a shock, which is the reason birds can land on high-voltage wires with impunity.
Electrical circuits need not be referenced to ground. They can “float,” as shown in Figure 1c, although floating circuits are rarely used for power distribution. A floating circuit is analogous to a closed pumping system in which the same water that leaves the pump outlet is returned to the pump inlet.
If a circuit is truly floating, a grounded person cannot receive a shock from touching any part of the circuit. This is the safety advantage claimed for floating power systems. In practice, though, a circuit must be at some voltage with respect to ground. It could float anywhere from zero (unlikely) to thousands of volts aboveground. If a grounded person touches it, a transient (brief) shock can result as the circuit is grounded through the victim. Floating circuits, therefore, can be a greater hazard than grounded systems. That is why grounded power systems are preferred in North America.
RESISTANCE AND OHM`S LAW
Just as the wall of a pipe or the jacket of a hose keeps water from leaking out, so electrical insulation–a nonconductor of electrical charge–keeps the charge from leaking away from wires. In electrical diagrams, the lines depict perfect electrical conductors, which offer no resistance to the flow of charge–the path taken by charge. In practice, conductors are not perfect. They exhibit resistance to the flow of charge. Electrical resistance is analogous to friction loss in hose and pipes. Like friction loss, which varies directly as the length of a hose and varies inversely as its cross-sectional area, electrical resistance varies directly as the length of the conductor (wire) and varies inversely as its cross-sectional area.
Most electrical calculations are easier than hydraulic calculations, though. Unlike friction loss, which varies with the flow rate, electrical resistance is usually constant–independent of the current. The electrical resistance (R), measured in ohms, of a device is defined as the voltage (V) across the device divided by the current (I) flowing though it, as shown in Figure 2. This relationship between voltage, current, and resistance is known as Ohm`s law.
Materials such as metal have very low resistance to the flow of charge and, therefore, are called electrical conductors–for example, 100 feet of #14 AWG (American Wire Gage) copper wire, used for residential wiring rated at 15 amperes, has a resistance of about 0.3 ohms; so with a current of 15 amperes flowing through it, the voltage drop across the wire is
V = I 3 R = 15 3 0.3 = 4.5 volts.
Materials such as dry wood, plastics, ceramics, and air have a very high resistance to the flow of electrical current and are called insulators. Their resistance is on the order of billions of ohms.
An open circuit is defined as one in which a very high resistance path reduces current flow to a minute amount. Examples are disconnected wires and open switches. A short circuit is defined as an extremely low resistance path across an electrical device, causing an abnormally high current to flow. Examples include a wrench across the terminals of a battery and live wires touching each other.
Typical values of electrical resistance through parts of the human body are given in Table 1 on page 58.
It is current flow through the body that results in injury. By using Ohm`s law, knowing the approximate resistance of a path through the body and the voltage across it, the resulting current can be estimated.
The total resistance through the body is the sum of the resistances at the two points of contact, as shown in Figure 3, which depicts a person standing on the ground touching a live electrical wire. The victim “completes the circuit” by providing a path for current flow between the wire and ground.
Suppose that the victim`s feet are dry and without cuts (intact), and he is standing on dry ground with bare feet. From Table 1, the contact resistance between his feet and ground is, say, 100,000 ohms. If he touches the live wire with a dry finger, the contact resistance of his finger might be, say, 500,000 ohms, his feet having lower resistance to the flow of charge than his finger because the feet have a larger area of contact. The total resistance between his finger and his feet then would be 100,000 1 500,000 ohms, for a total of 600,000 ohms.
If the live wire has a voltage of 120 volts aboveground (the usual residential service), by using Ohm`s law, the current can be calculated to be
A milliampere is one-thousandth of an ampere, so a current of 0.2 mA would flow between his finger and his grounded foot. Because it is AC, the current will alternate between flowing from his finger to his foot and flowing from his foot to his finger, 60 times per second.
Knowing the current makes it possible to estimate the effect of its flowing through the body. Although the threshold of perception for DC is about 5 mA, it is only about 1 mA for AC. Since only 0.2 mA is flowing, he should not feel a tingling sensation.
