May 10, 2005
|Cord- and plug-connected equipment without a grounded connector|
Electricity improves life. During power failures, much work and many routine activities grind to a halt. However, electricity can cause serious injury and even death if you fail to follow electrical safety practices, particularly in the workplace.
Electricity is recognized widely as a serious workplace hazard that exposes employees to electric shock, burns, fires, and explosions. According to the Bureau of Labor Statistics (BLS), 289 employees were killed by contact with electric current in 2002. Others were killed or injured in fires and explosions caused by electricity.
The U.S. Occupational Safety and Health Administration (OSHA) cites three possible factors as causing electrical accidents: work involving unsafe equipment and installations; workplaces made unsafe by the environment; and unsafe work performance (unsafe acts). The first two factors sometimes are combined and referred to simply as unsafe conditions. Thus, electrical accidents generally can be considered as being caused by unsafe conditions, unsafe acts, or, in most cases, combinations of the two. It also should be noted that inadequate maintenance can cause equipment or installations originally considered to be safe to deteriorate and create unsafe conditions.
OSHA also states that some unsafe electric equipment and installations can be identified by faulty insulation, improper grounding, loose connections, defective parts, ground faults, unguarded live parts, and underrated equipment. The environment also can contribute to electrical accidents. Environments containing flammable vapors, liquids, or gases; areas having corrosive atmospheres; and wet and damp locations are some unsafe environments affecting electrical safety. Finally, unsafe acts include the failure to deenergize electric equipment when it is being repaired or inspected, using obviously defective and unsafe tools, and using tools or equipment too close to energized parts.
General ways to protect employees from electric shock include insulating and guarding live parts. Insulation provides an electrical barrier to the current flow. To be effective, the insulation must be appropriate for the voltage, and the insulating material must be undamaged, clean, and dry.
Guarding prevents employees from coming too close to energized parts. The guard can be a physical barricade, or it can be provided by installing the live parts out of reach from the working surface. This technique is known as "guarding by location."
Grounding is another method used to protect employees from electric shock; however, it normally is a secondary protective measure. To keep guards or enclosures at a common potential with earth, they are connected by a grounding conductor to ground. In addition, grounding provides a low-impedance, ample-capacity path back to the source to pass enough current to operate the overcurrent devices in the circuit. If a live part accidentally comes in contact with a grounded enclosure, current flow is directed back to earth, and the circuit protective devices (fuses and circuit breakers) can interrupt the circuit.
If it draws too much current, electric equipment can overheat, which can result in fires. Protecting electric equipment from overcurrent helps prevent this from happening.
Designing and installing equipment to protect against dangerous arcing and overheating also are important in preventing unsafe conditions that can lead to fires, high-energy electric arcs, and explosions. Employers and employees usually cannot detect improperly designed or rated equipment. OSHA relies on third-party testing and certifying of electric equipment to ensure proper electrical design. This helps ensure, for example, that equipment will not overheat during normal operation and that equipment designed for use in a hazardous location will not cause a fire or explosion. It also helps ensure that equipment is rated and marked appropriately, allowing employees who design electrical installations and install electric equipment to select equipment and size conductors in accordance with those ratings. Many OSHA-required electrical standardsdepend on accurate equipment ratings.
In 1998 the National Institute for Occupational Safety and Health (NIOSH) published Worker Deaths by Electrocution: A Summary of Surveillance Findings and Investigative Case Reports, a monograph that highlights the magnitude of the occupational electrocution problem in the U.S.; identifies potential risk factors for fatal injury; and provides recommendations for developing effective safety programs to reduce electrocution risks . This publication also includes 224 electrocution case studies.
Among the case studies featured in the NIOSH publication is the 1985 account of a 29-year-old male production welder, a metal fabrication company employee who was electrocuted when he plugged a portable welding machine's cord into a defective extension cord.
The victim was assigned to work on a concrete platform immediately outside the facility. Using a torch, he cut brackets off a conveyor. He then re-entered the facility and wheeled a portable arc welding machine out onto the platform. No receptacle (electrical outlet) was nearby, so he used an extension cord. The cord's male end was four-pronged, and the female end was spring-loaded. The victim plugged the male end into a receptacle. He then plugged the welding machine's power cord into the extension cord's female end. The outside metal case of the machine's plug became energized, electrocuting the victim.
An examination revealed that the extension cord's female end was broken. The spring, cover plate, and part of the melamine casing were missing from the female connector's face. In addition, the welding machine plug ground prong was inserted 90 degrees from the ground terminal, so that the plug's normally grounded cover was electrified. It is impossible to insert the plug incorrectly when the plug and connector are complete and intact.
The welding machine's power switch was in the "on" position when the victim was discovered. The spring and melamine fragments were found at the site. The victim was totally deaf in one ear and suffered diminished hearing in the other. He may have dropped the extension cord at the site, and not heard the connector break.
The case study includes measures that could have prevented this accident:
In 1987 a male lathe operator died when he contacted the frame of a lathe that was energized by a ground fault.
In 1985 the company hired a contractor to install an additional 480Y/277-V transformer and three-phase wiring in an overhead busway to accommodate additional machinery. Between August and October 1986, new lathe and mill machines were installed by a contractor representing the machine manufacturer. During these installations, three grounding deficiencies occurred: (1) The electrical contractor failed to connect the grounding conductor on the transformer to the grounding conductor in the busway. (2) The electrical contractor failed to connect the grounding conductor on the transformer to building steel or to a grounded conductor on the supply side of the electrical service disconnect. (3) The machine contractor connected the grounding conductor on the lathe involved in this incident to the insulated neutral lug inside the switch box. The third deficiency interrupted the continuous ground for the lathe. The other two interrupted the ground for all equipment attached to the circuit.
Some time before the accident, a capacitor between the frame ground and the lathe servo power supply failed, energizing the lathe frame with 220 volts. While there were no eyewitnesses, it is assumed that while walking between the lathe and a neighboring lathe, the victim contacted the lathe frame and also a grounded metal air supply line running about 8 inches from the lathe. A supervisor found the victim a few minutes later lying face down between the two lathes, with his right arm touching the metal drip pan of the lathe. The supervisor received a slight shock when he contacted the victim's arm, but succeeded in pulling the victim away by the feet. Resuscitation failed. The victim had burns on the left side of his face.
To prevent this accident, NIOSH recommended that:
According to NIOSH, most of the 224 occupational electrocution incidents investigated could have been prevented through compliance with existing OSHA, NEC, and National Electrical Safety Code (NESC) regulations and/or by using adequate personal protective equipment (PPE). All workers should receive hazard awareness training so that they will be able to identify existing and potential hazards present in their workplaces and relate the potential seriousness of the injuries associated with each hazard. Once these hazards are identified, employers should develop measures that will allow for their immediate control. To reduce occupational electrocutions, NIOSH recommends that employers:
Don't wait until an electrical problem reveals itself by injuring or killing an employee or causing a fire that damages the workplace. Take the proper steps to ensure electrical safety.