What is an Arc Flash Study?

In this article we look at why an Arc Flash Study is useful, how it is carried out and what the outputs are

How is the risk posed by arc flash determined?

The United Kingdom's Electricity at Work Regulations 1989 and supporting guidance calls for suitable and sufficient risk assessment to be completed when working on or near live electrical equipment. This risk assessment should take into account all of the recognised hazards associated with electricity, including arc flash. The arc flash risk can be assessed by considering two factors:

  1. the potential severity of an incident, and
  2. the likelihood of that incident occurring.

Completing an Arc Flash Study is the best option for quantitatively assessing the potential severity of an Arc Flash incident.

We'll cover determining likelihood in a future article as this is usually a separate piece of work to understand how electrical equipment is used, how often people interact with it, and its condition, amongst other considerations.

What are the potential consequences of an arc flash event?

An Arc Flash incident that occurs on live electrical equipment is a highly energetic event that has the potential to produce numerous effects which can cause serious injury or death as well as equipment damage. These include:

  • A blast wave
  • Rapid vaporisation and expansion of copper
  • Release of debris, shrapnel and superheated ionised gases
  • Intense light (particularly UV) and a deafening sound
  • Heat energy

An Arc Flash Study focuses on the heat energy released during an arc flash incident. A recognised methodology is used to calculate the thermal energy that someone working on the equipment will be subjected to during the incident – this is called the Incident Energy (IE). The IE is determined at a specified distance away from the arc itself - this is called the Working Distance. This is the distance from the arc to the worker's head and torso, which are the areas of the body most at risk from severe burns. The IE, typically expressed in calories per square centimeter (cal/cm2) is a quantitative evaluation of the severity of an Arc Flash and can be used in a formal risk assessment process.

It is important to recognise that the Incident Energy will be greater at a distance less than the Working Distance.

The Arc Flash Boundary (AFB) is also calculated as part of an Arc Flash Study. The AFB is the distance away from the arc at which the heat energy experienced will be 1.2 cal/cm2. This value of heat energy will cause 2nd degree burns to bare skin when it is exposed for a duration of 1 second. Once inside the AFB, suitably-rated Arc Flash PPE should be worn to protect the worker for situations and activities where it has been determined that there is a likelihood of an arc flash occuring.

The Working Distance and Arc Flash Boundary are shown in the following illustration:

Illustration of the Working Distance and Arc Flash Boundary when working on live electrical equipment. An electrician in blue is shown working on a panel where an arc flash could occur. The energy of the arc flash is shown as a gradient from red to yellow coming from the working location. The Working Distance is shown as the distance to the electrician's torso. The Arc Flash Boundary is shown some distance further back. The energy from the arc flash extends beyond the Arc Flash Boundary in this example. An Arc Flash Study determines the energy and the boundary distance, using the working distance as an input.

Although more generally used as part of the arc flash risk assessment process, the calculated Incident Energy can also be used to determine PPE requirements. At a basic level Arc Flash PPE should be selected with an Arc Rating greater than the calculated Incident Energy the worker could be exposed to. However, PPE should always be considered as a last resort and it is important to explore other risk mitigation measures following completion of an Arc Flash Study. This is to ensure that the best safegaurds against arc flash have been employed and the Incident Energy lowered wherever practical. This allows for less restrictive and expensive PPE to be selected.

What are the recognised Incident Energy calculation methodologies?

Several methodologies for calculating arc flash incident energy have been developed over the years, including:

Why we use the IEEE 1584 Method

For industrial AC systems (typically operating below 15kV) the IEEE 1584 methodology, last revised in 2018, is recognised as being the most advanced and robust. It is based on thousands of laboratory arc flash tests followed by statistical modelling of the experimental data to produce best fit curves which allow for calculation of the arc flash incident energy for a wide range of equipment types, voltages, fault currents and other physical variables.

The IEEE 1584-2018 methodology has also been implemented in power system analysis software packages such as SKM Power*Tools for Windows, ETAP and DIgSILENT PowerFactory.

Safe Arc Solutions are experienced in using various power systems software packages to complete AF studies.

How is an Arc Flash Study completed?

An Arc Flash Study is completed by an experienced electrical engineer who is familiar with electrical distribution networks, power systems analysis software and the IEEE 1584 methodology.

