By Paul Grad, engineering writer.
Accidents in electrical installations have become of increasing concern, particularly those due to arc flash. Most types of electrical injury have been significantly reduced due to improved technology and safety. However, incidents relating to arc flash have become one of the leading causes of serious injury or even death among electricity workers.
An arc flash is the most extreme event in an electrical installation. It is also the most challenging regarding worker protection. It is a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or ground. It occurs when an electric current flows through an air gap between conductors. The cause of the short circuit usually burns away very quickly but the arc fault is then sustained by the creation of a plasma (an ionised gas). The energy discharged burns the bus bars, vaporising any metal and causing an explosion, the arc blast. Temperatures can reach 20,000°C.
The arc fault current is usually much less than the available bolted fault current and below the rating of circuit breakers. These devices will not normally trip and the arc flash will occur unchecked. The energy is voltage x current x time (v x i x t) and the arc flash increases in intensity with time. It is therefore essential to sense an arc fault current and shut off the voltage before it develops into an arc flash.
An arc flash can cause terrible injury, including third-degree burns. The blast can knock people off their feet, cause them to fall off a ladder or get thrown against a wall. It can even cause hearing loss and result in injury from flying debris.
About 50 per cent of the cases of death or injury due to electricity in Australia have been due to problems on the distribution network and about 50 per cent have been due to problems in consumers’ installations. Major causes of incident are exposed metal raised to dangerously high voltages because of electrical faults, contact with live parts, exposed metal raised to high voltages because of inadequate circuit design, defective insulation and electrostatic phenomena.
Governments have responded by introducing legislation and regulations mandating the use of safety switches and retrofitting these to buildings and portable equipment, and the use of personal protective equipment. A safety switch, also known as a “residual current device” or “earth leakage switch”, monitors electrical current flowing in the active and neutral conductors in a circuit. The safety switch responds to any imbalance or fault by cutting the power in less than 40 milliseconds.
Personal protective equipment (PPE), particularly arc flash PPE has become a major issue and many companies have developed a line of arc flash PPE, including fire retardant (FR) clothing, made of specially manufactured and treated fabrics, gloves, helmets and footwear.
The physics and technical aspects associated with arc flash are still imperfectly understood, and there has been concern that the industry standards of the US and other countries have been developed on a somewhat shaky basis.
That has been the framework behind the Electrical Arc Flash Conference held in Melbourne, on August 6-8, with the participation of leading experts, including Welcon Techbnologies (Queensland) senior power systems specialist Sesha Prasad, and US-based founder of e-Hazard and ArcWear Hugh Hoagland. Another arc flash technical conference is scheduled for Auckland, New Zealand, on October 22-23.
Several international industry standards have been developed to help prevent the occurrence of arc flash events or to adequately protect workers. They include:
NFPA 70E titled Standard For Electrical Safety in The Workplace – is a standard of the US’ National Fire Protection Association. The NFPA is best known for its sponsorship of the National Electrical Code (NFPA 70);
IEEE 1584 –Guide For Performing Arc Flash Hazard Calculations – is a standard of the US’ Institute of Electrical and Electronics Engineers;
OSHA –Occupational Safety & Health Administration – is part of the US Department of Labor. OSHA 1910.269 is titled Electric Power Generation, Transmission, And Distribution;
ASTM International, formerly known as the American Society for Testing and Materials, develops international voluntary consensus standards. ASTM F1506-10a is a “Standard performance specification for flame resistant and arc rated textile materials for wearing apparel for use by electrical workers exposed to momentary electric arc and related thermal hazards”; and AS4836:2011 is an Australian/New Zealand standard for “Safe working on or near low-voltage electrical installations and equipment”.
The standards stipulate that the materials used in PPE must be tested for their arc rating. Arc rating is the maximum incident energy resistance of a material prior to breakopen (a hole in the material) or necessary to pass through and cause – with 50 per cent probability – a second or third-degree burn. Arc rating is normally expressed in calories/square centimetre (cal/cm2). It is usually referred to as the arc thermal performance value (ATPV) for arc flash protection of a particular garment, when tested to ASTM F1959-2004.
The various standards establish hazard risk categories for PPE. For example, NFPA 70E breaks down hazard risk categories (HRCs) into 4 levels of acceptable ATPVs:
HRC 1: rated to a minimum of 4 cal/cm2 (FR shirt and FR pants)
HRC 2: rated to a minimum of 8 cal/cm2 (cotton underwear, FR thermals, FR shirt and FR pants or FR coveralls)
HRC 3: rated to a minimum of 25 cal/cm2 (cotton underwear, FR thermals, FR shirt, FR pants or FR coveralls)
HRC 4: rated to a minimum of 40 cal/cm2 (cotton underwear, FR thermals, FR shirt, FR pants or FR coveralls, and double layer switching coat and pants).