If the victim touches the wire with a moist finger, finger contact resistance would be about 1,000 ohms, so the total resistance would be about 100,000 + 1,000 = 101,000 ohms. From Ohm`s law, the current would become
Although 1.2 mA is below the threshold of perception for DC, in this AC case, tingling would be felt. If the victim is standing in water, the contact resistance of his feet could drop to, say, 1,000 ohms. If he grabs the wire with one moist hand, the contact resistance of his hand could be, say, 3,000 ohms, for a total resistance of 4,000 ohms. The current flowing between his hand and his feet would increase to
Above a certain current, the “let-go” current, the victim is unable to release his hand. The let-go current is about 15 mA for AC and 75 mA for DC. Since in this case, the current is about 30 mA AC, the victim would be unable to “let go of” the wire. If he was experiencing a shock from the same current flow of 30 mA DC, he would be able to let go.
Suppose the victim, standing in water, has a cut finger. If he touches the live wire with the open wound, the finger contact resistance would drop to, say, 200 ohms, for a total resistance of 1,000 + 200 = 1,200 ohms. The current flowing through his body would be
Ventricular fibrillation of the heart can occur in a fraction of a second with AC as low as 60 to 100 mA if the current path is through the heart, as in the above example. Therefore, the victim in the above example probably would suffer cardiac arrest. It takes between 300 and 500 mA of DC for fibrillation to occur.
Suppose that, instead of touching a 120-volt AC source, the victim in the above scenario touched the positive terminal of a 12-volt DC car battery, which has the negative terminal grounded. The resulting current would be
This is between the threshold of perception and the let-go current for DC. The victim, therefore, would feel a shock but would be able to let go without difficulty. Note that under normal circumstances, with intact skin, the current from 12-volt and even 24-volt batteries is imperceptible.
The current required for various physiological effects is shown in Table 2 on page 62.
Touching a 120,000-volt AC transmission wire, even with dry hands and feet, almost certainly would be fatal, since the current is proportional to the voltage and, assuming a body resistance of 600,000 ohms, would be
A bird, however, could safely land on a high-voltage wire because its body does not provide a current path to ground, air being an insulator.
POWER AND HEAT ENERGY
Fibrillation is not the only danger to a human body. The heat power (P) dissipated in a victim of electrical shock can cause serious injury, even if current does not flow through the heart. Electrical power is measured in watts. The power dissipated by a device is equal to the voltage across it multiplied by the current (in amperes) flowing through it.1 In this case, the heat power dissipated in the above victim would be
P = V 3 I = 120,000 3 0.2 = 24,000 watts.
A kilowatt (kw) is defined as 1,000 watts; in this case, 24 kw would be dissipated. The heat power concentrated inside the victim`s body is the same as that that would be given off by six electric baking ovens, each rated at four kw.
The heat energy imparted to the body is in direct proportion to the duration of contact with the current source. If the victim in the above example is in contact with the wire for, say, 300 seconds, the heat energy received by his body would be 24,000 3 300 = 7,200,000 joules, or 7 megajoules. This heat energy is equal to 6,600 Btus, which would raise the temperature of a 10-pound arm by roughly 660°F.
I responded to an incident similar to that described in the above example. A car rammed a power pole, bringing a live wire down onto the car. When the driver got out, the heat generated in the contact resistance of his booted foot when it touched the ground burned off several of his toes. The hand holding the car door also was burned off. He survived.
Although the above treatment of electricity and electric shock is brief, it provides firefighters with the ability to quantitatively predict the potential for injury from electrical shock in practical situations. n
1. For AC, power P = VI 3 Power Factor, but since the power factor usually is close to 1, P = VI is approximately correct.
I = V = 120 = 0.0002 amperes = 0.2 milliamperes (mA).
I = V = 120 = 0.0012 amperes ` 1.2 mA.
I = V = 120 = 0.03 amperes = 30 mA.
I = V = 12 = 0.01 amperes = 10 mA.
I = V = 120,000 = 0.02 amperes = 200 mA.
