Flashover - Prepare for the worst
The risk of exposure to electric arc is a hot topic when working with electricity. No
matter how many steps are taken to minimise the potential for an accident, it is crucial to be prepared for the worst, as Paul Reader, Empower's electrical training specialist reiterates week after week at Empower's purpose-built, £2 million training centre, housed beneath the cooling towers of Ratcliffe Power Station in Nottinghamshire
Empower, a leading provider of training services to a host of major UK utility and manufacturing companies, is the UK's only Centre of Vocational Excellence (CoVE) for Electrical Power. The company specialises in training technical staff and engineers in everything from safety management to cable jointing. Paul Reader explained, "There is a common misconception amongst people who are highly experienced in electrical work that an electric arc "won't happen to them" or if it does, it won't be powerful enough for them to come to serious harm. On the contrary, however low the risk, when working with low or high voltage systems, equipment failure and/or human error have the potential to cause an electric arc generating up to 30,000°C in the core plasma of the arc - in a split second. When an electric arc occurs, there is no warning, and in worst-case scenarios, if inadequately protected, people can suffer fatalities or burn injuries that can affect them and their families for years.
"Wearing personal protective equipment (PPE) as a last resort is as important as carrying out risk assessments and applying mechanical control measures to reduce the risk of accidents. It is vital for people at risk of exposure to electric arc or flash fire to put a final protective barrier in place by wearing the appropriate flame resistant clothing. As a CoVE, we strive for excellence in everything we do and that includes providing our delegates with the very best advice on the very best PPE."
The purpose of protective clothing is to address the thermal effects - i.e. burn injury hazard - of exposure to flash fire and/or electric arc. Layered garments, made of materials like Nomex from DuPont, can stand the arc blast, absorb the bulk of the radiant heat energy caused by electric arc or flash fire, and help minimise the burn injury level through its inherent flame resistant properties. Designed with a high level of safety and comfort in mind, garment systems are evaluated according to the International Electrotechnical Commission (IEC) in the Standard IEC 61482-1. In addition to meeting the performance requirements for CE-certification, fabrics and garments made of Nomex must be submitted to DuPont's testing facility in Geneva, in order to qualify for partnership in the Nomex Quality Programme (NQP) - which entitles manufacturers to attach a Nomex label to a garment. Here in Geneva, they undergo a series of special tests and evaluations for heat and flame protection, heat stress management, wear-life, comfort, aesthetics and design.
Arc-Man Testing
The DuPont Arc-Man, a set-up consisting of a mannequin or panels with built in temperature sensors on their surfaces (‘skin'), is used to test garments that are designed to protect against the thermal effects of electric arc. The mannequin, wearing the garment system in question, or panels covered with fabrics, is exposed to an electric arc in a controlled environment to test whether the systems or single fabric layers can prevent second-degree burns. The Arc-Man testing facility, uses a circuit generator that can produce 500 MVA, 60 MJ. Arc currents up to 15kA are possible with a duration in the range between 100ms and 2 second.
Under these circumstances, there are two typical test set-ups and procedures:
The international standard IEC 61482-1 (‘open arc test'), which is currently under revision (future IEC 61482-1-1) specifies two test methods to measure the arc thermal performance value (ATPV) of materials or garments.
The Method A is used to measure the fabric's response to arc exposure when tested in a flat configuration.
Method B is used to measure the clothing response to an arc exposure and shows the effects on the garment, sewing thread, fastenings, fabrics and other accessories when tested on a mannequin torso.
Testing is typically done with an open arc at 8 kA and of varying arc duration in order to achieve the desired incident energy onto the test specimen. The ATPV is the value of the incident energy (usually given in cal/cm2 or kJ/m2) on a fabric or material that results in sufficient heat transfer through the fabric or material to cause the onset of a second-degree burn injury based on the Stoll-Chianta curve model. On a basic level, when selecting a clothing system, the higher the ATPV, the better the protection.
