Online short circuit arc flash calculations example

This example is a continuation of the short circuit analysis described in the example 3. The arc flash study will be conducted using IEEE 1584 procedure from the IEEE 1584 Guide for Performing Arc Flash Hazard Calculations implemented in the online arc flash calculator presented on this web-site. Incident energies and flash protection boundaries at nodes a) to j) of the power distribution system shown below will be determined.

Arc flash analysis requires 3 phase bolted short circuit current value and system line-line voltage at each of the potential fault points. These can be obtained by running online short circuit calculator. System's radial representation in terms of database records programmed in the calculator is shown below. The system "tree" is broken into levels, with each level being more focused than the last.

Resulting 3 phase bolted short circuit MVA and current values are displayed below (see Example 3 for details):

• PSE[ 2000.00 (7.00X/R) +  17.13 (13.05X/R) =  2017.10 (7.03X/R)]
  • TX-1[ 222.33 (17.97X/R) +  18.40 (12.67X/R) =  240.72 (17.41X/R)]
    • C-5[ 229.13 (11.94X/R) +  0.00 (7.00X/R) =  229.13 (11.94X/R)]
      • TX-4[ 7.27 (3.79X/R) +  0.00 (7.00X/R) =  7.27 (3.79X/R)]
        • PNL-1[0.00(0.0X/R)]
    • C-1[ 231.10 (13.50X/R) +  4.13 (5.27X/R) =  235.20 (13.14X/R)]
      • TX-2[ 15.55 (5.93X/R) +  5.49 (5.15X/R) =  21.04 (5.71X/R)]
        • M-1[1.47(14.2X/R)]
        • C-2[ 15.66 (3.17X/R) +  1.48 (4.50X/R) =  17.14 (3.26X/R)]
          • MCC-2[1.48(4.5X/R)]
        • C-3[ 14.95 (3.29X/R) +  2.67 (4.50X/R) =  17.61 (3.43X/R)]
          • MCC-1[2.67(4.5X/R)]
    • C-4[ 222.85 (14.13X/R) +  14.31 (21.45X/R) =  237.16 (14.42X/R)]
      • TX-3[ 60.64 (12.98X/R) +  17.28 (25.10X/R) =  77.91 (14.54X/R)]
        • M-3[5.96(25.0X/R)]
        • M-4[8.94(28.4X/R)]
        • M-2[2.38(17.6X/R)]

Short circuits contributed by upstream (red) and downstream (blue) equipment are listed across each node. For a three phase fault, simply divide total SC MVA value by 1.73 * kV_LL to get short circuit current values in kA:

Also, the equipment configuration, namely gap between conductors, equipment class, grounding type, working distance, and the protective device operating time is needed for arc flash study.

The calculator uses bolted short circuit current values and the equipment configuration to calculate arcing current. The operating / arcing time is determined based on the arcing current, which can be considerably lower than the bolted fault current. The operating time can be obtained from the time-current characteristic of the upstream protective device of the faulted bus and the arcing current value estimated by online arc flash calculator.

For protective devices operating in the steep portion of their time-current curves, a small change in current causes a big change in operating time. Incident energy is linear with time, so arc current variation may have a big effect on incident energy. For circuit breakers, the instantaneous trip must be evaluated at its maximum setting so as to determine the worst case. However, instantaneous trip settings have a tolerance that can be as high as 25%. For inverse-time overcurrent characteristics of protective devices, the arcing time is greater for smaller currents than it is for larger currents. Since the incident energy of arc faults is more sensitive to arcing time than it is to arc currents, it is necessary to obtain a more accurate arcing time. To account for the above, the arcing current must also be calculated at 85% of the original calculated arcing current Ia. IEEE 1584 based online arc flash calculator allows you to consider two scenarios of arc currents. The arc energy is then compared using both values (100% and 85% of I_a) with the higher resulting value of incident energy being used.

If a main breaker or fuse protects the bus and this breaker or fuse is connected to the bus, then the arcing time would be equal to the trip time for this main device. This applies when a worker is working on the bus or on the load side of the main breaker/fuse. However, if energized work is required on the line side of the main breaker/fuse, the tripping effect of the main breaker must be excluded and the upstream trip device characteristic is used to calculate the arcing time.

^[1] - Bolted fault current the upstream protective device is subject to (see UPSTREAM KA column from table 4 above). This value is used to determine arcing current and arc duration the upstream protective device is subject to. Total short circuit current consists of current contributed both by upstream and downstream equipment (see RESULTING KA column from table 4 above).

Table 5 above shows the equipment configuration, arcing current determined by the calculator for the given configuration, system voltage and bolted fault current. Arc flash boundary, incident energy at specified working distance and risk category are determined based on IEEE 1584 procedure described in IEEE 1584 Guide for Performing Arc Flash Hazard Calculations and implemented in the online arc flash calculator.