Why Are My Arc Flash Values So High?!

Pieter Pijnenburg

2023 / 06 / 25

10778

Ever wondered why your arc flash values are higher than previously calculated and wonder why that has occurred? Then this guide is for you! This article is intended to inform about the arc flash value changes, whose calculation method and accuracy has changed significantly since the inception of the IEEE Guide for Performing Arc-Flash Hazard Calculations which is known as Institute of Electrical and Electronics Engineers standard 1584 or IEEE 1584 for short. The idea behind this article is to explain in layman terms:

Arc Flash Overview
Initial Arc Flash Analysis
IEEE 1584 Overview of Changes
1584-2018 Effect on Arc Flash levels

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Arc Flash Overview

Arc Flash is an ever-evolving industry that has not fully reached its maturity in contrast to other electrical principles. It has faced numerous challenges relating to accurately modelling incident arc flash explosions but has come a long way from its original inception. However, the primary driver behind the arc flash calculation is to determines to determine:

Incident energy is used to calculate the potential hazard of an arc flash which helps determine the type of Personal Protective Equipment (PPE) a person should wear when performing work near the piece of equipment. The arc flash boundary determines the circular boundary around a piece of equipment where a person can still receive third-degree burns. These critical pieces of information, along with shock hazard, are commonly printed on warning labels to clearly identify dangers with target equipment which allow any operator/employee to adequately dress in appropriate PPE.

Initial Arc Flash Analysis

Initial Arc Flash Analysis was initially quantified by Ralph Lee through the Lee method. Up to that point, it was recognized that electrical arcs and shocks were hazardous, but no attempt was made at modelling the arc. The Lee method uses a distance energy relationship to try and characterize the arc flash hazard. The Lee method was used for every voltage level until the first version of IEEE 1584 was published.

IEEE 1584 Overview of Changes

IEEE 1584 was created by the IEEE Industry Applications Society and has seen some changes in recent years. The sections below cover the inception of IEEE 1584 all the way to the present version of the standard.

IEEE 1584-2002

IEEE 1584 is used (together with the Lee method for systems above 15000 Volts) as the guiding principle for calculating and determining arc flash hazards. It has seen significant changes in its most recent version published in 2018 compared to that of the previous version. The initial iteration of IEEE 1584-2002 calculated the arc flash incident energy in by utilizing arcing current, conductor gap and a constant that changes value based on open or boxed configuration. It enhanced the understanding of lower-level voltage (<15kV) arcs due to its testing of several fault current levels at or below the threshold mark.

IEEE 1584-2018

The update related to the current iteration of IEEE 1584 published in 2018 has drastically changed the calculation methodology. Additional extensive empirical testing over many different voltage and configurations which totaled 1860 tests. The dramatic increase in testing data allows for more data available to validate and make changes to the original formula. The current iteration expands upon the arc flash calculation method by considering more critical components such as:

Conductor configurations (Table 9 IEEE 1584-2018)
Enclosure sizes/shapes
Using Arcing current for multiple voltage levels up to 15kV

IEEE 1584-2018 Effect on Arc Flash Levels

Due to the changes brought on by the expanded testing in IEEE 1584-2018, more accurate arc flash hazards can be calculated. Through effective utilization of the newer tests conducted, in addition to, formulaic changes based on more varying behavioral analysis, the updated version of the IEEE1 1584 results in certain hazards being higher and others being lower.

Conductor configurations can have big impacts as they impact arc flash explosion direction and magnitude. They can be broken down into the following configurations:

Vertical electrodes inside a metal “box” enclosure (VCB)
Vertical electrodes terminated in an insulating “barrier”, inside a metal “box” (VCBB)
horizontal electrodes inside a metal “box” enclosure (HCB)
Vertical electrodes in open air (VOA)
Horizontal electrodes in open air (HOA)

Where most HV system configurations related to transmission network systems are treated as VOA, HOA whereas, Motor Control Centers (MCC), electrical panels are typically vertically mounted and enclosed which would make them VCB or VCBB. Components like low voltage switchgear can be considered VCB, VCBB or HCB depending on the setup.

Enclosure sizes have been expanded in IEEE 1584-2018 and resulted in the model physical behavior changing for the established VCB (called in a box in 2002) as shown in the figure below (from IEEE 1584-2018) where arcing current is more pronounced in the lower voltage side then the higher voltage when compared to the 2002 iteration.

IEEE 1584-2018 Effect on Arc Flash Levels

Furthermore, IEEE 1584-2018 considers arcing currents for several voltage levels and then determining the correct current based on a combination of the voltage levels that are near the normal operating voltage (steady state).