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Arc flash – risk elements, studies and mitigation

17 September 2012

Legacy equipment presents higher risk of arcing due to system and equipment design, condition and operation, which is putting increasing pressure on industry. 

Emma Harrison, Business Projects Director at GSE Systems, looks at some of the major issues relating to Arc Flash, particularly risk assessment, mitigation and the essential elements of an Arc Flash Study.

Arc flash hazard is present in a wide variety of electrical plant, from transformer stations to 415v switchgear
Arc flash hazard is present in a wide variety of electrical plant, from transformer stations to 415v switchgear

Ageing infrastructure and equipment is one of the biggest challenges facing plant operators across industry.  It is not uncommon to find equipment that was given a 20-year lifespan exceeding that operational limit by one or even two decades.

Effective maintenance schedules, therefore, are paramount to ensure uninterrupted plant operations and, of course, the safety of personnel.   Among the key safety considerations that plant operators, engineers and maintenance professionals have to consider are the hazards related to electrical arcing.

It is also worth noting that, with the diversification of industry, together with the emergence of new energy sectors such as renewables and biomass, there are an increased number of high risk site environments. These include ships, chemical plants, COMAH sites and offshore platforms, where the consequences of a catastrophic event are even more serious due to the associated risks. 

There are, of course, some desired industrial uses of controlled electric arcs, such as welding, plasma cutting and electric arc furnaces, however, undesired, uncontrolled electric arcing may present a serious health and safety risk, and can cause serious damage and injury.

Consequently, the pressures on industry to address the issue of arcing are growing exponentially and require the support of professional, experienced electrical safety specialists who understand the dangers and mitigations of arc flash.

An electric arc is an electrical breakdown of a gas which produces an on-going plasma discharge, resulting from a current through normally non-conductive media such as air. Creating an explosive electrical event, it presents a hazard to people and equipment.

It can occur in electric power transmission and distribution systems and generates temperatures up to 19,500 degrees C, caused by switches, circuit breakers, relay contacts, fuses and poor cable terminations. 

It is often assumed that an arc flash incident will only occur on large-scale electrical systems, but in fact a hazard can exist at any voltage level.  In particular, analysis of a comprehensive collection of study results has revealed that around five percent of switchgear installations operating at 415v present a dangerous arc flash hazard.

Switching devices susceptible to arcing are normally designed to contain and extinguish an arc, but if a circuit has enough current and voltage to sustain an arc formed outside of a switching device, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire. 

Depending on how much short circuit current is available, the arc can result in nothing more than a minor embarrassment to a catastrophic explosion.

An arc flash hazard is especially severe for LV systems, due to the combination of potentially high arc currents and less effective protection systems and equipment characteristics. Combined with the potentially degraded condition of old legacy equipment and lack of fully-trained electrical personnel to operate and maintain the electrical system, this can present a serious health and safety risk.

Electrical arc flashes pose a serious hazard to personnel who operate and maintain electrical distribution switchgear. The almost instantaneous dissipation of large quantities of electrical energy causes thermal, mechanical and chemical emissions that have sometimes inflicted serious injuries and can cause significant levels of collateral damage and contamination.

The arc flash hazard has been known since the early days of electrical power systems, but there is still lack of awareness and understanding of the arc flash risks by many operators of electrical systems, in particular medium and low voltage industrial systems, marine systems and renewable electricity generation installations.

It has also only been relatively recently that methods to quantify the energy released by an arc fault and mitigate the risk of burns to personnel have been developed.

In 1982, Ralph Lee published a paper titled: The Other Electrical Hazard: Electric Arc Blast Burns. This landmark paper moved the arc flash hazard to centre stage. Subsequent research, papers, and conferences continued to improve the understanding of the arc flash hazard and in 2002, another landmark publication by the American Institute of Electrical and Electronic Engineers (IEEE), the IEEE 1584 - IEEE Guide for Performing Arc Flash Hazard Calculations, was released. 

Based on extensive tests and the best knowledge available at the time, the document is one of the few, if not the only, authoritative calculation guide available in the world and recognised as the best method for performing arc flash calculations and risk assessments. 

The understanding of arc flash and protection continues to evolve. New testing and research aims to answer the many questions that remain about arc flash, but often it also raises more questions. 

Currently, a team of experts from across the globe are working on an update to IEEE 1584, which will reinforce the guide’s position as a leading global standard. 

Among the research are the issues associated with DC arc flash, which is a less understood form of the hazard with potentially higher risk.  It is becoming more prevalent with the increase in battery power and applications, such as hybrid propulsion for ships and uninterruptable power supplies. 

Despite these developments, the application of effective Arc Flash Studies and mitigation processes are not yet universal, which results in unnecessary injuries, and in some cases, death. 

In addition to serious burns, personnel caught in an arc flash can receive lung damage from the inhalation of arc products, hearing damage, blindness and barotraumas, which is the effect of pressure waves on the brain, nervous systems and lungs.

According to research by Chicago-based Capelli Schellpfeffer, Inc., there are between five and 10 arc flash injuries every day in the United States.

In addition, the IEEE reports more than 2,000 US workers have been admitted to burn centres as a result of electrical incidents.  

There have been no specific UK studies of the number of arc flash incidents; however, based on the experience of GSE Systems’ Engineering Division, TAS Engineering Consultants, there may be more than 300 incidents in Britain every year and over 5,000 across Europe.

The fact that these figures are not definitive demonstrates that the UK and Europe are playing catch up with the US with regard to acknowledging and recognising the magnitude of the issue.

However, in the past two decades, GSE Systems’ UK division has amassed a considerable amount of data, experience and working practices to advance understanding through more than 100 Arc Flash studies.