If, for example, the incident energy is calculated to be 10 cal/cm2, NFPA 70E says that the minimum PPE for category 2 is 8 cal/cm2. The minimum PPE rating is the maximum exposure for that category, therefore it is necessary to wear at least 25 cal/cm2 clothing, because it is category 3 exposure.
It is necessary to decide which PPE to use, because a full category 4 suit is cumbersome to work with, it is hot and provides poor visibility. It would, therefore, be “nonsense” to use PPE for category 4 hazard, when PPE for, say, category 2 would be adequate.
In addition to the above standards, a hard hat with full-face shield and appropriate gloves are required. AS4836:2011 requires that arc flash suit and hood be rated at a minimum of 40 cal/cm2 protection. This aligns with NFPA 70E and the HRC4 rating.
Selection of appropriate PPE is done either by consulting a hazard category classification table, like that found in NFPA 70E, or by performing an arc flash hazard calculation to determine the available incident arc energy.
For example, IEEE 1584 provides a guide to perform those calculations once the maximum fault current, duration of faults, and other general equipment information are known. Once the incident energy is calculated it is possible to select the proper PPE combination.
However, extensive research done in Australia by Dr David Sweeting, of Sweeting Consulting, of Sydney, and Prof Tony Stokes, of Sydney University’s Department of Electrical Engineering, has raised some serious questions about the validity of IEEE 1584 equations for calculating the incident energy at electrical switchboards. However, they did not yet provide alternative equations.
It is essential to bear in mind that PPE provides protection after an arc flash has occurred and should be viewed as the “last line of defence”, in the words of IEEE. The first option should be reducing the frequency and severity of incidents through a complete arc flash hazard assessment and applying technology such as high-resistance grounding.
In the US, OSHA is the policing agency for electrical safety. Under the OSH Act, employers are responsible for providing a safe and healthy workplace. “OSHA’s mission is to assure safe and healthful workplaces by setting and enforcing standards, and by providing training, education and assistance. Employers must comply with all applicable OSHA standards. Employers must also comply with the General Duty Clause of the OSH Act, which requires employers to keep their workplace free of serious recognised hazards”.
Employers who fail to meet OSHA’s requirements are subject to citations, fines and penalties. OSHA relies on the consensus standards established by NFPA 70E. So while NFPA 70E is not itself law, it established the safety guidelines, which enable employers to comply with OSHA laws dealing with electrical workplace safety and required employee training.
In Australia, similarly, Standards Australia’s are voluntary documents with no legal status, and only become mandatory when they are referenced in regulation or law. Electrical operational safety codes in Australia are enforced by state governments through the Occupational Health & Safety Act and the Electrical Work Code. However, the Electrical Work Code includes only a brief reference to arc flash protection.
The original detailed guidelines for arc flash calculations and arc flash PPE are specified in ESAA NENS09-2004 developed by the Energy Supply Association of Australia (ESAA).
Lately a new national workplace health and safety legislation has been enacted across Australia. This is supplemented by the Code of Practice, AS/NZS 3017 (Electrical installations – verification guidelines) and AS/NZS 4836 (Safe working on low-voltage electrical installations).
The proper choice of PPE can a bewildering proposition due to the huge variety of equipment available in the market, and employers may require the services of a consultant.
Few companies in Australia or New Zealand manufacture PPE fabric or clothing. Most of them represent foreign suppliers. Exceptions include Elliotts Australia, of Virginia, Queensland, Hilec Energy Solutions, of Brisbane, and Jaedon Enterprises Ltd, of Auckland, New Zealand.
Elliotts markets FR and arc flash protective clothing manufactured from Banwear or UltraSoft materials. Each garment meets the requirements of NFPA 70E. The company’s safety gear range includes: arc flash protective apparel; FR protective workwear; safety eyewear; chem-tech workwear; firefighting apparel; furnace protection; welding apparel; welding protection; and safety gloves.
Its product includes ArcSafe apparel or FR Tecasafe Plus or Walls FR rated to hazard levels 2, 3 and 4.
Tecasafe Plus is a product from TenCate Protect BV, of Almelo, The Netherlands. Tecasafe Plus range in fabric weight from 197g/m2 to 288g/m2. The fibre is made of a blend of FR-modacrylic, lyocell, and aramid.
Modacrylic is a synthetic copolymer with properties making it especially suitable for use as a flame retardant material to protect against arc flash.
