When you review an accident, your objective should be to prevent another one like it. By reviewing the following accident, we hope to help prevent such an occurrence from happening in your facility. This accident had tragic consequences, and it happened despite the fact the employer had a solid safety program in place.
Unfortunately, history shows safety measures don't always produce accident-free performance. In essence, new accidents rarely happen. They are simply mistakes someone else has made before. If we study them carefully, they prove instructional to us. You can classify these mistakes into the following categories:
- Lack of alertness;
- Reckless or careless attitude;
- Personal factors such as poor coordination and physical disabilities;
- Lack of training;
- Inadequate techniques; and
- Lack of experience.
For example, a seasoned service technician suffered a fatal electrocution from an unquestionably avoidable accident. At the time of the tragedy, the reported level of industrial workers killed per year had reached an astounding 14,000 in job-related accidents across the nation. This particular accident was doubly tragic because it resulted from excessive error. We'll examine the safety measures that were in place and where the system failed.
Safety preparations and conditions. This technician worked for a service company that frequently conducted safety seminars. The company kept its well-written safety manuals updated and provided all necessary safety equipment. The training given to engineers and technicians satisfied regulations and specifications of all recognized organizations concerned with this topic [OSHA, NEC, NFPA, etc.]. In any situation related to site work involving installed equipment, the company's cardinal rule required working crews to determine the following items:
- The equipment is de-energized;
- Circuits are properly identified;
- Adequate working space exists;
- An effective grounding system exists;
- Lockout and key-interlocking provisions are enforced; and
- Close scrutiny is given to the as-built one-line diagram to ensure no backfeeds can occur from subsequent switching or other operations.
Unfortunately, service technicians had to work on the hospital's equipment far too often, mainly due to a lack of preventive maintenance at the facility. In fact, the mechanisms deteriorated to the point where they needed frequent troubleshooting and adjustment. The technician, who had an accident-free record, attempted to do some of this troubleshooting and repair work. He later died from electrocution. Sources maintain this man thought he was working on mechanisms external to any high-voltage area.
The fatal accident scenario. The incident occurred when the technician removed the motor-operating mechanism on a switch mounted in one cell of the emergency distribution system. The front door of the cell was open, providing access to the operating mechanism. This same door provided access to the switch, three buses running vertically from the bottom terminals of the switch, and load-side buses with fuses at the top terminals of the switch openings.
While removing bolts on the mechanism with a socket wrench, the technician noted that the socket on his socket wrench had loosened and fallen to the bottom of the cell, underneath an enclosure. As he tried to retrieve the socket, he made contact with an energized bus (located below the switch and back fed with 4,160V from an adjacent switch).
Investigations following the accident contained conflicting statements as to whether everyone present assumed the cell being worked on was live (or dead). Regardless of the assumptions, the working crew violated an essential maxim of safety: "Determine existing conditions by an inspection or a test before starting work. Consider electric equipment and lines to be energized until you determine otherwise by testing."
The hospital staff furnished a power system one-line diagram, which the service company's senior site engineer and hospital's engineering manager studied together. Apparently, the crew didn't understand the scheme or recognize the existence of the "hidden" swing bus. This would explain why they didn't anticipate the possibility of a back feed.
The fact that the crew barriered only the cell's generator-connected side suggests they considered the load side connection de-energized. One of the witnesses (a technician) testified the crew used a partial barrier only because they saw a need to provide protection in a high-voltage area that was close to the mechanism they were working on. Since the mechanism was located at the front of the panel, the senior engineer expressed it would be safe to work on the device.
The power system (as designed) makes provision for the isolation of high voltage from all exposed buses and conductors within the normal/emergency cells of the transfer switch. Although the power system design differs from the more regular emergency power supply systems, an understanding of the different switching possibilities does permit safe practice under all conditions. On the other hand, failure to understand the power system design should never preclude the use of alternate safety procedures.
The crew totally ignored the need for personnel protective equipment (PPE) and did not observe testing precautions. The measured clear working space to live parts was approximately 28 in. (NEC Sec. 110-34 specifies 4 ft; the service company specifies 8 ft.) The crew also failed to attach safety grounding cables or tag the switches.
Accidents don't just happen! Investigators of this tragic event considered these points:
- Why was there an unsafe condition?
- Did a maintenance system fail?
- Did the task require unusual skill or attention to detail?
- How can we assure such performance?
- How can we make the operation "human-error proof" procedurally?
We define safety engineering as the art and science of dealing with the prevention of personal injury and property loss. As such, it uses these two principles:
Eliminate the hazard. This implies that all electrical apparatus be de-energized before work on the apparatus proceeds. Disconnect feeder circuits, including back feed sources you can't switch open, and secure them from unauthorized use. After you make all disconnects, test all feeder and back feed sources with a line detector or appropriate meter to verify they are de-energized. Before beginning work, ground all de-energized sources. These sources include control circuit components, potential transformers, capacitors, synchronizing switches, and control transformers.
Guard, enclose, or control against the hazard. This is if you can't eliminate the hazard. For those unusual circumstances where you must work near energized apparatus, you must take responsibility to ensure these things happen. Whether you're the supervisor, the person doing the work, or a supporting crew member, lives depend on your thoroughness. You must consider hand tools, flashlights, keys, scales, metal rules, metal ladders, and anything else that could provide a path for electricity. Do not wear metallic jewelry, watchbands, or rings. If you use insulated tools, is the insulation intact and oil-free? Ensure you have an attendant, who remains in sight and within earshot to render assistance in an emergency. The attendant should be prepared with a radio and whatever PPE necessary to extract the worker safely. That PPE may include a wrist-strap and harness device on the worker or insulating clothing on the attendant.
To help overcome safety mistakes, consider this maxim: "You don't warn if you can guard. You don't guard if you can eliminate the exposure." These few words, as simple as they are, summarize the very foundation of established safety manuals. Safety is everybody's business. Safety is your business!