What is Arc Flash?
An arc flash is similar to a lighting bolt that occurs around live electrical equipment. This can occur spontaneously and often works simply when air moves when the electrical case opens. NFPA has recognized a significant arc flash hazard and is trying to protect workers through the latest implementation of NFPA 70E, the standard for employee safety in the workplace.
About 10-15 serious cases of arc flash occur in the USA every day. Most arc flash causes are caused by the operator.
Most technicians who usually work around electrical equipment with an electric drive are familiar with the arc flash when they see it first hand. It is believed that this is a major car accident: no one expects this to happen to them, so people tend to move with much less caution than they should. So it's with an arc flash, only worse. Like driving, you can make a mistake, or you can do it right when someone slaps you.
In particular, what is an arc flash?
An arc flash is an electric current flowing through an arc outside its normal path, where air becomes a conductor with high thermal energy (5000ºC% 2B) and generates a highly conductive plasma. The arc flash will conduct all available energy and generate an explosive volumetric increase in gases, which removes the electrical doors of the system and potentially creates shrapnel.
What causes arc flash?
An arc flash occurs when the gap between the conductors or conductors and the ground instantly closes. There is always a triggering event that almost always involves human intervention. Typical causes and contributing factors include:
- Accidental contact with live parts
- Insufficient short circuit rating
- Insulation Tracking
- Tools dropped on activated parts.
- Connection errors
- Contamination, such as dust on insulating surfaces
- Corrosion of parts and equipment contacts
- Incorrect working procedures
The vast majority of arc flash malfunctions occur when the door is open or open. The National Fire Protection Agency (NFPA) is the author of NFPA 70, also known as the National Electrical Code (NEC). This document is not intended to provide a comprehensive overview of the information available in the code, but simply to highlight some of the information that may be associated with thermography.
NFPA 70E is a standard for secure electrical installation work.
NEC is an electrical design, installation and control. It does not specifically cover topics such as maintenance and safe operation. National consensus was necessary to ensure safety when working with live electrical equipment. NFPA 70E is a standard for secure electrical installation work. NFPA 70E addresses four specific topics: safety-related practices, safety-related safety requirements, safety requirements for special equipment, and installation safety requirements. NFPA 70 offers to perform a hazard / risk analysis prior to starting work on electrical equipment. The analysis core is based on the limits of the shock and arc flash, which must be performed by a qualified electrical engineer.
Hazards of shock, flare hazards and personal protective equipment (PPE)
Before starting work with live electrical components, it is necessary to obtain an approved electrical work permit, which should include, but not be limited to the following:
- Description of the scheme, equipment for work and location
- Justification of why the work should be done in a stressful state
- Description of safe work practices
- Shock hazard analysis results
- Determining the limits of impact protection
- Explosion hazard analysis results
- Flash Protection Boundary
- Determine the necessary personal protective equipment (PPE) necessary to safely accomplish the task
- Means used to restrict access of unqualified personnel to the workplace
- Certificate of Completion of Work Briefing
- Approval of the performance of the responsible management, security officer and owner
NFPA 70E allows exemption from a safe work permit for qualified personnel who perform tasks such as testing, troubleshooting, voltage measurement, etc., if they use safe working methods and proper PPE. Before working with components in real time, the correct personal protective equipment and safe working practices should be determined by conducting an impact hazard assessment and explosion hazard analysis. The impact hazard analysis will determine the voltage to which personnel are exposed, the boundary requirements and the proper PPE necessary to minimize the potential for shock to personnel. The limits of impact protection are defined as limited, limited and forbidden for distances associated with different voltages.
Unqualified personnel should be informed and warned of the dangers of qualified personnel when working at or near the border of a restricted approach. When an unqualified person must work within a limited boundary, it is important that they are additionally notified of the risks and hazards and are constantly accompanied by qualified personnel. Under no circumstances should they be allowed inside the prohibited border. It is important that a burglary hazard analysis is conducted to protect personnel from injury from an arc flash. The analysis will determine the boundary of the Flash protection and determine the correct PPE. In this case, the flash protection limit is calculated at a distance from the excited parts, where the burn will be “recoverable” (2nd degree) and “incurable” (3rd degree). The guidelines dictate that the flash protection limit for systems with a voltage of 600 V or less is 4 for a cleaning time of 6 cycles (0.1 s) and the current available with a screw connection of 50 kA or any combination not exceeding 300 kA. For all other cleaning periods and bolt damage currents, the flash protection limit is usually determined based on the calculated incident arc energy, taking into account the system voltage, current and cleaning time (where the incident radiation energy is a measure of thermal energy at a certain distance from the fault). In cases where it is not possible to perform these analyzes (or they have not been performed), NFPA 70 provides recommendations (NFPA 70 Table 130.7-C9a) that can be used to determine the required PPE based on the task performed. Instead of researching Flash Hazard, the choice of PPE on assignment is usually allowed. However, for tasks that are not listed in the table, and for a cleaning time other than that listed, a full Flash hazard analysis is required. Using a hacking hazard analysis or task risk assessment, the following table can be used to determine the correct PPE:
Practice of thermographic control Infrared cameras have been used to identify problems in electrical systems for many years. Problems in electrical systems are manifested through connections and conductors that overheat as a result of increased stability due to loose or corroded connections or load imbalances. An infrared camera can easily identify these problems in a thermal image and is an excellent method to detect faults or problematic components before failure. Failure can turn off the electrical system and cause significant lost production, equipment damage and personal injury. Insurance companies use infrared electrical control to determine risk profiles and rates for industrial customers. More recently, thermographers have discovered that they can use ICs to prevent and predict failures, to help further reduce equipment failure time and increase overall safety.