In addition to farming information through practical applications of studies and mitigation processes for customers, GSE Systems is also working in the academic field to increase understanding of, and develop solutions for, mitigation and management of this serious electrical safety issue.

Through a partnership with Strathclyde University, we have instigated a PhD Research Project to further our knowledge of electrical arc flash hazards through experimentation, modelling and analysis in a way that will underpin moves towards a quantitative and scientifically-based evaluation of risk. 

The research to be carried out within this project is expected to provide a greater understanding of the arc flash hazard, its physical effects, factors that govern its severity, and ultimately, the control and management of risk. 

This is an increasing priority as switchgear ages and users seek to justify switchgear lifetime extensions. 

American standards such as NFPA 70E – Electrical Safety Code - which makes explicit reference to Arc Flash Hazard Studies, provide useful background and established guidelines to support the completion of Arc Flash Studies.

The extensive experience of electrical engineering consultants within the USA on these matters has much to offer those companies active within Europe seeking to bring their operations up to the highest standards. Honeywell, for example, has engaged GSE Systems to provide electrical hazard assessment analysis on their UK and European sites. 

Risk assessments and mitigation

All persons responsible for the safety of electrical installations should conduct an appropriate risk assessment to demonstrate that adequate measures are in place to mitigate any risks arising from electric arcs.

A key driver for the increased awareness has been the evolution of the Arc Flash Study field, which has led to greater recognition of the hazard and a move towards a risk-based approach to prevention.

A fully implemented Arc Flash Study is an important project, which will increase safety in the workplace.  However, it must be stressed that an Arc Flash Study is not an isolated activity and should be viewed as one element of a fully-integrated Electrical Safety Management System.  

Significant benefits are also delivered in terms of better documentation, an improved understanding of site electrical systems and their safe operation.

An Arc Flash Study, and the associated implementation, is a collaborative process.  Specialist engineering consultants work alongside a team consisting of site management, head of electrical maintenance, site electrical staff and the client corporate team.

To ensure a successful outcome is achieved from a study, it is important that this team works closely together.  Commitment from all team members is vital, particularly when considering the ‘human factors’ involved in improving well-established and proven working practices.  

Capturing the hearts and minds of personnel is a major priority as any new procedure or working practice is only as good as the people applying them. 

Broadly speaking, an Arc Flash Study has two elements. The first is to perform calculations to quantify the energy released by an electric arc and the potential impact of incident energy on personnel.  

The calculations provide information on the consequences of an arcing incident, specifically the thermal incident energy at a working distance and the distance at which the arc ceases to present a thermal hazard. 

To calculate arc flash energy levels requires knowledge of system fault currents and protection clearance times.  TAS, for example, employs specially-developed technology that is capable of calculating fault currents throughout a system and accepting protection relay characteristics. 

The second element of the study involves an Electrical Safety Review covering risk assessment and the mitigation of the risks posed by electric arcs.  The Electrical Safety Review is a qualitative assessment of the probability of an arcing incident, based on multiple factors including the site operation and maintenance regime, together with the physical features of individual items of switchgear.

A key feature of the Electrical Safety Review is to ensure that the full hierarchy of controls - as set out in The Health and Safety at Work Act, has been implemented before suitable Personal Protection Equipment (PPE) is recommended. 

Arc Flash Studies have helped industry make great strides in the field of PPE.  In an ideal world, risk mitigation measures would remove the need for the use of PPE totally. However, in some cases, after the hierarchy of controls has been applied, the residual risk is deemed sufficient to make the use of PPE appropriate.  

To correctly specify PPE it is necessary to calculate the arc incident energy to arrive at an appropriate Arc Thermal Performance Value (ATPV) value, which represents the capability for arc flash protection for a particular garment.  Continuous development has led to a new generation of lightweight, comfortable fabrics, which provide significant protection.

Each Arc Flash Study is specific to each individual facility and there are no ‘rules of thumb’ or generic guidelines.  

This is because the energy generated by an electric arc is directly proportional to the arc current and the arc duration.  Counter-intuitive results can often be obtained with, for example, higher calculated arc energy at the remote end of a feeder cable than the supply end.  This is due to the cable impedance reducing the arc current and increasing the protective device clearance time disproportionately.  

Similarly, many of the ‘worst case’ assumptions made in traditional short circuit calculations are not appropriate in the case of arc energy calculations.  

For many large and complex installations there is simply no alternative than to perform detailed calculations in order to obtain meaningful results.

These calculations are now seen as the starting point for a comprehensive risk assessment and mitigation process.  The increasing acceptance of the role Arc Flash Studies play in site safety within wider safety protocols ensures that greater knowledge is being gathered about arc flash.

Arc Flash Study elements

GSE Engineering has developed the following key steps for a comprehensive Arc Flash Study:

Step 1 – Know your system, which typically includes:
* High voltage cables, types, sizes and lengths.
* Details of high voltage over current protection, relay type and manufacturer, model, CT ratios, settings.
* Transformers, voltage ratio, KVA rating, impedance, vector group.
* Transformer secondary cables, type, size, number per phase, length.
* Main low voltage switchboard details, incoming switch type (if fitted), protection details.
* Outgoing circuit details for all circuits rated at 100A and above.
* System operating configurations.

Step 2 – Calculate incident energy at working distance and arc flash protection boundary using IEEE 1584

Step 3 – Reduce arc energy levels and potential exposure of personnel to arc hazards

Step 4 – Risk assessment including:

* A facility electrical safety management system audit.
* A switchgear risk assessment.

Step 5 – Implementation, covering:
* Engineering work.
* Electrical safety management system improvements.
* Training.
* PPE philosophy.
* On-going review.

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