The second test set-up and procedure is covered in the new IEC 61482-1-2:2007 (‘box test' or ‘arc in box test'), which supersedes ENV 50354:2001 and CLC/TS 50354:2003 (The two superseded standards did not require heat flux measurements, but based the performance rating only on visual evaluation of test results). The test set-up consists of a bipolar arrangement of electrodes which is surrounded by a box, which is open on one side only. The effects of the electric arc are thus constrained and directed towards the one panel or mannequin positioned at the open side in 30 cm distance from the centreline of the box. The test has been designed to be carried out in two fixed test classes (Class 1 or 2), selected by the amount of prospective short circuit current (4kA or 7kA). The test voltage 400V and the duration of the electric arc 500 ms are the same for both test classes. Materials and clothing will be tested with two methods:
The Material box test method is used to measure the material response to the exposure of an arc constrained by the specific box of this test method, when the material is in a flat configuration on top of a panel. A quantitative measurement of the arc thermal performance is made by means of the energy transmitted through the material. The Garment box test method is used to test the function of the protective clothing after an arc exposure including all the garment findings, sewing tread, fastenings and other accessories, no heat flux will be measured.
Designed to define
The essential difference between the test methods of IEC 61482-1 (or the future IEC 61482-1-1) (‘open arc test') and of IEC 61482-1-2 (‘box test') is that the ‘open arc test' has been designed to define and evaluate a protection parameter such as is the ATPV, which can be attributed to a material or garment as a product specific protection property, whereas the ‘box test' allows only a classification of materials and garments into two arc protection classes in case of exposure in front of a very specific box: Class 1 (4 kA), Class 2 (7 kA). In practice there can be higher risks than the Box-test Class 2, different arc enclosing boxes, etc. The most common classification of risks and protective clothing performance according to ATPV values is given by the NFPA 70E standard. It defines 4 risk categories and corresponding arc ratings for protective clothing: Category 1: ATPV > 4cal/cm2, Category 2: ATPV > 8 cal/cm2, Category 3: ATPV > 25 cal/cm2, Category 4: ATPV > 40 cal/cm2.
In any case, a risk analysis shall clarify the actual risk of exposure to an electric arc. And the testing according to both above mentioned IEC standards refers only to the thermal effects of an electric arc; it does not apply to other effects like noise, light emissions, pressure rise, hot oil, electric shock, the consequences of physical and mental shock, toxic or other influences caused by the decomposition of enclosures, etc.
But already the part of assessing the risk of thermal exposure in terms of incident energy is not an easy task. Elaina Harvey from DuPont Personal Protection, explained, "When talking about the incident energy relevant for the evaluation of the performance and the selection of arc-protective clothing, one is actually talking about values of incident energy per surface area, for example in units of cal/cm2. The calculated incident energy, against which protection is needed, comes from the end user's risk assessment. The incident energy will depend on:
- arc current i.e. fault value (kA)
- arc voltage (V)
- arc duration (milliseconds)
- distance from worker to arc (cm)
- electrode spacing (cm) and kind of electrode material (copper, aluminium, iron...)
- whether single or triple phase electrical circuit
- whether in an open or a box environment, and on the dimensions and materials of the box."
How to obtain the incident energy value
There are various ways end users can carry out a risk assessment to obtain this calculated incident energy value from simplistic free of charge computer calculators to in-depth statistical studies. Meanwhile several companies offer their help to carry out an arc flash survey.
One of them is EmPower Training which can provide advice in carrying out a risk assessment and training courses.
EA Technology is an organisation that can carry out an arc flash assessment to determine specific incident energy levels and this is tailored to the end user's network and operation. Therefore, PPE is one element of their arc flash study, other factors being assessed includes the reduction in electrical protection settings and network fault levels along with the operational practices.
TAS Engineering Consultants has been carrying out electrical arc flash hazard studies since 1995 and has extensive experience in this field; the work is tailored to suit the individual site and circumstances. A typical approach is to survey the site electrical distribution system, develop a single-line diagram, carry out a protection setting study, then calculate the electrical fault levels at each point in the system. With this in place, TAS then analyse the potential arc energy levels to IEEE 1584 parameters at each point in the distribution system and advise on the appropriate FR PPE to be used to safely operate the electrical equipment at each point in the system. Guidance is then given on rationalising all the information into one or two levels of FR PPE for use site-wide to cater for all circumstances.
