Arc flash: avoid the shock
Go beyond the NFPA 70E tables to do an analysis.
Whether to perform an arc flash analysis is a consideration for managers who have employees performing electrical work at their plant. But the consideration of the cost of performing a full-blown arc flash analysis is often enough to put this issue on a back burner. This is particularly true since managers may think it’s sufficient to use the Arc Flash PPE selection tables in the NFPA 70E standard.
But should managers keep relying on NFPA 70E tables?
The reality is that the problem of determining the precise hazard employees face, and what it takes to properly protect them from that hazard is not solved by using NFPA 70E tables. In more personal terms, the problem is ensuring that employees are not burned from exposure to an arc flash event and are able to return home safely each day.
Sure, the NFPA standards do allow the use of the tables. But one must keep in mind that the tables are not an exact science. And let’s face it, most employees may not use them properly anyway. It is the managers and employers who must know for certain if employees are protected from arc flash hazard levels at every location of the electrical system.
When industrial or commercial electrical distribution equipment fails, it can potentially result in an explosion. This explosion is the manifestation of incident energy. The only way to know exactly which personal protective equipment (PPE) is needed to protect employees from arc flash hazards is to perform an incident energy analysis.
Electrical system equipment and equipment configuration need to be evaluated to determine how much incident energy-heat generated from the explosion-can be produced during a worst-case failure. Incident energy is measured in calories per centimeter squared (cal/cm2). A calorie is the amount of energy it takes to raise the temperature of one gram of water one degree Celsius.
An arc flash analysis determines the most severe incident energy that can be produced at each location of the distribution system so that there is no doubt what level of arc flash PPE is required to protect workers from being burned during an arc flash event.
Performing an analysis
The first step of an arc flash analysis is data collection. A properly performed analysis requires information about every conductor, disconnecting device, protective device, transformer, capacitor, motor, and piece of electrical equipment associated with the electrical distribution system. Information from the electrical service provider also is needed. After the necessary data is collected, the entire electrical distribution system will be modeled either by hand or in a software program that will then be used to calculate the incident energy at any location where maintenance or operations activities could be performed. Keep in mind that the accuracy and completeness of the information collected and used to model a system will have a direct correlation to the accuracy of the incident energy calculations that will be used to determine the required level of arc flash PPE.
A common mistake companies make is rushing the data collection process and making assumptions just to get stickers on their equipment faster. The problem with this is that the quality of the end product may be impacted negatively because of a desire for a quicker schedule. An accurate and effective arc flash analysis will take time and money, but it could very literally save someone’s life.
There are often overlooked side benefits of having an arc flash analysis performed. The analysis will generate a thorough database of electrical equipment, their configurations, and their protective settings. Often the analysis leads to updating existing drawings to include a complete set of accurate one-line diagrams of the entire distribution system.
There also could be the benefit of a having a knowledgeable person from outside the company give advice on changes to help improve the system’s efficiency or offer ways to avoid some developing electrical failure. Many people identify equipment coordination issues that can lead to larger outages than necessary when a breaker trips. These are just a few examples that hopefully start to paint the picture that benefits can greatly outweigh the upfront capital investment.
One more step needed
The NFPA 70E requires that the arc flash analysis be reviewed at a frequency not to exceed five years. It also requires it to be updated when any changes are made to the system. There is a key principle to understand related to arc flash energy: it is determined by the available fault current at the location of the fault and the amount of time it takes to clear the fault. Assume there is a fault inside a motor control center and there is a breaker immediately upstream that clears the fault. The arc flash analysis makes the assumption that the breaker will open in accordance with the protective setting design and the contacts will separate at a speed for which it is designed. A poorly maintained breaker eventually will begin to respond more slowly and in extreme scenarios may not respond at all.
In these cases, the arc flash energy levels can greatly exceed levels calculated by the analysis. Therefore, an Effective Electrical Equipment Maintenance Program (E3MP) is an essential part of maintaining an accurate arc flash analysis program.
While it may seem like ensuring that electrical workers are protected from arc flash hazards is a cost that has no end, there are many studies available that verify the return on investment for electrical safety is a profitable endeavor. And let’s face it, there is no price you can put on avoiding a phone call to tell someone that his or her loved one has been burned or killed as the result of an arc flash incident.
Tommy Northcott is a senior power engineer with Jacobs.