Western Canada Mine Rescue Manual Chapter 4 Electrical Hazards PDF

Western Canada Mine Rescue Manual Chapter 4 Electrical Hazards 4-1 OBJECTIVES This chapter is intended to educate and protect rescuers who are called upon to respond to emergencies involving electrical systems. Upon completion of this chapter, the trainee shall be able to demonstrate competency...

Western Canada Mine Rescue Manual Chapter 4 Electrical Hazards 4-1 OBJECTIVES This chapter is intended to educate and protect rescuers who are called upon to respond to emergencies involving electrical systems. Upon completion of this chapter, the trainee shall be able to demonstrate competency in:  Concepts and definitions  Injuries caused by electrocution and factors affecting severity  Special considerations for emergencies involving electrical equipment  Guidelines for responding to emergencies involving electrical equipment Introduction The widespread use of electric power, carried by a vast network of energized wires, has resulted in many injuries and deaths due to exposure to electricity. Many factors influence the severity of electrical injuries. Although high voltages and amperages are dangerous, contact with low voltages can also be fatal. Moisture on the skin decreases the body’s resistance and increases the severity of the injury, whereas partial insulation by dry clothing lessens the effect. Electrocution at heights may result in a fall that can further injure the casualty. Electrical Installations Electricity is generated by power plants. This voltage is stepped up for efficient transmission over long distances to substations near the load centres. Transmission lines operate between 69,000 volts and 500,000 volts. At the substations, voltage is reduced and power is sent through distribution lines to industrial, commercial, and residential customers. These lines operate between 5,000 V and 25,000 V. Some mines, especially those in more remote locations, have their own on-site power-generating capabilities. These facilities present unique circumstances during emergencies. CONCEPTS AND DEFINITIONS Voltage is the difference in electrical potential between two points in an electrical field. It is the force that causes the flow of electricity, and it is measured in volts (V). Because mines require high voltages, kilovolts (kV, 1 kV = 1,000 V) are often used to express the difference in electrical potential. Current is a flow of electrical charge. It can be compared to the rate of water in a pipe. Current is typically measured in amperes (A). (1 ampere = 1000 milliamperes (mA)).  Alternating Current (AC) refers to when a current in a circuit reverses polarity or changes direction in current flow 60 times per second (60 Hz).  Direct Current (DC) refers to a current flowing only from positive to negative. Resistance is similar to the effect of friction on the flow of water in a pipe. Water flows more freely in a large pipe than in a small one, and different materials have different resistances to the flow of electricity. Resistance is measured in ohms (Ω). 4-2 Grounding is the process of mechanically connecting isolated wires and equipment to the earth, with sufficient capacity to carry the fault current and to ensure the wires and equipment remain at the same potential (same voltage) as the earth (ground). Bonding is the process of joining together two conductors that do not carry currents. These may be two wires, a wire and a pipe, or these may be two pieces of equipment. Bonding is done by connecting all the metal parts that are not supposed to be carrying current during normal operations, thereby bringing them to the same electrical potential. Grounding is still required after bonding as bonding itself does not protect anything. Insulators are materials of high resistance that conduct electricity in such small quantities that it cannot normally be detected. Examples of insulators include glass, ceramic, and porcelain. Conductors are materials of low resistance that conduct electricity in large amounts. Examples of conductors include copper, aluminum, iron, salt water (brine), and most other metals. Semiconductors are materials that have a value of resistance between those of insulators and conductors. Examples of semiconductors include wood, earth, and rubber tires. Arcing: An electrical arc is a sudden release of electrical energy bridging a gap between two conductors. An arc can be extremely hot. Arcing is usually associated with a short circuit, a current interruption at a switch point, or loose terminal. Overheating: Loose connections and overloaded electrical conductors or motors cause overheating. Exceeding the amount of current that conductors and equipment are designed to carry is dangerous and can be avoided by using properly-sized overload and short-circuit protection devices. Low Voltage: Most electrically caused fires originate in equipment operating below 750 V. In the electrical industry, anything below 750 V is commonly referred to as low voltage or secondary voltage. High Voltage: Electricity can arc through the air to a person, tool, or other conductor if they get too close. All rescuers, tools, and equipment including aerial devices and extension ladders must maintain a minimum distance known as the safe limit of approach. Canadian Electrical Code’s Safe Limits of Approach Voltage