UNDERSTANDING ELECTRICAL POWER
WIRING BY C. BRUCE EDWARDS
n C. BRUCE EDWARDS, consultant and research director of FireTech Engineering, Inc., in Vancouver, Canada, was appointed deputy chief of the Wabasca (Alberta) Volunteer Fire Department in 1978. He is currently coordinating a joint U.S.-Canadian research project to develop and quantitatively evaluate advanced structural and wildland fire suppression systems. He is a graduate of the Institution of Fire Engineers (Great Britain) and holds a bachelor of applied science degree in electrical engineering and a master of applied science degree in medical engineering, both from the University of Toronto.
I = V = 120 = 200 amperes
ELECTRICAL SAFETY AND FIREFIGHTERS BY GEORGE BROWNE
In July 1995, the National Fire Protection Association (NFPA) reported that 2.4 percent of the firefighters killed in the line of duty during 1994 died as a result of electric shock. Firefighter injury statistics for the past several years list few, if any, electric shock injuries for firefighters. On the surface, this is unspectacular information. However, it suggests a misunderstanding of electricity and the types of injuries it produces. Looking only for injuries such as burns, cardiac arrest, and nerve damage may cause us to miss other associated effects of electric shock that can cause serious injuries or fatalities.
HOW SHOCKS OCCUR
Electric current will flow only in a closed circuit (see “Electricity and Electric Shock,” page 57). We can become part of the electrical circuit in three ways:
Contact with both wires of a power source. How many times have we seen a bird sit on overhead electrical wires? Have you ever wondered why the bird isn`t electrocuted? Very simply, the bird is not completing a circuit, since it is in contact with only one wire, the surrounding air is an insulator and there is no path for the electricity to follow. A firefighter plugging in an electric floodlight and contacting both of the live metal prongs on the plug will complete the electric circuit with the fingers of one hand and receive a shock even though he is not grounded.
Contact with one live wire and the electrical ground. Contact with actual earth is not the only way to be electrically grounded. Touching the “grounded” metal case of electrical equipment has the same effect. A person or device that is electrically “grounded” has a low resistance path to ground for current. A firefighter on a metal ladder, grounded on damp earth, who touches a bare wire or contacts it with a conductive tool will provide the path to ground and can receive a shock. If the ladder is placed on an insulated mat, he will not; but if another person with conductive footwear standing on the ground touches the ladder, he can receive an unexpected shock and give the person on the ladder a shock too, since both are in the current path. Rubber boots in good condition can act as insulators as long as they are clean and dry. They can prevent the wearer from being grounded unless he touches a grounded object.
An ungrounded metallic object that becomes “hot” while the person is in contact with the ground. This is extremely dangerous. Raising an aerial ladder and having it come in contact with an overhead wire is a good example: In addition to the ladder`s being hot, the rest of the truck is also hot. Its rubber tires prevent it from becoming grounded. If a grounded firefighter touches the hot truck, the circuit will be completed and the electricity flowing through the firefighter to the ground will cause an electric shock. That is the reason ladder operators must not operate aerial equipment while standing on the ground.
SEVERITY OF SHOCK
In discussing electricity, we all can think of a time when we received a shock or know of a person who received one. Yet, we don`t know of many people who were electrocuted. The severity of a shock determines the severity of the injuries received. Three factors affect the severity of a shock:
The amount of current passing through a body. The higher the current, the more potential for injury. A current as little as 50 milliamps–that`s 50/1,000 of an amp–can cause death. Whereas low currents between a hand and a foot cause only a tingling sensation, there have been cases where the victim of an electric shock lost an arm and a foot due to electricity`s boiling the synovial fluid in the shoulder and ankle joints, causing an explosive amputation.
The path of the current through the body. A shock that takes a path through one finger and out another finger on the same hand (such as when touching the prongs on a plug) might cause only a painful, temporary injury. On the other hand, the same current flowing through the chest can cause death through ventricular fibrillation.
The length of time that current flows through the body. Obviously, the longer the duration of a shock, the greater the potential for an injury.
BURNS AND OTHER INJURIES
The most common injury caused by electricity is a burn. There are three classes of burns:
Electrical burns. They are caused by current flowing through tissue or bone, generating heat. Electrical burns may appear minor on the skin surface but can be very serious, and they require immediate attention.
Arc burns. An electric arc can produce temperatures in excess of 3,000°F. Direct contact with an arc can cause serious burns even if the electricity does not pass through the body. A welder`s arc is a good example of an electric arc; it is hot enough to melt steel.
