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August-September 2004 |
Electrical Utilities, Part III
In addition to encountering normal electrical hazards in various types of structures, other electrical hazards can be encountered. In this final article, let’s examine some of these specialized hazards and consider possible mitigations.
Generating-Distribution Facilities
These facilities can include steam-powered plants, receiving stations, distribution stations, and similar facilities. Personnel should not enter any of these facilities unless accompanied by appropriate utility personnel. If dispatched to a physical rescue within one of these facilities, do not enter while it is energized or until it is declared safe by utility personnel. If necessary, use the apparatus PA system to instruct conscious victims inside hazardous areas, since the power within these facilities can present extreme hazards. Do not direct hose streams into these areas unless authorized by utility personnel.
Emergency Operations
Consider apparatus placement and overhead lines.
If necessary for a fire, secure a water supply and position lines. Protecting exposures may be the primary objective until utility personnel declare the involved area de-energized and safe to enter.
When confronted with an electrical problem, always consider the reliability of a verbal notification from an appropriate responsible person who is not "on-scene" that the electricity has been turned off and it is safe to proceed. A safe rule to consider in these circumstances would be summarized as follows: "Unless a responsible utility person is on scene and is willing to go first, consider waiting."
Transmission Towers
Transmission towers and lines can be found throughout numerous municipalities. Each tower should have an identification number near ground level that can be given to a dispatch office or utility company for identification purposes in case of an emergency. Transmission lines carry between 115,000 volts to 1,000,000 volts.
Emergency Operations
Instant electrocution can occur upon direct contact or by being in close proximity to transmission lines.
If a rescue is necessary on a transmission tower, do not climb or place an aerial device to a tower until approval is obtained from an appropriate utility company. If a conscious victim is on a tower, provide instructions (for the victim) to remain in position until it is safe to effect a rescue.
Transmission towers present a unique hazard due to a static electricity charge that is present (in varying degrees) in each tower. The high voltages traveling through transmission lines generate a magnetic field that varies and is dependent upon the voltage and atmospheric conditions. The magnetic field can easily generate significant static electricity charges (12,000 volts have been measured) that are present on or near transmission towers. Therefore, transmission towers can be deadly to personnel who may be near or on a charged tower.
Poles
Power poles can be constructed from metal, wood, or concrete. In some municipalities, a vertical metal or plastic identification tag will indicate the utility company (power, telephone, etc.) responsible for the pole. Power poles will normally have a ground strap on the exterior of the pole for grounding purposes.
Emergency Operations
Remember that metal poles will conduct electricity. Wood and concrete are considered non-conductors. However, treated wood poles, and metal rebar inside concrete poles can conduct electricity. If the grounding strap is broken (e.g., by an automobile striking a power pole), personnel who touch the auto or pole can serve as the grounding strap with predictable results.
Wires Down, Pole To Pole
Determine the type of wires that are down. They may be transmission lines or sub-transmission lines, primary conductors or secondary conductors. Remember, the best method to determine approximate voltages is by the size of an insulator. Additionally, due to different types of routing, power can flow either way in a conductor. Therefore, downed wires should always be assumed to be live.
Emergency Operations
Secure the area. Keep unauthorized people at least one pole span away from downed wires.
Consider all downed wires live.
Consider the possibility of hidden hazards associated with downed wires, such as energized vehicles, metal patio roofs, chain link fences, telephone lines, etc.
Consider potential electrocution and fire hazards when high voltage lines are down.
If it is necessary to cut downed wires during wind conditions, it may be advantageous to cut the wires in small, manageable sections to prevent the wires from wrapping around personnel cutting the wires. Coil all cut wires to prevent other personnel from thinking the wires are charged.
Wires Down, Pole To Structure
Determine if the wires are down pole to structure or structure to pole. Pole to structure will usually be energized and structure to pole will usually not be energized. However, check to see if all wires in the service are down. As an example, if one wire is down in a three-wire service, power can travel from a pole to the structure, through an appliance, and back out of the service to an apparent dead wire on the ground, energizing the wire. This is called feedback.