of Live Power Minimum Distance 0–750 V 1 M (3 ft) 750–150,000 V 3 M (10 ft) 150,000–250,000 V 4.5 M (15 ft) Greater Than 250,000 V 6 M (20 ft) Voltage Gradient on the Ground Because electricity always seeks the path with the least resistance to the ground, electrical systems use conductive grounding rods to ensure that any stray current is returned to earth safely. These rods are typically driven 2.5 m (8 ft) or more into the ground to ensure good contact with the ground. However, if 4-3 electricity is released onto the surface, such as when a "live” wire lies on the ground, the electricity will fan out from the point of contact. During a fault to the ground, there is a rippling effect that can be likened to dropping a pebble into calm water. In the pool of water, the wave created at the point of contact gets smaller as it spreads outward. Similarly, in a "pool" of electricity, the energy is at full system voltage at the point of ground contact, but as you move away from the contact point, the voltage drops progressively. This effect is known as ground gradient. Step and Touch Potential The ground gradient, or voltage drop, creates two problems: step potential and touch potential. Assume that a live downed wire is touching the ground and has created a pool of electricity. If you stand with one foot near the point of ground contact (at x voltage) and your other foot a step away (at y voltage), the difference in voltage will cause electricity to flow through your body. This effect is referred to as step potential. If rescuers find themselves within a ground gradient, they must safely exit it. To do so, keep both feet in contact with each other and hop or shuffle out of the affected area. When shuffling, make certain that the feet are always in contact with one another. Similarly, electricity will flow through a body if it touches an energized source with the hands, but the feet are at some distance from the source. The difference in potential voltage in this case is referred to as touch potential. 4-4 INJURIES CAUSED BY SHOCKS AND ELECTROCUTIONS WARNING: Electricity always seeks the easiest path to the ground. People, who place themselves between any two energized conductors, or any energized conductor and ground, will become part of an electrical circuit that can kill or cause serious injury. Effects of Electricity on the Body The path electricity takes through the body is critical. For example, current passing through the heart or brain is more life-threatening than current passing through the fingers. The expected effects from just a fraction of this current for a few seconds are illustrated below. Current level=mA (Milliamperes) Probable Effect on Human Body 1 mA Slight tingling sensation. 5mA Slight shock felt; not painful but disturbing. Average individual can let go. However, strong involuntary reactions to shocks in this range may lead to injuries. 6mA–16mA Painful shock, begin to lose muscular control. Commonly referred to as the freezing current or “Can’t let go" range. 17mA–99mA Extreme pain, respiratory arrest, severe muscular contractions. Fractures can occur. Individual cannot let go. Death is possible. 100mA–2000mA Ventricular fibrillation (uneven, uncoordinated pumping of the heart.) Muscular contraction and nerve damage begins to occur. Burns will occur. Death is likely. > 2,000mA Cardiac arrest, internal organ damage, and severe burns. Death is probable. Any electrical hazards must be controlled before approaching a casualty. Electrical energy casualties will require prompt and appropriate medical treatment. Factors Affecting Severity of Injury It is the current (amperage) that kills or injures. But the voltage, which pushes the current through the body, also has an important effect. Persons exposed to household voltages may suffer a muscle spasm and become locked-on to the electrical source until the current is turned off, or until they are dragged clear by the weight of their body falling away from the contact. Relatively long periods of contact with low voltage current cause many electrical fatalities. At very high voltages, such as from power lines, the casualty is often quickly blown clear of the circuit. This results in less internal damage, such as heart failure, but serious surface burns where the current enters and leaves the body. Exposure to a large electric arc can result in injury from the intense heat or from ultraviolet rays, which can cause serious eye damage. Path of electricity through the body Degree of skin resistance Length of exposure Pressure of body against source Current Voltage Frequency AC/DC 4-5 SPECIAL CONSIDERATIONS FOR ELECTRICAL EMERGENCIES Combustible Materials Fires involving electrical equipment often result from the presence of combustible materials. For example, most fires that break out in electrical generating plants originate in fuel systems, oil systems, flammable gaseous atmospheres, combustible dust, accumulated waste material, or in buildings constructed of combustible material. Faulty Electrical Equipment Electricity is safe in normal operating conditions. However, hazards are created when electrical equipment or wires have become faulty due to:  Wear or other deterioration  Improper installation  Inadequate maintenance  Improper use  Damage or breakage  Lightning Any one of these factors may cause arcing or