Thermal burns. These burns are caused by contact with hot equipment. Have you ever grabbed a hot light bulb? The source of that heat is the electrical power used to raise the filament temperature until it is white hot. Hot electrical equipment can cause skin burns and even get hot enough to be an ignition source.
All three types of burns can occur simultaneously. Victims may complain of a great deal of pain where the contact with electricity was made–even though there is little or no visible injury to the skin.
Our nervous system is an electric circuit that can be affected by outside electrical stimulation. These effects can include pain, involuntary muscle reactions, respiratory arrest, and cardiac arrest.
SECONDARY OR INDIRECT INJURIES
Very often, victims of electric shock suffer secondary or indirect injuries. These secondary injuries can be so severe as to cause death by themselves. One can survive the electric shock but die from an infection, fall, or collision resulting from the shock. Most of us have heard of people being “thrown” by electricity. What really happens is that the electric current causes very strong involuntary muscle reflexes that cause the person to fall or have a collision. Being “thrown” by electricity can result in bruises, fractures, and even death. A firefighter on a ladder can contact an electric source and be thrown from the ladder. In looking at the small number of electrical injuries reported in the NFPA statistics, one may ask whether it is possible that in some instances electricity provided the initial force that caused the firefighter`s fall and death. Since the electric contact was not witnessed, the cause might have been reported as a spontaneous fall.
Electric arcs can cause indirect as well as direct injury. An electric arc, for example, can ignite a combustible atmosphere and cause an explosion or fire. High-energy faults can damage equipment, which in turn conceivably could cause equipment parts and molten metal to fly in all directions. Being struck by a piece of equipment-related shrapnel can cause serious injury or death.
We can take steps to reduce the hazards of electricity, which we use daily in our homes, at work, and on the fireground.
Insulation. One way to reduce risk is to check the insulation on electric wires. How often do we do this? Electric-powered tools have insulated cords that can wear and break down. Look at your power tools and extension cords. Is the insulation cracked or damaged? Are the cords intact where they enter the plugs or equipment? Are the plugs loose or damaged? A few minutes worth of repairs not only will improve safety but can ensure that a critical tool will function when it is needed.
Grounding. We are familiar with the ground connection on the electric service and the telephone system in our homes. This grounding normally is a secondary protective measure. The term “grounding” refers to bonding the frame or case to an electrical ground through wires that ultimately lead to a grounding rod or plate in the earth.
Ground connections may be intentional (connecting electrical equipment through a ground rod) or accidental. The jacks of aerial apparatus may create an accidental ground but do not necessarily provide an adequate ground connection to prevent electrocution. The ground contact resistance of jacks placed on dry ground can be decreased by sprinkling salt on the ground and then wetting the area, but the truck and aerial device must always be assumed to have an uncertain ground connection.
The third prong on the plug of power cords connects the tool or equipment to the ground conductor, colored green or bare. How often do we work with a damaged or missing ground prong even though it is designed to protect the user and the equipment? If a live wire touches the equipment case, a high current flows (a “short circuit”), which trips an overcurrent protective device (a fuse or circuit breaker), thereby deenergizing the equipment. Note that grounding is not always desirable. Although a grounded ladder truck can save a grounded person touching it if the ladder contacts a live wire, it could kill a firefighter who touches a live wire while on the ladder. “Double insulated” power tools provide safety from electric shock by being ungrounded. The key to electrical safety is not memorizing rules by rote. It is understanding basic electricity.
Overcurrent protection. Overcurrent protection limits or shuts off the flow of electricity should a ground fault, overload, or short circuit occur. Examples include fuses, circuit breakers, and ground fault circuit interrupters. There are some important differences in the type of protection provided by each type of overcurrent device. Fuses and circuit breakers are placed in a circuit to limit the amount of current a circuit can carry. When the current exceeds that limit, the circuit breaker or fuse will open or break the circuit. Fuses and circuit breakers are meant to protect conductors and equipment–not people. Check the circuit breakers or fuses in your home and workplace. They work only if they are of the correct rating, which is 15 amperes for most 120-volt branch circuits. Check for pennies under fuses!