Emergency Operations
If wires are down, notify all persons in the area of the potential hazard. Be aware of other hazards such as charged chain link fences, metal sheds, telephone lines, etc.
If wires need to be cut, use the same techniques as cutting drip loops. Cuts should be staggered and above head height.
If a triplex service is down and needs to be cut, it must be cut at the pole where it separates from the pole. If triplex cannot be cut, place traffic cones around the hazard.
Always coil cut wires to prevent other personnel from thinking the wires are charged.
If an energized wire or wires are on the ground, personnel who step near or between the wire(s) can be electrocuted by a condition known as step potential. The step potential distance around or between a wire(s) is dependent on various factors such as the voltage in the wire(s), type of dirt (sand, any impurities that are present, etc.), moisture in the ground, and other similar considerations. If it is necessary to walk near a wire(s) on the ground, hop or use numerous short steps.
Always remember the principle of apparent dead wires can suddenly become live wires.
Pole-mounted Transformers
Pole-mounted transformers usually reduce higher voltages to lower voltages for appropriate applications by consumers. Pole-mounted transformers generally provide a voltage drop of 34,500 volts to 4,800 volts or 4,800 volts to 480-240-120 volts. Transformers may be mounted alone or in groups of two or more. It is possible for transformers to explode and burn. In this case, the oil used for cooling inside the transformer is the burning combustible. Although most utility companies have removed pole-mounted transformers containing PCB's, some transformers containing PCB's may still be located within a municipality.
Emergency Operations
Advise dispatch office and have utility company notified. Secure the area.
Water (in a fog-spray pattern) may be applied from a safe distance. (This concept is expanded under Hose Streams on Energized Equipment at the end of this article). If PCB's are present, consider the hazard from water runoff from suppression lines.
Pole-top Switches
Pole-top switches are used to switch 34,500-volt lines for sectionalizing purposes. They are padlocked and are to be opened only by utility company personnel.
Emergency Operations
Secure the area.
If a switch is burning, advise the dispatch office and ask for utility company assistance.
Capacitor Banks
A capacitor or banks of capacitors are periodically placed in lines to assist in "pushing" and enhance proper "phasing" of the electricity. Capacitors will store electrical energy.
Emergency Operations
As capacitors will store electrical energy, they can be charged to the full voltage of the line they are connected to (i.e. 4,800 volts) even when disconnected or power has been removed from a conductor. Capacitors can be oil filled and may contain PCB's. The same hazards and emergency operations that apply to transformers apply to capacitors.
Street Electrical Vaults
Underground electrical vaults can be located throughout a municipality, are covered with manhole covers, and are placed at convenient points in underground electrical systems to permit the installation and maintenance of cables, transformers and other electrical equipment. Voltages between 34,500 and 4,800 volts can be encountered in vaults.
Emergency Operations
A square manhole cover (sewer manhole covers are round) usually indicates an underground transformer vault.
If smoke is issuing from holes in a manhole cover, do not attempt to remove the cover. Gas may have formed in the vault as a result of burning insulation or oil and may be toxic and explosive. The sudden entrance of air could cause an explosion. Establish a safe perimeter around a manhole opening. Manhole covers can weigh 300 pounds and have been blown 75 feet into the air and up to distances of 125 feet. In some locations, there is a second, inner, watertight cover under the top manhole cover.
Make no attempt to extinguish fires involving underground electrical equipment until notified by on-scene utility personnel.
Never put water into a vault. Use only non-conducting extinguishing agents and only when requested by utility personnel.
Do not enter a vault until a qualified utility representative confirms that it is safe.
Street Lights
Street lights may be either low- (under 750 volts) or high- (over 750 volts) voltage. For identification purposes, newer street lights with a photoelectric cell are fed by 240 volts, and older street lights without a photoelectric cell can be fed by up to 6,800 volts. Older street lights use a "constant current transformer" that will try to increase voltages as the perceived load increases. Therefore, severed wires can result in a sudden dramatic increase in voltage. Utility companies consider street lights energized at all times.