overheating of electrical equipment. Substation and Generator Fires Substations and generating facilities contain transformers, large quantities of oil, energized electrical equipment and, in some cases, cylinders of compressed gas. Some older transformers still in service might contain polychlorinated biphenyls (PCBs), many of which release toxic by-products when heated. Upon arrival at a substation or generator fire, rescuers should stand ready to protect adjacent properties. Authorized personnel will inform rescuers when the substation has been made electrically safe. Once electrical energy isolation is completed and locked out, rescuers can then proceed to extinguish the fire. Electrical Arc Flash Hazard An arc flash hazard can exist when energized electrical conductors or circuit parts are exposed or are within equipment in a guarded or enclosed condition. The hazard is present when a person is using electrical equipment improperly, or when someone breaches the safe limits of approach. Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained should not pose an arc flash hazard. 4-6 Vehicles in Contact with Live Wires Emergency Situation A fallen wire lies under a vehicle with occupants… The operator is unhurt and can move the vehicle… A fallen wire lies across a vehicle with occupants… If the operator is injured and cannot move the vehicle… Action to be taken by emergency personnel Do not touch any part of the vehicle. You could be electrocuted, even if you are wearing rubber gloves. Instruct occupants to stay where they are until electrical crews arrive. Instruct the operator to move the vehicle clear of the wire, and clear of any pools of water which may be energized by the live wire. Make sure you are not in a position to be injured if the wire springs up after being released when the vehicle moves. Make sure no one else is standing in a dangerous location. Do not touch any part of the vehicle. Instruct occupants to stay where they are until electrical crews arrive. Instruct the operator to stay in the vehicle until electrical crews arrive. Direct contact with power lines is not necessary to pose an arcing hazard as power can arc from the lines to a crane or other piece of equipment. 4-7 ELECTRICAL HAZARDS ENCOUNTERED BY SPECIFIC WORK GROUPS Work Groups Welders Hazards Responders should know all welders use electrical systems to “Weld, Cut, or Braze”. They must be aware of the electrical hazards and take positive steps to eliminate and/or mitigate those hazards. Crane Operators Contact with overhead power lines is a major cause of fatalities in the industry. Electricity can travel from a power line to a worker touching any part of the crane or the load. Haul Trucks and Other Heavy Equipment Tires can explode during or after contact with power lines / lightning. If a vehicle contacts overhead power lines there may be a massive electrical current flowing through the vehicle and its tires: • This can cause the tires to explode on contact or could cause the tires to start burning inside. Rescue teams must consider their approach angle, safe distances, and the size of the tire. • This creates a build-up of gases and heat which could cause the tire to explode at a later time, even as much as 24 hours after the incident. • The resulting explosion could potentially injure persons in the proximity with flying debris. • The vehicle should be isolated for a period of time at a safe distance to avoid injury. Ground Engagement Tools (excavators, dozers, graders, etc.) Buried power and communication lines pose a hazard to operators of equipment used during trenching and excavation activities. Operators need to be aware of the hazards posed by penetration of energized power lines and take positive steps to eliminate the hazard before digging. Photo 4-8 GUIDELINES FOR ELECTRICAL EMERGENCIES Always assume that all electrical wires and equipment are energized until proven otherwise. Mine rescue teams must ensure that energy isolation is complete prior to conducting rescue operations.             When arriving at the incident scene, stage response vehicles at a distance that avoids exposure to electrical hazards. Control the incident scene to eliminate unauthorized access and prevent exposure to electrical hazards. Wait for authorized personnel to isolate power. Use lock-out/tag-out devices when working near energy sources as per site-specific isolation procedures. Guard against electrical shocks, burns, and eye injuries from electrical arcs. Establish an exclusion zone equal to the length of the distance between two poles (i.e., one span) in all directions from downed power lines. Be aware that damaged electrical lines can move significant distances by themselves when energized or as a result of the wire’s coil memory. Be aware that other wires may have been weakened and may fall at any time. Exercise caution while raising or lowering ladders, elevated work platforms, and booms near power lines. Do not touch any vehicle or apparatus that is in contact with electrical wires. Do not use solid or straight water streams on fires in energized electrical equipment. Be aware that wire-mesh, chain-link, barbed wire, and steel-rail fences can be energized by wires outside of your field of view. Where wires are down, heed any tingling sensation, as this indicates a ground gradient. 4-9