Ground-fault circuit interrupters (GFCIs). These devices shut off current in as little as one-fortieth of a second. They work by comparing the amount of current going into a device with the amount returning. Since the current (flow) must be the same coming in and going out (unless there is a ground fault), when a difference is detected, the GFCI trips. You will find GFCI outlets and breakers protecting areas where there is moisture or the potential for contact with the outside (pools, garages, kitchens, and bathrooms). GFCI breakers and outlets also are found on portable generators and extension cords. How many fire departments have ever thought of protecting their members by using GFCI-protected circuits on extension cords or generators? You should also be aware of regulations requiring GFCI protection for on- and off-the-job situations.
Guarding. Another way to reduce electrical hazards is to mandate guarding of electric circuits and service. The Occupational Safety and Health Administration (OSHA) requires private industry to guard electric service in excess of 50 volts. Guarding does not affect us as directly as other forms of electric protection; but when we see it, we must recognize the danger it signals. This type of guarding is accomplished in several ways, including the following:
–locating the equipment in a room, vault, or similar enclosure accessible only to qualified persons;
–using permanent partitions or screens;
–locating equipment on a balcony, gallery, or elevated platform arranged to exclude unqualified persons;
–elevating exposed equipment eight feet or more above the floor (not applicable to electric utility sites);
–marking entrances to rooms, and so on, with exposed live equipment; and
–enclosing in a metal vault or an area controlled by a lock–and marking–indoor installations over 600 volts located in an area that is open to unqualified persons.
Should you encounter guarded areas, you must act as if the equipment inside is live and has the potential to electrocute. Do not assume that the equipment has been turned off. A qualified person must check the equipment to verify it has been turned off. In areas such as rail tunnels, electric substations, factories, and so forth, the presence of live electrical equipment is a strategic factor to be considered when developing tactics. If your jurisdiction has electric installations that are of special concern, preplan a response to those areas.
HOT STICKS, LINEMAN`S GLOVES, AND OTHER EQUIPMENT
Even when an incident involves electricity as a strategic factor, we often feel that we must intervene to save a life or extinguish a fire. To handle an incident involving electricity, many fire companies use hot sticks (insulated poles), rubber and leather lineman`s gloves, and a variety of other insulated tools. When used properly by trained, certified, and qualified people, this type of equipment will protect the user under known conditions and with limitations. In the hands of the untrained (hot dog), they alone will not prevent serious contact accidents. If an unsuspecting firefighter is allowed to walk into a ground gradient condition with his conductive fire boots (see “Boots as Insulators” on page 88), the hand and stick protection will be of no avail. In fact, the availability of such equipment might encourage firefighters to go up closer to a downed wire or other electrical hazard.
Also, insulated tools and equipment must be retested on a routine basis to ensure that their electrical resistance has not been compromised or lost. This testing is not something the fire department can do; it must be performed by a qualified testing lab and be documented. As far as that pair of lineman`s gloves goes, when was the last time the pair of gloves that has been lying in the compartment since you were a rookie was tested? Do you know how many volts your department`s protective equipment was created to handle? Did anybody ever show you how to use the gloves correctly? How many of us have ever used a pair of insulated wire cutters? How do we use the cutters? Should we cut wires? Should all the wires be cut at once, or should they be cut separately? (See “Cutting Live Wires” on page 79.) Simply put, even if the tools are ready for us to use, we often are not prepared to use them.
Don`t make the situation worse. Admit that there are limits to what you can do, and base those limits on fireground tactics, not electrical line work. Prepare ahead. Before an incident arises, locate qualified people who can help. Large industrial facilities in your area may have on-site electricians. Electrified railroads often have electricians available 24 hours a day. Don`t forget about your local power company–it has the experts.
