Generation, Transmission and Distribution
There are three stages of electric power supply; generation, transmission and distribution. Each of these stages involves distinct production processes, work activities and hazards.
Most electricity is generated at 13,200 to 24,000 volts. The hazards of the electrical power generation process include explosions and burns resulting from unexpected equipment failure. Accidents can also occur when proper lockout/tagout procedures are not followed. These procedures are in place to control energy sources. Before performing maintenance on equipment where the unexpected energizing, start up or release of stored energy could occur and cause injury, the equipment must be isolated from the energy source and rendered inoperative. Failure to properly isolate these energy sources (lockout/tagout) can result in serious injury or death.
After electrical power is generated, it is transmitted over distances using transmission lines. Transmission lines are constructed between transmission substations located at electric generating stations. Transmission lines may be supported overhead on towers or they may be underground. They are operated at high voltages. They send out large amounts of electrical power and extend over considerable distances. When electricity comes out of a generating station, the transmission substation located there steps up the voltages to the range of 138,000–765,000 volts. Within the operating area, transmission substations reduce the transmitted voltage to 34,500–138,000 volts. This power is then carried through lines to the distribution systems located in the local service territory. The major hazards present during the transmission process are electrical. Failure to maintain proper approach distances or use appropriate protective equipment (rubber gloves and sleeves) can result in serious injury or death. Falls also are a source of serious accidents and can occur during maintenance work on overhead lines and while working from poles or bucket trucks.
The distribution system connects the transmission system to the customer’s equipment. The distribution substation reduces the transmitted electrical voltage to 2,400–19,920 volts. A distribution transformer further reduces the voltage. Hazards related to distribution work also are electrical in nature. However, there is the additional hazard of working in enclosed spaces (manholes and vaults) when dealing with an underground distribution system.
Transmission and distribution substations are installations where the voltage, phase or other characteristics of the electrical energy are changed as part of the final distribution process. Electrocutions represent the primary safety hazard in substations. Such accidents are generally caused by failure to maintain proper approach distances to live electrical equipment and/or failure to use appropriate personal protective equipment, including rubber insulating gloves and sleeves.
Safety Hazards of Generation, Transmission and Distribution
The Electric Power Generation, Transmission and Distribution Standard, also known as the Electric Maintenance Standard Codified at 29 CFR 1910.269, was promulgated by the US Occupational Safety and Health Administration (OSHA) on 31 January 1994. The Standard covers all electric utility workers involved in the operation and maintenance of electric power generation, transmission and distribution equipment and associated equipment. In addition, contract lineworkers, contract line clearance tree trimmers and independent power producers are also covered by the provisions of 1910.269. Other countries and regions have similar regulations.
The hazards that are directly addressed by the OSHA standard are those of an electrical nature which would cause electrocution and injuries resulting from electric shock. The consequences of inadvertent contact with high-voltage electricity are often death or serious injuries such as second- and third-degree burns, amputation of limbs, damage to internal organs and neurological damage.
The standard also addresses fatalities and injuries associated with four other types of accidents—struck by or struck against; falls from ladders, scaffolds, poles or other elevations; caught in or between as a result of the accidental activation of machinery during routine maintenance work; and contact with temperature extremes which can occur when high-pressure steam is inadvertently released during maintenance work on boilers. The Eastern Research Group (ERG), who prepared the Economic Impact Study for the proposed OSHA regulation, reported that “there were more accidents associated with transmission and distribution lines than with substations or power generation installations”. ERG reported that in the transmission and distribution line category, line workers, apprentice line workers and working line supervisors experience the most fatal and serious lost-time accidents. Within the substation and power generation category, substation electricians and general utility mechanics experience the most accidents.
OSHA has estimated that in the United States an average of 12,976 lost workday injuries occur annually to electric power generation, transmission and distribution employees. They also report that 86 fatalities occur to these workers annually. OSHA estimates that 1,633 lost workday injuries and 61 deaths can be prevented annually through compliance with the provisions of this standard and the other standards referenced in the final rule. OSHA breaks down the reduction in lost-workday injuries and fatalities into two categories. The greatest benefit is expected to be achieved in the electric utilities, which account for approximately 80% of the fatalities. Utility contractors, including electrical contractors and line clearance tree trimmers, and non-utility establishments account for the other 20%. OSHA also expects the greatest reduction in lost workday injuries to be experienced by the electric utilities. The second category of reduction relates to the referencing of existing standards within 1910.269. For example, OSHA expects the employer to provide medical services and first aid as specified in 1910.151.