As an example, if the end user risk assessment requires clothing to protect from 22 cal/cm2 then DuPont with the garment and fabric manufacturer, can recommend layered systems to achieve the protection required. If underwear is taken in to account when recommending the system, steps must be taken to ensure that employees are supplied with and wear the undergarments. If not, it is important to advise that, worn in high risk environments, undergarments made from fabrics like polyester and nylon (e.g. football shirts) can stick and melt onto the skin and cause further burn injury in the event of a flash fire or electric arc."
TAS uses two products that assist in the analysis, and detection, of arcing faults. These are the Arc Flash PTW software module, and the Falcon Arc Detection System, both supplied by CEE Relays, part of a protection relay manufacturing group.
CEE Relays has been supplying the Power Tools for Windows (PTW) power system analysis software for the last 20 years. The software consists of several modules, including the arc flash module, allowing users to quickly, and easily, calculate the incident energy caused by an arcing fault, investigate ways of reducing this energy, and identify suitable grades of protective clothing. Using the full suite of PTW software, CEE Relays also carries out all types of power system analysis, including arc flash analysis, and can also train engineers in the use of the software. As well as a wide range of protection relays, CEE Relays also supplies the Falcon Arc Detection System. The arc detectors, which can be retrofitted into existing switchgear, detect the light created by an arc, and issue trip signals within 1ms. This fast response time significantly reduces the damage caused by an arcing fault.
Thermo-Man Testing
It is also important to protect from flash fire, which could occur after the arc incident, for example - if there are ignitable contaminates on the garments or around the working environment. DuPont Nomex systems can be assessed on Thermo-Man, a life-size mannequin equipped with 122 heat sensors and exposed to a flash fire with temperatures rising to 1000°C. Data collected from the heat sensors, both during and after exposure to flames, predict the amount and location of second and third-degree burns. The chance of survival rates can also be predicted across certain age groups, which is important information that can be used in helping companies if they have an ageing workforce.
Harvey continued. "PPE worn for protection against the thermal effects of electric arc should provide permanent flame resistance, not melt or ignite, resist break-open during exposure and insulate the wearer from heat so as to minimize burn injury. The principle of selection of PPE is that the arc rating of PPE exceeds the calculated incident energy of the arc for each specific working environment identified during the risk assessment. Garments made of Nomex are designed to offer excellent protection and comfort. The meta-aramid fibres do not drip or melt at high temperatures. The Limiting Oxygen Index is approximately 28. Thus, when exposed to flame at room temperature in a normal environment, Nomex will not continue to burn and will self extinguish, when the flame is removed. At temperatures above approximately 427°C Nomex carbonises and forms a tough char. Garments have a high resistance to ignition and break open and because they are inherently flame resistant (as opposed to having only a flame retardant coating), this protection is permanent and cannot be washed out or worn away."
Reader from Empower said, "As electrical training specialists, we are keen to develop technical partnerships with solutions providers and manufacturers of safety clothing and equipment intended for use in the utility and manufacturing industries. That way we can understand how and why products are developed, learn about the science behind the products and gain first hand experience of how they react in various environments, applications and situations. This information can then cascade through us to end users to improve their level of understanding of the risks and their effects.
"We were delighted to be invited to DuPont's European Technical Centre in Geneva to learn more about the technology behind Nomex and watch garments made being put through rigorous electric arc and flash fire tests. In comparative tests, flame resistant, protective utility garments made of Nomex scored very highly on the safety scale in relation to some garments made of conventional fabrics, and often exceeded the relevant European and International Standards. Through working with their customers to develop new solutions for the safety and protection of people at work, companies like DuPont are instrumental in improving the safety culture in today's working environments. Large power and manufacturing companies are driving the trend for better employee safety and thanks to this, many more companies are starting to recognize the importance of supplying protective workwear garments that offer high performance characteristics."
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