Electricity provides a unique set of problems to firefighters and other emergency responders. We must respect the potential for danger electricity holds, but we must also be able to work around it safely. We can do this by using safe work practices. The idea that simply “pulling” an electric meter will solve all our problems is naive. We must learn to identify qualified people to deenergize, test, and ground electric circuits. We must treat all wires as live and dangerous. It is critical to keep all the objects we handle during the emergency from contacting electrical lines. Most importantly, we must keep our limited knowledge of electricity from becoming a liability. If you need more information about electricity, talk to your local power company. Electric utilities often will conduct, on request, training programs without cost. n
I would like to acknowledge the assistance of the following in the preparation and review of this article: William E. Hering, corporate safety director of S. M. Electric Co., Inc., has been a qualified journeyman electrician for 30 years (a member of Local Union 52, IBEW); is a certified OSHA/MSHA trainer for the U.S. Department of Labor; and is an emergency medical technician instructor for the New National EMT Curriculum Certified New Jersey Department of Health.
Al Saharic retired as safety manager at Jersey Central Power & Light Co. after 36-plus years of experience at all levels. He is safety officer and a past fire chief of the Lebanon (NJ) Volunteer Fire Company and OE&M coordinator for the borough of Lebanon and a certified OSHA trainer for the U.S. Department of Labor.
(Above) The three transformers on this utility pole indicate “three-phase” electrical power service, usually supplied to heavy industry, by which power is provided by three high-voltage distribution wires (one in each transformer). The voltage of each wire is out of phase with the others by 120 degrees. For safety purposes, three-phase power can be treated as single-phase power but with three “live” wires instead of one. (Right) Electric line service comes down the pole (note conduit) and goes under the road to a substation. Transformers at electrical substations step down voltages from power transmission lines for overhead and underground distribution to consumers. (Photos by author.)
Electric substations generally are protected by guarding (the fence, in this photo) and “danger” signs. If there is no sign, don`t assume there is no danger. Electromagnetic fields near high-voltage wires can result in a shock without even touching a conductor connected to the power system.
Consultation with the local electric utility as a component of preplanning for responses in jurisdictions with electric installations can increase safety and avoid surprises for first responders. As an example, although this substation in an upscale neighborhood (left) appears to be a residence, a rear view (right) reveals that it is actually a guarded electric installation.
OPERATIONS AROUND ELECTRICAL EQUIPMENT
Operators should use appropriate operating platforms (pull-out, for example).
Aerial devices should be at least 15 feet from overhead electric utility conductors unless the power company certifies otherwise.
Straight-stream applications into overhead electric wires can energize the nozzle.
Keep ground personnel clear of the aerial device when rotating or operating adjacent to high-voltage (over 600 volts) systems.
Trip subpanel breakers and main circuit breakers and/or switch disconnects off to shut down electrical energy in areas of overhaul.
To ensure electrical shutdown, use appropriate testing devices.
Motor Vehicle Accidents Involving Electrical Systems
Preplan with the local power company.
Assure occupants in the vehicle that the situation is under control, and instruct them to remain in the vehicle.
Contact the appropriate power company immediately, informing of the serious nature of the incident, including the fact that human lives are at risk.
Should occupants have to leave a disabled, energized vehicle (tires begin to burn, for example), instruct them to leap from the vehicle from the door farthest from the point at which the wire is contacting the vehicle. In any case, emphasize that the victims must not be in contact with the vehicle and the ground at the same time.
In all cases, ground-gradients can create a hazard to firefighters who may approach the energized vehicle.
Personnel Receiving Electric Shock
Any victim of electric shock (no matter how small the shock) shall be immediately referred to a medical treatment facility for physician review and possible electrocardiogram testing.
Keep the following in mind: Senseless
K ills. n
Electricity presents a grave hazard to crews on the fireground. While I have not been able to discover a single case of a firefighter being electrocuted by current conducted through a hose stream, I do know of four deaths brought on by ladders in contact with charged wires. In one of these, the man was on an aerial ladder and came in contact with the wire. In the others, the victim was on the ground. One firefighter had a foot on the running board of a metal aerial truck and the other on the ground when the ladder touched a high-voltage line. In both of the other cases, a ground ladder hit a wire while men were moving the ladder. In each of these accidents, one of the four men at the base of the ladder was killed and the other three were not injured.
— William E. Clark, Firefighting Principles and Practices, Second Edition (1991)
CUTTING LIVE WIRES
It is not recommended that firefighters cut live wires except in extreme emergency, and then it should be done only with approved wire cutters (not bolt cutters or other tools) and wearing lineman`s gloves that are in good condition. When it is necessary to cut a wire, care should be taken to keep loose ends from lashing around when released.
