The Secret to Understanding Arc Flash Calculations

Informative Annex H, Guidance on Selection of Protective Clothing and Other Personal Protective Equipment

Table H.3(b) provides guidance on selection of arc-rated clothing and other personal protective equipment (PPE) for use when incident exposure is determined by a hazard analysis. By calculating the incident energy, you can determine the HRC. Then use table H.3(b) to determine PPE.

Table H.4(a) for low-voltage systems introduces maximum 3-phase bolted fault-current limits at various system voltages and fault clearing times of circuit breakers for recommended use of 8 cal/cm² and 40 cal/cm² PPE in an “arc-in-a-box” situation.

Table H.4(b) for high-voltage systems introduces maximum 3-phase bolted fault-current limits at various system voltages and fault clearing times of circuit breakers for recommended use of 8 cal/cm2 and 40 cal/cm² PPE in an “arc-in-a-box” situation.

Tables H.4(a) and H.4(b) can really help during the design and review stages of an arc flash study. These two tables can be used in several ways, as follows:

• Knowing the maximum 3-phase fault current, a maximum value of the upstream protection fault clearing time can be established in order to achieve an HRC value of 2 or 4.
• Knowing the 3-phase fault current at a point in the system and upstream circuit breaker clearing time, you can use the Tables to check the calculations if you are reviewing a study without actually performing the calculations yourself.
• You can establish a maximum for the 3-phase short circuit current in a new system, and use it as a criterion for the design.

In conclusion, arc flash regulations have brought a great deal of challenge to the industry, but also present a great opportunity to improve the electrical safety and the quality of a power distribution system design. Electrical designers and project reviewers alike should look to arc flash calculations as a tool for continued improvement.

Elgazzar is a senior electrical power engineer for the federal government and is currently registered as a professional engineer in the states of Virginia and South Carolina. He can be reached at mgelgazzar@gmail.com.