Saturday, August 30, 2014

Comparing An Arc Flash Hazard Analysis to An Alternative Approach [NFPA 70E Table 130.7(C)(15)]

My topic on how often you are required to conduct an Arc Flash Hazard Analysis has opened some eyes of facility engineers and those responsible for electrical safety (e.g. Safety Managers).  Many organizations have the resources, either financial or manpower, to conduct an Arc Flash Hazard Analysis.  However, there are some organizations that do not have those resources.

Luckily, the Standard for Electrical Safety in Workplace (NFPA 70E) provides some guidance for those organizations that need an alternative approach to conducting an Arc Flash Hazard Analysis.  NFPA 70E, Article 130.5 requires an Arc Flash Hazard Analysis be conducted and that analysis shall be reviewed and/or updated every five years or whenever a major modification occurs [1].  There is an exception that allows for the use of NFPA 70E Table 130.7(C)(15) and Table 130.7(C)(16) be permitted instead of conducting an Arc Flash Hazard Analysis to determine what protective methods are to be used by Qualified Employees who are working on or near exposed live (energized) circuits [1].

Table 130.7(C)(15) provides a description of various tasks for particular energized equipment and provides corresponding levels of arc flash personal protective equipment (PPE), whether or not insulated gloves (for shock protection) are required, and whether or not insulated tools are required.  Table 130.7(C)(16) provides a description of the levels of arc flash PPE.

The advantage of using Table 130.7(C)(15) and Table 130.7(C)(16) is that it can be easier to determine the PPE requirements for a particular task and location as compared to the arc flash hazard analysis.  When using these tables the notes associated with each section shall be followed.  This requires that the short circuit current and the opening time of the upstream overcurrent protective device be determined.  In many cases, this will require an engineer to conduct the evaluation.

The disadvantage of using the Table 130.7(C)(15) and Table 130.7(C)(16) is that this method can result in a more conservative level of PPE required for working on or near exposed live (energized) circuits than what would be required if an arc flash hazard analysis was conducted.  Additionally, if the short circuit current (SCC) or the opening time of the overcurrent protective device exceeds the values denoted in the notes of the table in NFPA 70E, then the tables cannot be used.  This condition would then require an arc flash hazard analysis.

Probably the best way to show the limitations of Table 130.7(C)(15) and Table 130.7(C)(16) is through an example.

This example will determine the incident energy, the arc flash boundary, the Class of Shock Hazard PPE, and the Category of an Arc Flash Hazard PPE at point FA, the power control cabinet.  The example will compare the data obtained from performing an arc flash hazard analysis to using the tables in NFPA 70E.


The SLD is shown in Figure A1.  A 2 MVA transformer steps down utility power voltage, sourced from a 20 MVA substation located 1 mile away, from 34.5 kV to 480Y/277 three-phase, 4W+G.  The impedance of the transformer is five percent.  The output of the transformer supplies two 480 V circuits through circuit breakers CB1 and CB2 located 10 feet away by two 1000 MCM conductors paralleled per phase (C1).  Circuit breaker CB2 is connected to a power control cabinet by fifty feet of # 4/0 AWG conductor.  The power control cabinet is protected by a 200 A thermal-magnetic circuit breaker (CB3) with shunt trip capability.

Through a short circuit current study, the short circuit current at location FA is 42,300 A and the calculated arcing current is 19,760 A.  The bolted fault power is 35.2 MVA.  The clearing time of circuit breaker CB2 is 0.1 seconds.

The calculation for the Flash Protection Boundary can be accomplished using EQ1.


where DFPB is the flash protection boundary in feet, MVABF is the bolted fault power, voltage and current (applicable between 16,000 A and 50,000 A), and t is the duration of the fault (less than 0.6 seconds).

The calculation for the Incident Energy at 18 inches can be accomplished using EQ2.


where EINC is the incident energy in Calories/cm2, FA is the short circuit current (fault current) in kilo-amperes (applicable between 16,000 and 50,000 A), and t is the duration of the fault in seconds.

Performing the arc flash hazard analysis utilizing the above information, EQ1 and EQ2 yields a Flash Protection Boundary of 3.1 inches, and an Incident Energy of 11.7 calories/cm2.  Based on these calculations, the PPE level required for the Power Control Cabinet is Category 3.

A summary is of the Arc Flash Hazard Analysis is shown in Figure A2:


As noted, an alternative to conducting an arc flash hazard analysis is to use Table 130.7(C)(15(a) from NFPA 70E.  Using NFPA 70E Table 130.7(C)(15)(a), the applicable task is “Panelboards or Other Equipment Rated > 240 V and up to 600 V”.  The particular applicable section is “Work on energized electrical conductors and circuit parts of utilization equipment feed directly by a branch circuit breaker of a panelboard”.


The notes indicate that the maximum short circuit current is 25,000 A, the maximum clearing time is 0.03 seconds (2 cycles) and the minimum working distance is 18 inches.  In this case, Table 130.7(C)(15(a) cannot be used as the short circuit current and the trip time exceeds the limiting parameters.  Trying to use Table 130.7(C)(15)(a) when the short circuit current and/or the trip time of the overcurrent protective device exceeds the amplitudes and/or duration stated in the notes can result in incident energy that exceeds the level of PPE stated.  This can create a hazardous condition that could result in an injury as severe as not wearing any PPE at all.