— William E. Clark, Firefighting Principles and Practices, Second Edition (1991)
Small Class C fires can be extinguished safely with carbon dioxide, dry chemical, or halon [-replacement] extinguishers. Water in the form of fog is also safe. Dry chemical and water, however, may create a clean-up and restoration problem for some kinds of electrical equipment, although in many cases the fire has already done enough damage to require rebuilding the equipment.
On large fires that are beyond the capability of extinguishers, water in fog form is indicated. This method always raises the question of whether water conducts electricity. Pure water is, of course, a nonconductor; it is actually the dissolved minerals in water that make it a conductor. A straight stream of water, therefore, can conduct electricity, but not very well. There is no documented case of a firefighter being killed by electricity conducted through a stream of water, either solid or fog.
It is possible for a person holding a nozzle to get a low-order shock through so-called solid stream or a straight stream from a fog nozzle, but the danger would not be so much that of electrocution as losing control of the nozzle due to reflex action.
Because there are air spaces between the water drops in a fog stream, the electrical conductance is limited; and such a stream is generally regarded as safe to use on energized equipment, even up to fairly high voltages. There are still, however, indirect hazards which should be avoided. For instance, it is not advisable to approach a high-voltage source closer than one`s body length, so that a forward fall would not cause direct body contact. Another precaution is to stay out of puddles; current may travel up one leg and down the other on its way to the ground. It is not necessary that the puddle be there first. It could be formed at one`s feet from the water applied to the fire.
There may be cases where a straight stream is accidentally directed onto high voltage. Therefore, it would be well to know what might be expected. I was working at a lumberyard fire when an engine company nearby struck an 11,000-volt overhead wire with a stream from a 118-inch nozzle. The result was thunder and lightning–a bright white flash accompanied by a big noise. It was at night; the light was visible for miles, and the sound was heard over a considerable distance. As for the firemen holding the nozzle, all they suffered was surprise; they did not feel any current and continued operating without harm. The distance from the nozzle to the wire was about 40 feet.
Several universities and fire departments have conducted tests to discover safe distance limitations for straight streams on electrical equipment. But, results vary somewhat; and there are differences of opinion on what is the most amperage an individual holding a nozzle can safely sustain….
From the various test data available, it appears that, with the exception of a power plant, fires in buildings would rarely present voltages that would be highly hazardous if accidentally struck by a water stream. Even if an outside stream strikes a high-voltage transmission line, the distance from wire to nozzle is usually sufficient to preclude a dangerous level of conductivity.
— William E. Clark, Firefighting Principles and Practices, Second Edition (1991)
FACTS FOR FIREFIGHTERS
Ground gradient: The ground soil/surface is a conductor and will allow current to travel a distance from an energized source (fallen wire). Step into this area, and electric shock will result.
Boots: They can be conductive if they are not pure rubber, have steel arches and toe protection, or contain carbon substance.
Water: Straight streams or continuous patterns are conductive. Fog and spray are not conductive; however, runoff must be considered.
Carbon: When Class A materials burn, they create smoke and char. This is conductive carbon. (Diamond is pure carbon, but it is an insulator.)
Equipment: All firefighters` equipment becomes covered or impregnated with conductive carbon over a period of time.
Tires: They are made of materials similar to your boots and usually have steel belts. The belts are conductive. The material itself may be conductive.
Vehicles: To exit from an energized vehicle, you must leap or jump off, to eliminate a path to ground.
Extinguishers: The discharge of dry chemical or CO2 units can create a static charge, which you can feel.
There are times when an arc may ignite the oil in a transformer. The transil oil is of high flash point and can be extinguished with fog. If this task can be done from the ground, there is no great danger except that of being struck by a falling wire, but firefighters should not climb the pole to attempt extinguishment at close hand.
When a wood pole is wet, it should be regarded as a conductor. Firefighters should not only steer clear of it but also keep others away.
