Seven decades ago, Edward, an American aerospace engineer, was working on a special Air Force project involving a rocket sled. The sled was hooked up to a series of sensors to measure the G-forces it experienced when hurtling down a set of rails. However, during the sled’s run, the sensors were not functioning properly and an investigation revealed the sensors had been wired backward. Irritated, Edward made his thoughts known about the technician allegedly responsible, saying something along the lines of, ”If there are two ways to do something, and one of those ways will result in disaster, he’ll do it that way.”
From that moment, Edward became legendary, but not for his engineering work. Today, we all know him by his last name. The phrase Capt. Edward Murphy used was condensed and reworked, eventually becoming Murphy’s Law, which states, “Anything that can go wrong will go wrong.”
Let’s face it, things can go wrong.
That is certainly the case with electrical power systems. Whether it is from an incorrect installation, lack of maintenance, misapplied equipment or mistakes by electrical workers, the list always goes on. When a failure occurs, the results may just be a minor inconvenience. However, there is always a of chance serious injury or even death.
Although Murphy’s Law is not directly referenced in NFPA 70E, it seems to permeate throughout the standard that focuses on preventing what can go wrong and protecting the worker if it does.
One example is establishing and verifying an electrically safe work condition. This multistep process includes verifying that the circuit is de-energized with an adequately rated test instrument. But what if the instrument fails during the test? That is why the “live-dead-live” method is also used and requires checking the instrument on a known source before and after use.
Another example is the requirement to wear arc-rated clothing and protective equipment when there is a risk of an arc flash. Although the occurrence of an arc flash is not common, if it does happen the results can be catastrophic. Wearing this protective equipment can help contractors be prepared if anything uncommon does happen.
Since you never know when things will go wrong, NFPA 70E’s approach regarding electrical hazards is to perform a risk assessment defined as: An overall process that identifies hazards, estimates the likelihood of occurrence of injury or damage to health, estimates the potential severity of injury or damage to health, and determines if protective measures are required.
Both a shock risk assessment and arc flash risk assessment must be performed before any person is exposed to the electrical hazards. One more important step is to estimate the likelihood of occurrence. How do you know if you are likely to encounter an electric shock or an arc flash?
For the Shock Risk Assessment, the Restricted Approach Boundary is used to define an approach limit from an exposed energized electrical conductor or circuit part where there is an increased likelihood of electric shock. For the arc flash hazard, the likelihood of occurrence can be evaluated using the aptly named Table 130.5(C) Estimate of the Likelihood of Occurrence of an Arc Flash Incident for AC and DC Systems. This table provides a list of tasks and equipment conditions that are used to evaluate the likelihood of the occurrence of an arc flash.
My mentor and good friend loves to quote Dirty Harry when the subject of risk assessment comes up. In the 1971 movie, rule-breaking cop Harry Callahan is looking down the barrel of his gun at a suspect. Neither man is sure if Harry has any bullets left, and Harry lays out the suspect’s life-or-death decision: “You’ve got to ask yourself one question: ‘Do I feel lucky?’ Well, do ya, punk?”
What do Edward Murphy and Dirty Harry have to do with electrical safety? Quite a bit. When judging the likelihood of occurrence of an arc flash or electric shock hazard, keep in mind Murphy’s Law. Then consider Dirty Harry’s warning and ask yourself the question: “Do I feel lucky?” Well, do ya?