Published · Vili Power Solutions
Safety first
An arc flash label is a warning, not permission. Work on or near energized equipment is only for qualified people who have been trained, who are wearing the protective equipment the label calls for, and who have a reason the equipment cannot be de-energized. The safest arc flash is the one that cannot happen because the equipment was locked out.
What an arc flash is
An arc flash is an electrical fault that jumps through the air instead of staying in the wiring. It can be started by a dropped tool, a loose part, corrosion, or contamination. The arc releases an intense burst of heat and light in a fraction of a second, along with a pressure wave and molten metal. It can cause severe burns several feet away from the equipment, and it happens most often while someone is working on or near energized parts.
Which equipment needs a label
The electrical code requires a field-applied arc flash warning on equipment outside of homes that is likely to be examined, adjusted, serviced, or maintained while energized. In a plant or shop, that typically means:
- Switchboards and switchgear
- Panelboards
- Motor control centers
- Industrial control panels
- Meter socket enclosures and disconnects that get worked on
The basic code warning just says that an arc flash hazard exists. Most plants should go further and have labels with real numbers on them, which is what OSHA expects employers to provide to anyone working on the equipment.
How to read a detailed label
Under NFPA 70E, the workplace electrical safety standard, a detailed label shows:
- The nominal system voltage.
- The arc flash boundary. The distance from the equipment at which an unprotected person could receive a second-degree burn. Anyone inside it needs arc-rated protective equipment.
- Either the incident energy or the PPE category. Incident energy, in calories per square centimeter at a stated working distance, tells you how much heat an arc would deliver. A PPE category points to a table of required clothing and equipment. A label shows one or the other.
- Often, the shock approach boundaries, the equipment name, and the date of the study.
Where the numbers come from
The numbers come from an arc flash risk assessment, usually performed with a short-circuit and protective-device coordination study. The process looks like this:
- Data collection. An electrician walks the system and records the utility service, transformers, conductor sizes and lengths, and every protective device with its settings. This is usually the biggest part of the job.
- Calculation. An engineer models the system and calculates fault currents, how fast each breaker or fuse clears, and the resulting incident energy at each piece of equipment.
- Labels and recommendations. Labels are printed and installed, and the study often points out simple changes, such as faster breaker settings, that lower the hazard.
Labels go stale
The numbers on a label are only true for the system as it was when the study was done. NFPA 70E calls for the assessment to be reviewed at least every five years, and any time there is a major change. Changes that commonly make labels wrong include:
- A new or upgraded utility transformer, which can raise the available fault current
- A service upgrade or new switchgear
- Replaced breakers or fuses, or changed trip settings
- New feeders, motor control centers, or large motors
A label with the wrong number is worse than no label, because people trust it.
Where we come in
We collect the system data for arc flash studies, work with the engineer who performs the calculations, install the labels, and make the protective device changes the study recommends. We also keep track of what changes as we work on a plant, so the next review is straightforward. It fits naturally with our motor control center and commercial electrical work. Our MCC warning signs article covers the maintenance side. Contact us or call (970) 685-2068.