— William E. Clark, Firefighting Principles and Practices, Second Edition (1991)
BOOTS AS INSULATORS
Fireman`s rubber boots have some insulating quality because they are made of rubber, but are not reliable insulators because carbon black (a conductor) [generally] is used in the rubber to give it strength. Thus the insulating value is… decreased. Furthermore, firefighters` boots pick up much metal in the form of small nails, especially the boots of ladder company members who do the overhauling. When I was a fireman, I would periodically use pliers to remove this metal from the soles of my boots. Many times I would remove 30 to 40 nails from each boot; this amount of metal in the sole could make it conductive.
— William E. Clark, Firefighting Principles and Practices, Second Edition (1991)
POWER LINES DOWN–A REMINDER BY MARTIN GRUBE
When power lines are severed or when they are separated from their connectors and fall to the ground, they can become thin lines of death.
In September, 1989, I traveled to Charleston, South Carolina, with a contingent of Virginia Beach firefighters to help with the recovery efforts in the aftermath of Hurricane Hugo. Our main concern was to clear the streets for fire equipment and for the power company. Almost every electrical wire in the Charleston area was damaged by fallen limbs, high winds, or debris shearing the wires from the poles. Since Hugo, the entire country has experienced dozens, if not hundreds, of serious weather events that have damaged electric power systems to one degree or another.
But the danger from fallen power lines is everpresent, even on a beautiful summer day. Tall trucks have ripped down wires, vehicle accidents have caused utility poles to collapse, and fire has burned lines from their service connections. Firefighters responding to a call are especially vulnerable to this danger when they are looking for a house address, sizing up a fire, or determining where to spot the apparatus. Unless the company is dispatched specifically to a “wires down” call, many firefighters never give the potential electrical danger a second thought. This could be a serious oversight: A quick scan of utility lines–just a few seconds on every call–could save your life.
If your area has experienced some type of environmental damage by high winds, tornadoes, an ice storm, and so forth, you and your crew should scan both sides of the street along your response route. Suspect fallen wires. Many fire departments establish and follow standard operating procedures when electrical lines are involved. If a wire is lying across the street or a pole is down, establish a danger zone. Stop the apparatus a full span of wires (or two poles` distance) away from the downed wire or pole. Stop all traffic from entering the danger zone. Obtain an address and pole number closest to the damaged pole and ask the fire dispatcher to notify the power company. Pole numbers can be repeated in different grids, so specify both the pole number and the nearest street address to the downed wire.
Normally, the lowest wire on a pole is the television cable. It is one-quarter-inch round cable wrapped in black insulation. The next wire up normally is the telephone service wire, also wrapped in black insulation. All other wires above that–or the highest up on the pole–are electrical wires. These wires have ceramic insulators that look like saucers near the pole. Remember, these wires carry up to 20,000 volts in single phase (residential usage) and 24,000 volts in three-phase (commercial usage).
When responding to a vehicle accident where a pole is down and wires are on or near the vehicle, do not approach the vehicle. Have the fire dispatcher notify the power company and tell them to expedite, for you have victims trapped by the wire. Provide the pole number and street address. Advise victims not to exit the vehicle, and do not allow any rescue workers near the vehicle. Be aware that the vehicle could be resting on a downed wire, transformer, or underground service box.
Power crews will arrive and use a fiber glass “hot stick” to remove the wires or will shut down the grid so you can begin patient care. Never approach energized wires, and do not attempt to remove downed wires from a vehicle. Do not touch the vehicle–the vehicle could be energized and, by touching it, your body will complete a circuit to ground and you will sustain a serious electric shock.
The fire department should remain at a location to prevent entry into the danger zone until relieved by a police unit or the utility company. If a search of the area is necessary or there is a need to evacuate, wear full protective gear, keep a comfortable distance from the downed wire(s), and watch your step. n
(Opposite page) This power pole was tossed through a window during Hurricane Hugo. (Top left) Never leave the scene unprotected. Call for a police unit to block the roadway at both ends of the affected area. (Top right, bottom left) Once tree limbs cause the electrical lines to fall, heavy snow falls could quickly cover them, and they may never be seen. (Bottom right) Electrical lines are always located on the highest part of a pole. Single-phase lines are wrapped in insulation, whereas three-phase service cable is grounded by the saucer-like ceramic insulators. Downed power lines often are caused by fallen tree limbs resulting from ice storms or heavy snowfalls. (Photos by author.)