Although simple studies can be completed by hand or using a spreadsheet, completing an Arc Flash Study using power system analysis software allows more complex networks and multiple operating scenarios to be efficiently analysed. Having an accurate model of the distribution network also allows for other power system studies to be run in the future and for arc flash mitigation measures (such as adjusting protection settings or retrofitting relays) to be explored and their benefits determined.

For an Arc Flash Study we generally complete the following key steps:

  1. Identification of Study Scope and Scenarios - a key first step is to define the scope of the study and the scenarios to be considered. The scope may encompass your whole network, or may focus in on a new part of the facility. The scenarios should be selected to represent the most common ways you operate the network, as this has an influence on arc flash results. This may be a normal operation and an emergency operation with reduced load, but could also include network configurations used during maintenance.
  2. Data Collection - gathering accurate data for all of the key electrical equipment in your network is crucial. At a minimum this means gathering data related to generators, transformers, switchgear, protection, cables and loads and your site's grid connection. We'll also need to understand how the network is configured and operated. Unless you are in possession of a power system model, or have a good document library, this step is often the most time consuming part of a study and may require a site visit or a series of information requests.
  3. Power System Network Modelling - the data gathered is used to construct an accurate model of the electrical network using power system analysis software. This involves building up the network of equipment and inputting the data that will used in the studies.
  4. Calculation of Short Circuit Fault Currents - the system model is then used to determine the short circuit currents available throughout the network in any number of operating scenarios. Fault current is a key input to the arc flash incident energy calculation. Typically three-phase short circuits are considered as the worst case for an arc flash study, but the software can also provide other fault types to ensure your equipment is adequately rated.
  5. Calculation of Arc Flash Incident Energy (in cal/cm2) and the Arc Flash Boundary - the short circuit currents are used along with other data to determine the IE (at the Working Distance) and AFB at each switchboard within the network. The software utilises the IEEE 1584-2018 methodology to determine these values and will take into account the variables involved such as enclosure size and electrode configuration.
  6. Compilation of an Arc Flash Study Report - once the results have been generated a technical report is compiled detailing the data, scenarios and the results, along with a discussion of the findings and initial recommendations. This document will detail the IE and AFB for each switchboard within the scope of the study, and for each scenario. This gives you a detailed understanding of the arc flash severity throughout the network.

What is the output of an Arc Flash Study?

Safe Arc Solutions will compile a detailed technical report that will explain exactly what was studied, how the methodology was applied, and the Incident Energy and Arc Flash Boundary results. Our reports typically include the following main sections:

Executive Summary A short summary of the report detailing the key results and recommendations which is suitable for sharing with non-electrical personnel
Background on Arc Flash An overview of what arc flash is, how it can happen and what the Arc Flash Study provides. We also provide an introduction to Arc Flash PPE covering the relevant standards
Methodology An overview of how we complete Short Circuit studies and the subsequent Arc Flash calculations, including the variables considered by the IEEE 1584 methodology. This section also presents the Study Scenarios that will have been agreed with you
Data and Assumptions A summary of the key information that has been used to build up the model of your power system network. Any assumptions (typically due to missing data) will be detailed here
Results The Short Circuit and Arc Flash results will be presented for each scenario. Arc Flash results will include the key outputs of the calculation including the arcing current, the time to trip, the resulting Incident Energy and the Arc Flash Boundary. To make it easier to digest the results we also provide summary tables focusing in on just the Incident Energy and Arc Flash Boundaries
Discussion The Short Circuit and Arc Flash results will be discussed with any results of particular note highlighted. Often this might be where, for example, overcurrent settings are causing a long trip time which is resulting in a very high Incident Energy. In this section we will also discuss Arc Flash mitigation measures relevant to your network and the initial results
Recommendations Based on the results initial recommendations will be made. This section will reflect the results and discussion sections and will provide clear and concise next steps to be taken

What happens once an Arc Flash Study is complete?

Once the severity of the arc flash hazard is understood, a complete risk assessment can be undertaken by also factoring in likelihood information. The results of the arc flash risk assessment will then be used to determine what risk mitigation measures are required in order to achieve a safer electrical environment. This can then be formalised in an overall arc flash management strategy which will encompass risk assessment, mitigations, awareness, training and competence.

How can Safe Arc Solutions help you?

We can gather data, build models and run the necessary studies to determine the potential severity of an arc flash across the electrical equipment at your site.

Contact us to arrange a discussion about arc flash at your site