An arc flash occurs when short circuit current jumps across an air gap between energized conductors. The event is normally the result of initial (and inadvertent) contact between energized conductors which creates the short circuit. During the event, the conductors or a conducting object may either melt back or be blown back producing an air gap. When current flows across the gap, it ionizes the air resulting in a conducting plasma and thermal cloud. IEEE 1584 – IEEE Guide for Performing Arc-Flash Hazard Calculations refers to this as the Arcing Fault Current which is defined as:
A fault current flowing through an electrical arc plasma. To calculate the arcing fault current, IEEE 1584 equations require many input variables such as: bolted three phase short circuit current, bus gap distance, electrode configuration and voltage.
The gap distance is the physical distance between conductors that the current flows across during an arc flash. Measuring the actual gap distance may be considered impractical since it would require evaluating each piece of equipment which may be energized requiring it to be placed in an electrically safe work condition. In addition, there may be several different gap distances in the same piece of equipment depending on where the arc flash occurs.
IEEE 1584 provides a range of valid gap distances from 0.25 inches to 3 inches for voltages from 208 V to 600 V. The standard also lists typical gap distances for equipment classes such as 1 inch (25 mm) For low voltage motor control centers and panelboards and 1.25 inches (32 mm) for low voltage switchgear. These values are commonly used for arc flash studies in the absence of measurements. The example problem used for the calculations was defined in Part 1 of this series and is based on a bolted fault current of 28,500 Amps at 480 Volts.
Table 1 lists the calculated arcing fault current using different gap distances. It illustrates how the arcing current increases as the gap distance decreases and vice-versa.
The magnitude of arcing fault current is also voltage dependent. As the voltage decreases the arcing current also decreases as shown in Table 2. The difference between the bolted and arcing fault current is greater at lower voltages. This is because the equivalent impedance of the bolted fault current is smaller at lower voltage so the arc impedance has a more significant impact.
The introduction of different electrode configurations was one of the most significant additions to the second edition of IEEE 1584. Five different electrode configurations are included with three configurations used for equipment inside an enclosure. These include: VCB – vertical conductors inside a metal box, VCBB – vertical conductors terminated in an insulating barrier inside a metal box and HCB – horizontal conductors inside a metal box. See the February 2019 article for details regarding electrode configurations.
IEEE 1584 provides some guidance regarding the selection of electrode configurations however, it is up to the qualified person performing the study to make the final determination. Table 3 lists the results of arcing current calculations for our example using different electrode configurations. It is possible to have several different electrode configurations in a single piece of equipment depending on where the arc flash occurs. Also, just because equipment has a horizontal, does not automatically mean the electrode configuration is HCB. HCB is for horizontal bus/electrodes aimed at the worker. Similarly, if the equipment bus is in a vertical orientation, that does not necessarily translate into VCB or VCBB. What if there are stabs pointed out towards the worker from the vertical bus. That may result in an HCB configuration.
The electrode configuration affects the arc trajectory and the arc impedance. VCBB results in the lowest arc impedance due to the concentration of plasma at the insulating barrier and yields a greater arcing fault current. Each electrode configuration can result in a different arcing current, incident energy and arc flash boundary.
The calculations for our panelboard example defined in Part 1 would likely use both VCB and VCBB. Why? When conductors terminate into a main protective device such as the top of a panel, it may behave like a VCBB configuration at that location. However, if an arc flash occurs on a branch device, this may act more like VCB. Why analyze both? Because either one could result in the worst-case incident energy. If the arc duration is the same for each case, VCBB will result in the greater incident energy. However, VCB can result in a lower arcing current which could cause an upstream protective device to take longer to operate. The longer arc duration could result in a greater incident energy for VCB.
This article was originally published in the November 2021 Edition of Electrical Contractor Magazine.