Is It Off? Verifying absence of voltage with large incident energy

Shutterstock / AnaTolir / Wacomka
Shutterstock / AnaTolir / Wacomka

The 2021 Edition of NFPA 70E, Standard for Electrical Safety in the Workplace, introduces a new informational note to address the age-old dilemma: How do you verify absence of voltage when the calculated incident energy is greater than commercially available personal protective equipment (PPE) ratings?

With few exceptions, NFPA 70E requires working only on systems placed in an electrically safe work condition. However, as part of the process, the equipment is still considered energized, which requires adequate shock and arc flash protection until the absence of voltage has been verified.

What? The hazard may still exist until I get up close with a test instrument to verify the hazard does not exist? How can the hazard still exist if the disconnect is open? Although rare, I have seen this a few times where the equipment appeared isolated from the supply, yet it was still energized. One case was a simple switch failure. The handle was visibly in the off position, but the switch mechanism inside failed in the closed position. Verifying the absence of voltage indicated the equipment was still energized, and two contractors that were about to make contact lived to tell about it.

Classic problem

It is not unusual to find the calculated incident energy is greater than the arc rating of available PPE. In this case, verifying absence of voltage could potentially expose a person to this extremely dangerous condition. One location where this regularly occurs is at the main service equipment. Although PPE ratings continue to push the envelope, with some arc ratings reaching well above 100 calories per square centimeter as part of a risk assessment, large incident-energy values should be avoided. In fact, energized work in general should be avoided. But you still need to verify absence of voltage, which is the dilemma.

The most common cause of a large calculated incident energy is a long clearing time for the upstream protective device used to define the arc duration. For service equipment, the upstream device is often the electric utility fuses or relays. Based on my electric utility experience from early in my career, I often jokingly say these devices are intentionally selected to interrupt “next Tuesday.” Why so long? Utility devices are selected to avoid nuisance interruptions to customers. However, that also means the devices are slow to respond to faults, including an arc flash on the customer’s system.

The service isn’t the only location where this can be a problem. It is also common on sites such as transformers’ secondaries and equipment served by devices with long clearing times. Although the industry has risen to the occasion with equipment such as arc-energy-reduction methods, there are still locations where a large incident energy can still exist.

New guidance to the rescue

To provide better guidance for this situation, a new informational note was added to 130.7(C)(1). This note states: “Where the estimated incident energy exposure is greater than the arc rating of commercially available arc-rated PPE, then for the purpose of testing for the absence of voltage, the following examples of risk reduction methods could be used to reduce the likelihood of occurrence of arcing event or the severity of exposure.”

Note the word “reduce” is used. It does not say “eliminate” the likelihood of occurrence or severity.

The four examples

1. Use a noncontact proximity test instrument(s) or measurement of voltage on the secondary side of a low-voltage transformer mounted in the equipment before using a contact-test instrument to test for absence of voltage below 1,000V. These methods provide an alternative for what may be considered a preliminary test before moving closer with a contact test instrument.

2. If equipment design allows, observe visible gaps between the equipment conductors and circuit parts and the electrical source(s) of supply. Some devices can be examined to determine if the blades are open or if the circuit breaker is racked out.

3. Increase the working distance, which is the distance from the potential arc flash to a person’s chest and head. This is used as part of the incident-energy calculation. Increasing the working distance decreases the incident-energy exposure and reduces the hazard.

4. Consider system design options to reduce the incident-energy level. Many design alternatives can reduce the risk. A new standard known as IEEE P1814, Recommended Practice for Electrical System Design Techniques to Improve Electrical Safety, is under development. Once published, it will provide guidance for safer design alternatives.

Since introduced in 1979, each new edition of NFPA 70E continues to improve, all with the goal of increasing electrical safety in the workplace. The 2021 edition is no exception.

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