Let’s Consider Incident Energy: Electrode Configuration in 2018 IEEE 1584, Part 2

0719 Arc Flash Safety Image Credit: Jim Phillips
Image Credit: Jim Phillips

Continuing the topic of major changes from part 1 to the 2018 Edition of IEEE 1584 - IEEE Guide for Performing Arc Flash Hazard Calculations, this article explores the effect new electrode configurations have on the calculated incident energy involving enclosures.

The primary focus of performing incident energy calculations as part of an arc flash study (risk assessment) is to determine the prospective incident energy at a specific working distance for each piece of electrical equipment that is part of the study. The results are provided in terms of calories per square centimeter (cal/cm2) and used to select appropriate arc-rated clothing and personal protective equipment.

The original 2002 Edition of IEEE 1584 was based on arc flash tests with electrodes oriented in a vertical configuration in a metal box/enclosure (now known as VCB) and vertical electrodes in open air (VOA), which is not part of this discussion.

Subsequent research indicated the orientation of the electrodes can affect the magnitude of incident energy. As a result, the 2018 edition provides additional electrode configurations for enclosures including VCBB and HCB. These electrode configurations are illustrated in the figure below.

1. VCB—Vertical conductors/electrodes—inside a metal box— During an arc flash, the arc travels away from the source toward the tips of the electrodes and plasma “spills” out toward the worker (also included in the 2002 edition).

2. VCBB—Vertical conductors/electrodes terminating in an insulating barrier inside a metal box. The plasma hits the insulating barrier and is forced out of the enclosure more aggressively.

3. HCB—Horizontal conductors/electrodes inside a metal box. The arc travels along the horizontal electrodes and plasma is directed out of the box.

The new electrode configurations provide greater flexibility for modeling actual electrical equipment with some application guidance provided in Clause 6.6, Annex C and Annex G.2 of the 2018 edition of IEEE 1584.

Image Credit: Jim Phillips
Image Credit: Jim Phillips

Incident energy vs. electrode configuration—enclosures

To illustrate the effect the electrode configuration can have on the calculated incident energy, an example problem based on low-voltage switchgear is provided in Table 1. Four different electrode configurations were analyzed and the results are listed in Table 2.

In addition to the voltage and bolted fault current being defined for the example, additional factors were also required. An arc duration of six cycles was used ,which defines the total time of the arc flash. It is typically derived from the clearing time of an upstream protective device.

Based on data from IEEE 1584 for low-voltage switchgear, a bus gap of 32 millimeters (mm) was selected as well as a working distance of 24 inches, which defines the distance from a potential arc source to the worker’s face and chest. An enclosure opening of 20 inches by 20 inches was selected. It should be noted that the person performing the study may select other values based on individual site-specific parameters.

Image Credit: Jim Phillips
Image Credit: Jim Phillips

For this one specific example, comparing the VCB configuration from both the 2002 and 2018 editions indicates there is a minimal difference in the calculated incident energy (Cases 1 and 2). If a different electrode configuration is selected, such as VCBB or HCB (Cases 3 and 4), the calculated incident energy will be different (and typically greater) due to the effect the configuration has on the direction and behavior of the arc and resulting plasma.

Please note that this is only a single, very specific example used to illustrate how different electrode configurations can affect the calculated incident energy. It is not intended to provide an overall trend since each case is unique and needs evaluated on an individual basis.

The equations in the 2018 Edition are based on more than 1,800 new arc flash tests and provides greater modeling flexibility. For additional details regarding the equations used in the calculations, a copy of the 2018 Edition of IEEE 1584 should be obtained.

This article is the author’s view and may or may not necessarily reflect the views of any particular standard organization.

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