Excavation operations shall comply with Subpart P of 1926; personal protective equipment shall meet the requirements of Subpart I of 1910; personal fall-arrest equipment shall meet the requirements of Subpart E of Part 1926; and ladders shall comply with Subpart D of 1910. These are a few examples of the many other OSHA standards referenced in the Electric Power Generation, Transmission and Distribution Standard. OSHA believes that these references will foster an increased recognition of the various applicable safety standards and, together with employee training and emphasis on hazard recognition through job briefings, an additional 2 fatalities and 1,310 lost-workday injuries will be prevented annually.
The Electric Power Generation, Transmission, and Distribution Standard provides a comprehensive approach for the control of hazards found in the electric utility industry. This is considered a performance-based standard, where the employer has the opportunity to implement alternative programmes provided he or she can demonstrate that they provide a level of safety equivalent to that specified in the standard. General provisions of the standard include: training requirements, hazardous energy control (lockout/tagout) procedures for power generation, transmission and distribution; enclosed space entry procedures and procedures for working safely in underground installations; requirements for working on or near exposed energized parts; requirements for working on overhead lines; grounding requirements; line clearance tree trimming; procedures for working in substations; and requirements for live-line tools, hand and portable power tools, and ladders and personal protective equipment.
The standard is comprehensive and addresses all aspects of the operation and maintenance of power generation, transmission and distribution equipment.
Some of the most significant provisions of the Standard include requirements for employees to have emergency aid training, job briefings, and training in safety-related work practices, safety procedures, and emergency procedures including manhole and pole-top rescue. There are also specific clothing requirements for working on energized equipment, and requirements for entry into underground structures, as well as the control of hazardous energy sources. Another significant element of the standard requires employers to certify that employees have been appropriately trained and can demonstrate proficiency in the work practices specified in the standard. A few of these elements are discussed in more detail below.
OSHA requires that employees performing work on or associated with exposed lines or equipment energized at 50 volts or more be trained in first aid and cardiopulmonary resuscitation (CPR). For field work involving two or more employees at a work location, at least two employees shall be trained. For fixed work locations such as a generating station, a sufficient number of employees must be trained to ensure that an employee exposed to electric shock can be reached within 4 minutes.
The lead employee in a work group must conduct a job briefing with the employees involved in the work before they start each job. The briefing must cover the hazards associated with the job, work procedures involved, special precautions, energy source controls and personal protective equipment. For repetitive and similar jobs there must be one job briefing before the start of the first job of each day or shift. When significant changes occur, another briefing must be conducted. Reviewing the task at hand requires job planning, and job planning helps to reduce accidents.
OSHA also has required that the employer certify that each employee has received the training required to be qualified and competent. The certification shall be made when the employee demonstrates proficiency in the work practices, and shall be maintained for the duration of an employee’s employment. Training alone is inadequate. Proficiency must be demonstrated, generally through testing an employee’s knowledge and understanding of the subject at hand. This will help ensure that only qualified workers work on energized equipment.
There are clothing requirements for workers who are exposed to the hazards of flames or electric arcs. The section requires that the employer ensure that each employee who is exposed to the hazards of flames or electric arcs not wear clothing that, when exposed to flames or electric arcs, could increase the extent of injury that would be sustained by the employee. Clothing made from acetate, nylon, polyester or rayon, either alone or in blends, is prohibited unless the employer can demonstrate that the fabric has been treated to withstand the condition that may be encountered. Employees may choose among cotton, wool or flame-retardant clothing, but the employer must determine, based on the exposure, whether or not a natural fibre such as cotton or wool is acceptable. Cotton or wool could ignite under certain circumstances. Although this section of the standard has caused much controversy throughout the industry, prohibiting the use of synthetics is a significant step towards reducing injuries to electrical workers.