Machine & Operator Safety
Arc flash compliance: What you need to know
June 28, 2011 by Steve Craig
As a controls integrator, I have had the opportunity to work in different facilities across the globe. The majority of these facilities have one thing in common – the concept of arc flash is largely an unknown. This is no surprise, as arc flash standards and awareness have only recently become publicized and enforced.
I was in a plant a couple of years ago to fix some wiring in the 24-volt area of an enclosure. I informed the supervisor of the work to be done, de-energized the panel, and placed my lock on the disconnect. The supervisor then handed me a pair of double-insulated gloves and a face shield. I’m all for safety, but at some point work needs to get done. Have you ever tried to wire a panel wearing a pair of double-insulated gloves and a face shield? I asked him why he felt the need for the extra precautions, and his reply was, “We don’t know what else to do yet.”
The point is that even though the company was just learning about arc flash, they still took action to err on the side of caution rather than do nothing at all. Of course, it was not practical to continue this way, and they came up with an appropriate electrical work procedure shortly thereafter.
This article is not intended to be an all-encompassing arc flash source, nor should it be solely used to develop standards within your company. The intention is to get you thinking about arc flash safety in your workplace.
Arc flash occurs when electrical energy passes through air from a high voltage down to a low voltage (usually ground) conductor, causing a possible explosion with extreme heat and light (much the same as lightning). Arc flash is usually caused by deteriorating insulation in aging equipment, poor installations, dust and debris in the electrical components, and improper or accidental connection of tools. It is one of the leading causes of fatalities among electricians and needs to be taken seriously.
An arc flash can happen in milliseconds, resulting in temperatures that can cause immediate, irreparable damage to the skin. The bright flash can be blinding, followed by a potentially deafening sound. Flying shrapnel can damage equipment and injure others nearby. Negligent companies that don’t require workers to wear the appropriate personal protective equipment (PPE) could be fined in the event of an injury.
The prevention of injury from arc flash starts at electrical design. There are many considerations that must be taken to allow safe maintenance access to critical components while minimizing downtime in the event of a failure. NFPA 70E – the standard for electrical safety in the workplace – deems anything under 50 volts electrically non-hazardous and, therefore, not requiring any special precautions pertaining to arc flash. This is one advantage to using 24-volt control rather than 120-volt control in your facility. Panel design should also take advantage of this by isolating the low voltage circuits from higher ones.
Separate disconnect enclosures or power distribution panels are a good means of isolation. Some panel manufacturers have also integrated disconnect isolation bays that keep the line side of the disconnect enclosed. Also, keeping frequently accessed items like programming ports and ground fault circuit interrupter (GFCI) receptacles outside of the panels, and having remotely resettable items like electronic overloads, will reduce exposure while decreasing downtime.
Current-limiting fuses or short circuit-rated breakers, if used correctly, can also aid in reducing the protective circuit reactive time and, therefore, lower incidental energy; however, detailed analysis of components is required to ensure that they are operating in their optimal fault current range.
In addition to proper panel design, what else can you do to minimize an arc flash incident? Preventative maintenance within panels – such as cycling breakers that haven’t been operated in a long time, checking that connection points are tight and clean, checking for damage to insulation and clearing out any loose debris or dirt from electrical enclosures – can reduce the chance of an arc flash. Panel labels should be fixed as per NEC section 110.16 to make workers aware that an arc flash hazard exists. Panel labels are also a good place to state the PPE required before working within a live panel; however, service to a control panel is usually done with the power off.
Many companies are starting to specify that the load side of disconnects have voltage indicators mounted outside of the panel, which will tell you if the panel is dead before opening the door. This still requires a secondary (but safer) meter check before service begins.
Finally, keeping maintenance personnel knowledgeable about hazards is always the key to safety.
How do you determine the proper PPE? The first step is to review your electrical panels and components to calculate the risk and required level of worker protection. This analysis will result in an incident exposure level measured in calories/cm2 – calculated by voltage level, available fault current levels, protective device reaction times, working distances and a few other factors. A detailed summary of this calculation can be found with the IEEE Standard 1584-2002, in NFPA 70E or by using arc flash calculation software available through several different vendors. With the incident exposure level, the minimum level of PPE can be determined for various maintenance tasks as per NFPA 70E.
An easier but less accurate way of complying with PPE requirements is the table method from NFPA 70E (Table 3-3.9.1). The table is divided into panel types, voltage levels and maintenance tasks. For example, if you need to work on a live 120 VAC+ component, including voltage testing in a 600-volt motor control centre with a maximum available short circuit current of 65 KVA, the table will tell you that this is a hazard risk category of 2* (an extended version of category 2). Looking up this risk category shows that the worker should be wearing a fire retardant long-sleeved shirt and pants, or fire retardant coveralls. Required PPE would be a hard hat, hearing protection, arc-rated face shield and a stocking hood with an eight cal/cm2 rating or a multi-layer switching hood. Gloves and tools rated for the maximum line-to-line voltage are the final minimum requirement.
In this example, the PPE becomes very cumbersome; so if your process permits, shutting the power source down before servicing might be a better option.
Assessing your potential arc faults can be a daunting task, but it is imperative to keep personnel safe. Don’t wait until someone gets hurt. Contact your controls provider to see what arc fault services they may be able to provide.
Steve Craig has worked for controls integrator D&D Automation as a controls designer for 10 years. He can be reached at firstname.lastname@example.org.