Emergency Operations
Secure the area and request assistance from the utility company responsible for street lighting.
Do not cut exposed conductors due to constant current transformers that are used to power street light circuits.
Be aware of the possibility of charged vehicles that have collided with street lighting poles.
Traffic Lights
Traffic lights do not ordinarily present a significant hazard as they normally operate on 240 volts. However, if mounted on a streetlight pole, there could be high voltage conductors in the pole.
Emergency Operations
Secure the area, ask dispatch for appropriate utility (power, traffic department, etc.) assistance.
Telephone Lines
Telephone lines may be found as a separate system or in conjunction with power lines. If in conjunction with power lines, telephone lines will be located lowest on the pole. Power for these lines is provided by a telephone company, independent of power lines. Voltages will vary between 45 to over 100 volts (which is used for the "ringing circuit" of a telephone).
Emergency Operations
Advise dispatch and request telephone company assistance.
Treat the wires (and any wires contacting telephone wires) as carrying a minimum voltage of 110 volts (provided they are not in contact with wires of higher voltages).
Only cut wires if absolutely necessary. Since all lines are enclosed in a weather-proof covering, it will be necessary to cut all wires at the same time which may result in a quick short. Additionally, cutting telephone cables can create major damage to telephone communications over a large area.
Separating Persons From Live Wires
This consideration demands cautious, deliberate actions. Personnel who attempt to remove a victim without taking proper precautions may only compound a hazard and can become part of the problem.
Emergency Operations
Try to determine the approximate voltage that is being contacted by the victim.
Try to de-energize wires (if this can be accomplished in a timely manner) before separating victim from the wires.
Take appropriate safety precautions if cutting the wires.
Use a fiberglass pike pole or similar non-conductive tool in combination with rubber gauntlet gloves to separate victim from the energized wire(s).
Energized Vehicles
Vehicles are insulated from the ground by their rubber tires but may be energized by live wires as follows:
Contacting an object that has been energized by a wire.
Direct contact with an energized wire.
Aerial or antenna contacting an energized wire.
Personnel dismounting an energized vehicle must jump clear of the vehicle and not touch any part of the vehicle when standing on the ground. When responding to an incident that may consist of a vehicle that may be energized, personnel should consider and evaluate the potential of an electrical hazard, as touching an energized object can result in the person becoming the ground circuit which can result in serious injury or death. As an example, assume a car has struck a metal power pole with considerable force. If you were the first responder to this incident, how would you know if the vehicle were energized before touching the auto or trapped occupants in the vehicle? In this scenario, do not let your injury or death be the first indicator the vehicle is energized!
Emergency Operations
Keep aerials/antennas at least 10-feet from electrical conductors.
All personnel should be aware of any electrical hazards.
Do not touch a charged (energized) vehicle.
If personnel are on a charged apparatus, they should remain in place and not touch anything. If it becomes necessary to abandon the apparatus, do not step off but jump clear. Once clear, post a member to prevent other personnel from touching the apparatus until the hazard has been eliminated.
Hose Streams On Energized Equipment
The use of water on energized equipment presents a potential problem of electric current traveling from the source back to a nozzle by using the water as a conducting medium. While the resistance of a water stream is relatively high, dangerous amounts of current can be carried through a solid stream under certain conditions. Fog-spray steams are recommended as the water is broken into droplets surrounded by air which greatly increases resistance and minimizes conductivity.
Emergency Operations
Water should only be applied to energized equipment after considering the appropriate risks and when absolutely necessary. If necessary, use fog or spray streams in concert with the following considerations:
As the distance between the nozzle and energized equipment increases, so does the resistance of the spray-fog stream and safety of the operation.
Do not continuously apply water to energized equipment. Use short-quick applications.
This article was adapted from Truck Company Operations, by John Mittendorf, published by Pennwell. Click here to purchase.
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