There is a plethora of papers written on the “thou shalt’s” for performing an arc flash study, including references to OSHA, NFPA, IEEE, et al. However, this will be on a different topic that should be talked about much more than it is. The purpose of this article is to help maintenance and safety folks understand the reams of reports and data produced by an arc flash study.
An arc flash study is NOT a single study but rather a series of four studies. These studies are all required to get the predicted incident energy results. Listed below are the studies required in the order which they are completed.
- Short Circuit Study
- Equipment Evaluations
- Coordination Study
- Arc Flash Study
A short circuit study calculates the maximum current flowing through a given point in an electrical distribution system. It does this by calculating the maximum available symmetrical short circuit current (assuming zero impedance connection between conductors) and the X/R ratio at that point in the system. The X/R ratio is used to determine the multiplier (1.414 to 2.828) of symmetrical current used to calculate the peak current.
This information is critical when adding components to the system. All components in a distribution system must be capable of withstanding the thermal and magnetic stresses available at the point of connection. Additionally, overcurrent protective devices must be capable of safely interrupting the available current at the point of connection. In the United States, interrupting ratings are published for a specific maximum voltage and a specific X/R ratio. If the X/R ratio at the point of connection is greater than the published test ratio, the published interrupting rating will be reduced.
Therefore, when purchasing panels, disconnects, fuses, circuit breakers, contactors, etc., you must tell the vendor the available short circuit symmetrical current and the X/R ratio in order to ensure the purchased equipment can be safely integrated into the system. You paid for the data, so use it and prevent future problems.
Equipment Evaluation
The equipment evaluation study compares the values produced by the short circuit study against the manufacturer’s published data for existing equipment. Busbars in cabinets that fail may experience a catastrophic failure in the event of a short circuit. Overcurrent devices that fail may not be able to safely interrupt the current, causing additional damage to themselves and of downstream equipment.
Having this knowledge allows you time to investigate, plan, and execute a solution to bring the equipment that failed the evaluation into compliance before you experience a catastrophic failure. You paid for the data, so use it and prevent future problems.
A coordination study compares the opening and clearing times, at various levels of short circuit current, of overcurrent protective devices in series to determine which will interrupt the current. In a perfectly coordinated system, the overcurrent protective device closest (furthest downstream) to the short circuit will be the only device that opens, limiting the blackout to the smallest amount of equipment or area impacted.
Although perfect coordination is rarely achieved in existing facilities, any improvement will increase the reliability of the system and reduce the blackout impact during a short circuit event. It may be the solution is as simple as tweaking the settings on some of the circuit breakers or changing a fuse size. You paid for the data, so use it and prevent future problems.
An arc flash study has the sole purpose of identifying the level of personal protective equipment required to reduce the thermal trauma to a worker’s head and torso to a second-degree burn. To accomplish this, you need three pieces of information: the amount of current flowing through the arcing short circuit, the length of time the current flows through the arc, and the distance the absorbing entity is from the arc. The current flowing through the arc is calculated based on the current from the short circuit study and the arc impedance. The length of time the current flows is based on the clearing time of the first overcurrent device upstream of the arc.
If there is mis-coordination, the clearing time of the fastest device going upstream one or two levels is used. If the appropriate overcurrent protective device failed the equipment evaluation, the clearing time of the next device upstream is used. The distance is published in IEEE 1584 IEEE Guide for Performing Arc-Flash Hazard Calculations and varies depending on the type of equipment.
From the above information, you can readily see that there are so many variables that the probability of the same energy existing at different points in the system is extremely low. To reduce the number of PPE sets required, a review of the incident energy report is necessary to identify what level is required to cover to the most situations. You paid for the data, so use it and prevent future problems.
Last Thought...
Most arc flash studies include the production of a set of single line drawings exported from the software model used to calculate all the above. These drawings can be used to assist in lockout tagout, troubleshooting, and designing modifications or additions to the system. As with everything else, these drawings should be updated whenever changes are made to the system. You paid for the data, so use it and prevent future problems.