Lyocell is a cellulose fibre made by an organic solvent spinning process. Its properties are similar to those of other cellulosic fibres such as cotton, linen and rayon. Lyocell fibres are soft, absorbent, very strong, and resistant to wrinkles.
Aramid fibres – also called aromatic polyamides – strong and heat-resistant synthetic fibres. They were first commercialised by Dupont as HT-1 and then under the trade name Nomex. Nomex has excellent resistance to heat. It neither melts nor ignites in normal levels of oxygen. Dupont also developed a para-aramid called Kevlar, used for bullet-proof body armor fabric.
Elliotts line of products also includes FR fabrics from Walls Apparel Canada, Inc, of Edmonton, Alberta, Canada, and from Westex Inc, of Chicago, Illinois, US.
Walls FR clothing is made from Banwear and Banox fabrics by Itex Inc of Englewood, Colorado, US.
Westex FR fabrics include UltraSoft and Indura fabrics. UltraSoft is 88 per cent cotton and 12 per cent high tenacity nylon. UltraSoft fabrics weight from 237g/m2 to 440g/m2. The Westex engineering process forms a long chain FR polymer impregnated into the core of each cotton fibre.
Jaedon Enterprises offers its ArcPro range of FR clothing. Many of the company’s garments are cotton-based with an inherent FR built into the fibres before the fabric is made and therefore lasts the lifetime of the garment. The ArcPro range goes from underwear through to cold and wet weather garments all arc rated to level 2 or greater.
The company has just released its ArcPro FR T-shirts. These 100 per cent cotton garments are rated to NFPA 70E for high voltage electric arc, and have an ATPV of 9.6 cal/cm2. They are thus suitable for being worn on their own in long sleeve form, or under other level 2 arc-rated clothing in the ArcPro range.
The company said the fabric passes all NFPA 2112 requirements for FR garments to protect against flash fire.
It has also released a new FR wool coat that is 100 per cent New Zealand wool with FR merino fleece lines.
Another recently released product is the ArcPro range of under garments, including XP ArcPro thermals, long-sleeved tops and long-johns. The fabrics are a blend of merino wool and FR Viscose. The garments are rated to NFPA 70E level 2, with ATPV of 8.2 cal/cm2.
The company said when combined with level 2 PPE, these undergarments will more than double arc protection.
The switching coats, trousers and hoods are at 45 or 65 cals – level 4 are made to order so they are made especially for your size.
Another major supplier of PPE is Hard Yakka. The company does not have a manufacturing operation in Australia, but has one in Fiji.
It has just launched Tecgen Select, a range of inherent FR workwear comprised of modacrylic, FR Viscose, carbonized acrylic, para-aramid, and high tenacity nylon fibres. The garments are certified for NFPA 70E (HRC2) and NFPA 2112 (flash fire).
The company recently announced a high visibility FR fabric to meet the demands of the Australian market.
The Tecgen brands of FR apparel were originally made by the Ashburn Hill Corporation, headquartered in Greenville, South Carolina, US. Ashburn Hill Corporation had a manufacturing operation in Angleton, Texas, where it produced its TecGen brands of FR apparel. Another company, Invista, of Wichita, Kansas, US, a wholly-owned subsidiary of Koch Industries, Inc, has acquired Ashburn Hill Corp.
Hylec Energy Solutions, of Brisbane, markets arc flash switching suit items from Stanco Manufacturing, of Atlanta, Texas, US, as well as other PPE items. Its products include a new lightweight FR 74.4 cal/cm2 HRC4 arc flash kit from Stanco.
The company’s workwear focus is on arc flash protection for electrical workers. It offers locally fabricated workwear in short production runs using Westex fabric.
The company’s line includes arc flash FR gloves, and arc flash ventilated hoods and visor.
Sicame Australia, of Yatala, Queensland, offers arc flash protective kits rated at 25, 40, 55, 65 and 100 cal/cm2, high visibility arc flash parka rated at 19 cal/cm2, arc flash jacket and protective coverall kits rated at 8 or 10 cal/cm2, arc flash protective gloves, face shields, and insulating boots.
The huge variety of PPE in the market, as well as the tricky task of calculating the available incident arc energy and of understanding the requirements of applicable standards, may pose a formidable problem to the employers responsible for the safety of their workers. A properly qualified consultant will be needed in most cases.
Australian workers and employers have been devoting increasing attention to the hazards posed by arc flash and related phenomena, especially in the mining, utilities and manufacturing areas. There has lately been much activity in all related areas, including research to better understand the phenomena involved, in the development of appropriate industry standards, and in the ways to mitigate or hopefully eliminate arc flash hazards.