Subsection (b) of Section 2940.11 requires employers, in certain situations, to select protective clothing and other protective equipment with an arc rating that is greater than or equal to the incident heat energy estimated under Section 2940.11(a)(2). For example, an employer that provides employees with arc-flash protective equipment rated at 8 cal/cm 2 can use the table to determine if any system area exceeds 8 cal/cm 2 by checking the clearing time for the highest fault current for each voltage range and ensuring that the clearing times do not exceed the values specified in the 8-cal/cm 2 column in the table. Alternatively, an employer could select a base incident-energy value and ensure that the clearing times for each voltage range and fault current listed in the table do not exceed the corresponding clearing time specified in the table. Thus, the available heat energy for this part of the system will be 4 cal/cm 2 or less (from the column heading), and the employer could select protection with a 5-cal/cm 2 rating to meet Section 2940.11(b)(2). For rubber glove work, this system falls in the 4.0-to-15.0-kilovolt range the next-higher fault current is 10 kA (the second row in that voltage range) and the clearing time is under 18 cycles (the first column to the right of the fault current column). The power line can supply a maximum fault current of 8 kiloamperes with a maximum clearing time of 10 cycles. For example, an employer might have a 12,470-volt power line supplying a system area. 3 To use the table, an employer would use the voltage, maximum fault current, and maximum clearing time for a system area and, using the appropriate voltage range and fault-current and clearing time values corresponding to the next higher values listed in the table, select the appropriate heat energy (4, 5, 8, or 12 cal/cm 2) from the table. ![]() Table 6 also assumes that the arc length equals the sparkover distance for the maximum transient overvoltage of each voltage range. The table assumes that the employee will be 380 millimeters (15 inches) from the electric arc, which is a reasonable estimate for rubber insulating glove work. 2 Table 6 presents estimates of available energy for employees using rubber insulating gloves to perform work on overhead systems operating at 4 to 46 kilovolts. ![]() Table 6 and Table 7 provide incident heat energy levels for open air, phase-to-ground electric-arc exposures typical for overhead systems. Incident heat energy for single-phase-to-ground exposures. The employer will need to estimate other parameters, such as the length of the arc and the distance between the arc and the employee, because such parameters vary widely. The employer can precisely determine some of these parameters, such as the fault current and the clearing time, for a given system. Each method requires the input of various parameters, such as fault current, the expected length of the electric arc, the distance from the arc to the employee, and the clearing time for the fault (that is, the time the circuit protective devices take to open the circuit and clear the fault). ![]() The Division of Occupational Safety and Health does not endorse any of these specific methods. ![]() Table 2 lists various methods of calculating values of available heat energy from an electric circuit. Subsection (a)(2) of Section 2940.11 provides that, for each employee exposed to an electric-arc hazard, the employer shall make a reasonable estimate of the heat energy to which the employee would be exposed if an arc occurs.
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