Arc flash protection

July 24th, 2009, Published in Articles: Energize

by Zarheer Jooma and Hugh Hoagland, e-Hazard

South Africa averages at least one arc flash incident with lost time injuries per month and at least one fatality every two months.  This statistic is based only on cases reported in the Highveld media and is not necessarily representative of the actual number of incidents which is believed to be greater.

Standards South Africa is publishing the first standard which is dedicated to electrical arc flash safety and will soon follow with standards which aim to reduce the number of arc flash incidents and the effects of such incidents in the workplace.  This article introduces the soon to be published SANS 724 “Personal Protective Equipment: Protective clothing against the thermal hazards of an electric arc” and conclude with further developments in the field of electrical arc flash.

Electrical arc flash

An electrical arc flash originates from a breakdown in the insulation medium creating conductive plasma. Heat propagation from the plasma is predominantly infrared radiation with temperatures exceeding 20 000°C. Workers operating on electrical equipment posing an electrical arc flash hazard should be protected against such high temperatures. 

The Occupational Health and Safety Act (Amended Act, No. 181 of 1993) [OHSA] requires the employer as per section 8,2 (b)  “taking such steps as may be reasonably practicable to eliminate or mitigate any hazard or potential hazard to the safety or health of employees, before resorting to personal protective equipment”.  Ideally, based on this clause, the first standard published should mitigate the risk rather than provide personal protective equipment (PPE).  However, with a number of suppliers already trading PPE from local and international markets, an urgent need to develop a standard to regulate arc rated clothing existed.  In order to fulfil the requirements of the OHSA, further developmental work is underway to mitigate the electrical arc hazard.

Existing personal protective equipment


Companies already provide hard hats, face shield, overalls, shoes and gloves so why regulate clothing for electrical arc flash?  The intent of generic PPE was mechanical impact withstand, chemical and dust ingress protection, minimising cuts and bruising amongst a host of other workplace hazards.  Consider a face shield, which displays excellent mechanical withstand capabilities, being exposed to a 20 000°C electrical arc flash. The face shield will melt and the molten matter will ensure that the heat of the electrical arc remains attached to the skin of the worker long after the electrical flash has cleared.  Most over-the-counter overalls protect against small welding/cutting arcs and fires which are not of the same magnitiude as an electric arc flash.  These types of clothing will most often ignite, leaving the worker with a 50% chance of survival and severe burns. At present 80% of all injuries result from the ignition of clothing.

The SANS 724 working group was convened under the umbrella of SC67E and mandated to standardise PPE which would protect users against the thermal effects of an electric arc. The working group used the standard to introduce the electrical arc flash phenomenon and regulate the arc rated PPE clothing market.

Understanding burns and arc energy

An electrical arc flash results in energy propagation. The energy varies according the system fault level (kA), construction of the electrical apparatus, fault clearing time and other protection characteristics. The incident energy received by the worker depends on the distance between the worker, the arc energy and directionality. Arc energy is measured in calories, although Joules are also acceptable. The unit of measure of the available energy and the incident energy is cal/cm2 (or J/cm2 or kW.s/cm2). These factors will be discussed in more detail in a subsequent paper in the electrical arc flash series.

If the incident energy from an electrical arc flash is completely dissipated by the personal protective equipment then no further energy can be transmitted to the body of the worker.  This is the ideal case. In reality, heat transmission to the workers body does occur (measured in terms of a Stoll curve differential).  Human tissue can dissipate a measure of heat before the onset of second degree burns. In the early 1960s, Alice Stoll conducted studies on humans (academics and sailors) and animals [2]. A predetermined amount of energy was exposed to skin for a certain period of time which gave rise to the definition of a second degree burn (also termed a blister burn).  Referring to Fig. 1.[1], this is the point at which the epidermis burns and separates from the dermis. In terms of the thermal energy, second degree burns will occur at energies on human skin exceeding 1,2 cal/cm2. In other words, PPE should then dissipate (or reflect) enough energy so that the residual energy transmitted to the worker never exceeds 1,2 cal/cm2.  The work of Stoll was very important as it provided a measure for human tissue burns as a function of heat flux, time and change in temperature. 

Fig. 1: A cross section view of human skin.

Third degree burns often result in permanent disability and death but so can second degree burns if over a large extent of the body. The ignition of clothing is usually the main culprit.  In the case of a third degree burn the epidermis and dermis is completely destroyed.  The complete destruction of capillaries, nerve and muscle makes recovery highly improbable.  Fig. 2, taken on 2009-03-12, shows a recent burn victim with extensive third degree burns.

Fig. 2: Electrical worker after an electrical arc flash incident (taken 2009-03-12).

The worker was unfortunately not protected by arc rated PPE and only survived for a few days. 

Fig. 3: Electrical arc flash incident on an 11 kV system.

Fig. 3, an unrelated case, shows equipment failure which provided arc energy of
50 cal/cm2 during an arc blast incident. The incident energy is believed to be around 20 cal/cm2 according to arc rated clothing expert Hugh Hoagland from ArcWear.com. The worker used a 90 cal/cm2 flash suit jacket and non-arc rated pants as shown in Fig. 4. The brunt of the arc flash was over his right hand with very little energy over the trouser. The worker did not suffer any effects on the right hand, but did experience sun burn type effects on the right leg covered by the non arc rated trouser. If the trouser had ignited the effect would have been catastrophic.

Fig. 4: Electrical worker exposed to 50 cal/cm2 arc blast.

SANS 724 – inclusions and exclusions

SANS 724 “Personal Protective Clothing: Protective Clothing Against the Thermal Effects of an Electric Arc” covers the design, manufacture, marking and testing of arc rated PPE. As the title suggests, the standard regulates PPE used for thermal protection only and does not address electrical shock, projectiles, shock waves, hot oil release, or other hazards of an electrical arc.  

The two exclusions regularly questioned are the protection against hot oil release and projectiles. Hot oil release has resulted in fatalities previously and is a valid concern. Tests have shown that oil increases the ignitability of almost any garment. It is also accepted that little can be done to protect workers who are engulfed in flaming hot oil. SANS 724 requires that under such conditions, special precautions should be taken such as remote operation and de-energised work amongst others. The employer should assess each application uniquely and ensure that workers are not exposed to hot oil release.

When addressing projectiles it is imperative to separate two phenomena namely arc flash and arc blast. Arc flash is a violent eruption of thermal energy, usually below 40 cal/cm2. Arc blasts occur when sufficient pressure is created by the arc to cause trauma. This has been shown to be a function of the fault current. If the arc energy is greater than 40 cal/cm2 the engineer must consider the arc blast or shockwave element in determining the risk. An arc blast creates an explosive environment producing projectiles which can reach speeds of 1000 km/h. Ballistic rated clothing is available, however, at present no test method exists for combination testing of arc rating and ballistic protection. Some specialised suits have been shown to work but there is little test data to permit standardisation. The mass and rigidity of this type of clothing can inhibit movement and create new hazards.

In the cases investigated, the injury mechanisms are always burn wounds. So the question remains, if an arc blast creates projectiles, perhaps more fatalities should be attributed to projectiles but the evidence is unclear. To examine the effects of projectiles, the arc blast should be categorised into a series of events. The plasma creates infrared radiation at the speed of light.  The rapid temperature increase over a fixed volume of air creates a shock wave beyond 200 kPa (29 psi) at the speed of sound which can throw a worker several meters. Lastly projectiles are emitted from the source at 1000 km/h. So the worker is usually struck off their feet before projectiles can penetrate clothing and then the workers body. SANS 724 requires that if an arc blasts potential exists, the risk must be reduced through accepted engineering means. Suggested engineering means would include remote operation, replacement of apparatus with internal arc proof apparatus and de-energised work.

Items that have been excluded fell into two broad classes. In general where insufficient information was available in the international arena and cases where no test method exists for verifying a design requirement.

Personal protective clothing and personal protective equipment

Arc rated protective equipment should protect a user who is completely engulfed in an electrical arc.  In terms of SANS 724 personal protective equipment consists of all items including head, face, neck and chin protection, eye protection, hearing protection, body protection, hand and arm protection, foot and leg protection.

A bee keeper’s hood, long sleeved shirt, innerwear or underwear, full length trouser, coverall, jacket and rainwear are categorised as personal protective clothing.  Arc rated personal protective equipment refers to face shields, balaclavas, hard hats, hearing protection, fall arresting equipment, gloves and shoes. Welding aprons and flame retardant coats are not regarded as arc rated personal protective clothing.

The philosophy of SANS 724

Very little research has been performed in SA on the topic of arc rated PPE.  When the working committee was tasked with standardising PPE, the focus had to shift outside of SA. Although SA has a Memorandum of Understanding (MOU) with the IEC, it is actually the United States and Canada who are seen as global leaders in terms of standardisation and research work in this area. South Africa had to accommodate both the European Standards as well as the North American Standards since local retailers offer PPE from both sides of the world.

A vast and diverse array of standards is available in the international arena. This prompted the working committee to use SANS 724 as a front end document which references various standards (e.g. NFPA, IEEE, IEC, EN, ISO, SABS and ASTM) and allowed flexibility to include requirements unique to the South African application. It is therefore a requirement that SANS 724 be read in conjunction with the NFPA 70E – 2009 and IEEE 1584a – 2002. 

The requirements of SANS 724


Head protection

Head protection can be achieved by using a bee keeper’s hood, face shield, goggles or balaclava.  The design requirements of the visor on the face shield and the hood are regulated by EN 166, Personal eye protection – Specifications and ANSI Z87.1, Occupational and Educational Personal Eye and Face Protection Devices. The test to determine the arc rating of head protection is according to ASTM F 2178-02, Standard test method for determining the arc rating of face protective products.

Hand protection


The general approach to hand protection in SA is the use of the red leather welding glove.  Although this glove offers protection, its arc rating is not known.  The debate regarding finger dexterity is often a contentious issue. Many countries have enforced the use of arc rated gloves while others leave it to the end user. SANS 724 has standardised the use of leather gloves or gloves manufactured from an arc rated fabric.  A standard test method for determining the arc rating of gloves is currently in draft format in the US.  Once this standard is approved and provides testing and certification, the working group will discuss its possible inclusion into SANS 724.

Miscellaneous PPE Shoes, socks and hearing protection are usually not directly exposed to the arc.  If exposed to an electric arc, fairly good arc ratings were attained from standard issue work wear.  Leather shoes performed well when exposed to 50 cal/cm2 while yellow hearing protection inserts withstood 25 cal/cm2. These items of PPE should be manufactured to the relevant South African National Standard and may be arc tested to the relevant American Standard Test Method (ASTM).

Fall arrestors were not discussed in detail during the working committee meetings and hence excluded.  However, with the recent focus on fall protection in terms of fatality prevention a renewed focus is required. ASTM F887 Standard for Personal Climbing Equipment provides the electrical arc testing guideline for fall protection equipment.

Clothing


Electrical arc flash hazards are present at low voltage, medium voltage, high voltage and extra high voltage applications. Most industry, mines and small commercial installations consist of many electrical systems which will offer a wide range of available arc energies. For obvious reasons, a customised garment cannot be provided for each calculated incident energy level. Most companies have adopted a simple two garment approach.  The daily work wear will be used for frequent daily tasks where the incident arc energy is usually below 25 cal/cm2. For higher incident energies, usually greater than 40 cal/cm2, a full flash suit is used.

The use of melting fibres such as polyester, nylon, acetate and spandex are prohibited irrespective of whether exposed to the arc or used as an innerwear or underwear.  Melting fibres will stick to skin if exposed to an arc. This phenomenon will result in the melted fabric retaining heat on the skin and further damaging the human skin. Some arc rated materials contain low levels of these fibres but these are propriety designs included to offer the user certain benefits. These are acceptable if tested to the requirements of SANS 724.

In the event of a garment being exposed to an arc, all items of personal protective clothing exposed to an electric arc shall be permanently withdrawn from service. SANS 724 also requires that the minimum number of laundering cycles shall be specified, before the arc rating of the garment decreases from its original rating. The laundering cycles should comply with the laundering requirements from the manufacturer. Once the number of laundering cycles has been attained, or if the laundering decreases the original arc rating of the garment, such garments should also be withdrawn from service. In general fabric softeners and bleach should not be used when laundering arc rated clothing.

The testing requirements of arc rated clothing are governed by either the ASTM or IEC standard. South African laboratories are not equipped to perform tests which determine the arc rating of a fabric. The Kinectrics High Current Laboratory in Toronto, Ontario, Canada is set up to perform tests required by SANS 724. Over the next few articles in the electrical arc flash series, more details will be provided on various test methods proposed by SANS 724.

Electrical arc flash – The way forward in South Africa


PPE manufacturers must take the lead and ensure that all PPE will comply with the requirements of SANS 724.  This will ensure that the last barrier a worker has when exposed to an electrical arc flash will perform when required. The employer needs to also be proactive and comply with extract 8.2 (b) from the OHSA quoted above.  Electrical arc flash safety should be seen as a multi-tiered program with PPE being the last line of defence. 

The next series of articles will discuss Hazard Risk Categories, Testing Methods, Approach Boundaries, IEEE 1584a – 2002 and the NFPA 70E. These articles will provide guidelines to perform arc flash calculations and determine the level of protection (cal/cm2) required as well as discuss hazard risk mitigation and safe work practices. 

References


[1] University of Virginia, Health System http://www.healthsystem.virginia.edu/uvahealth/peds_burns/thirdeg.cfm#
[2] Dr. Tom Neal, Oberon, Understanding the Stoll Curve, 2005.
[3] SANS 724 Personal Protective Equipment, Protective Clothing against the Thermal Hazards of an electric arc, 2009.
[4] E-Hazard.com, Low Voltage Qualified Training on US OSHA and NFPA 70E, 2009.
[5] Occupational Health and Safety Act No. 85 of 1993 as amended by the Occupational Health and Safety Amendment Act No. 181 of 1993, Republic of South Africa.

Acknowledgements

Pictures of the fatally injured worker used with the kind permission from the family of the deceased.
Sincere thanks the Mr. S G Hennop (Pr. Eng.),  ArcelorMittal SA; Mr. A Maharaj, Du Pont and Mikhail Golovkov, ArcWear.com (US) for their technical input.

Contact Zarheer Jooma, e-Hazard, Tel 083 336-3084, zarheer@e-hazard.com

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