Often, during thermographic checks, panel covers are removed and subsequently replaced, which is contrary to the requirements of the NFPA70E.
Like visible cameras, infrared cameras require a direct representation of an object in a straight line. In most cases, cabinets interfere with cabinet designs that obscure target components to be tested, and thermographers are at risk by opening cabinets or doors trying to gain access to internal components. IR studies of electrical systems are best carried out when the system is under heavy, if not peak electrical load, which requires the thermograph to inspect inside and around living electrical components. As a rule, the electrical system covers are removed during thermographic checks and subsequently replaced. This method of operation contradicts the requirements of NFPA 70E.
NFPA70E recommendations related to thermal inspection
NFPA 70E recommends that only “qualified” personnel be able to perform work within the outbreak protection boundary. Thermographs must be accompanied by “qualified” people if they intend to remove the panels. Both the thermograph and the extra person must be in full. One of the ways the NFPA 70E identifies danger and risk, and the required PPE is based on the activities you conduct around equipment. Risk potentials are determined on a scale of 0-4, where 4 indicates the greatest risk. For example, removing a bolted cover on a 600 V equipment has a hazard / risk classification 3 and which corresponds to a value of 4 at voltages above 600 V. Since this work occurs within the Flash protection boundary, it is necessary to use an appropriate PPE. The required minimum cost of PPE for working with danger / risk 3 must withstand 104.6 J / cm², and the required minimum PPE for working with danger / risk 4 must withstand 167.36 J / cm². Since most of the work done for IR inspection requires the removal of bolted covers, this will be the required PPE.
Infrared windows: eliminating controlled risk
The first rule in any risk assessment is to eliminate the risk, if possible. Infrared windows eliminate many of the risks associated with conducting real-time inspections, as they allow you to directly view infrared cameras of live electrical components without having to open electrical cabinets. They provide an excellent means of accessing electrical equipment efficiently and safely. In addition, a second qualified technician is not required to open and unscrew the shells. The infrared viewing window is basically an infrared transparent material with a holder / mounting housing. Thermographs may even decide not to use the window when inspecting components at a certain distance from the lid and use a protective grill in place of the window. The grill must be IP2X certified (the grid size must provide protection against foreign objects with a diameter greater than 12 mm). This method can significantly reduce the cost of a window, as well as provide an additional advantage for conducting state-of-the-art inspections of an electrical switchgear. However, when using grills, operators will be exposed to living electrical components, and they must wear an appropriate level of PPE, as determined from an analysis of the hazard of the Arctic flash switchgear. Infrared windows eliminate many of the risks associated with live infrared checks because they allow you to directly view infrared cameras of live electrical components without having to open electrical cabinets. The design of the optics holder depends on a number of parameters: the field of view, the equipment lens and the window size are all the functions of the design and must comply with all the parameters that the thermograph requires before the holder is manufactured. In addition, a protective cover should be included in the design, since the crystals are very expensive and in some cases extremely fragile. Infrared windows are available in various sizes and can be custom made to replace dead facades on switchboards and switchboards. The larger the window size, the larger the field of view, which can be seen using an IR camera.
Recommendations for installing infrared windows
In order to properly install infrared windows, it is necessary to identify targets that require verification. As a rule, traditional research concerns only bolted connections in a switchgear. They are usually considered the “weakest points” or “points that are most likely to fail.” These may include:
- Cable connections
- Bus connection
- Insulators or connectors
The required window size will depend on the following:
- size of viewed objects and their distance from the panel cover;
- angular field of view of an infrared camera and transparent apartheid;
- the ability of the camera to focus on nearby objects and be placed next to the window.
View through infrared window
The infrared window allows the camera operator to check the inside of the electrical cabinet to check the physical state of the components you have chosen to test. As with traditional thermographic checks, we can see the temperature differences very clearly. You must have confidence in the infrared windows that you use. They are designed to transmit infrared energy through them at a known transmission rate; therefore, if there is even a slight difference in temperature, you can see it with an infrared camera and be able to record images for an infrared inspection program.
Recommendations for installing infrared windows
Installing an infrared window requires cutting holes into a very expensive switchgear. Therefore, it is very important to be sure that they are installed in the right place and that the switchgear ratings do not deteriorate in any way. Before installation, consider the following factors:
- NEMA or IP address of switchgear and IR windows: Remember to never install an IR window with a lower rating than the switchgear rating.
- Test certificates: Убедитесь, что ИК-окна были протестированы и одобрены органами по сертификации в качестве распределительного устройства, для которого они предназначены (например, UL, IEEE, Lloyds).
- Внутренние препятствия: Перед снятием внутренних покрытий или кабелей Perspex / Plexiglas убедитесь, что вначале рассматривается утверждение местного менеджера по безопасности. В некоторых случаях вы не сможете полностью удалить крышки и сможете изменять их только путем сверления или пробивки отверстий, чтобы сохранить требования IP2X для некоторых распределительных устройств.
- Взрывоопасность (если применимо): Некоторые панели расположены в искробезопасных зонах и поэтому никогда не могут быть изменены в полевых условиях.
- Диэлектрические зазоры: В тех случаях, когда ИК-окна используют решетки или контрольные отверстия, они должны соответствовать IP2X (13 мм 0,5 дюйма), и клиенты должны были знать о безопасных диэлектрических зазорах для типа распределительного устройства, которое они намерены установить в окно, показанное слева ( из таблицы A.3 IEEE C37.20.2) указаны минимальные расстояния от живых компонентов, и рекомендуется, чтобы они были рассмотрены в качестве стандарта для грилей / смотровых отверстий.
При использовании инфракрасных окон важно исправить потери при передаче окна и коэффициент излучения компонента, который должен быть проверен через ИК-окно.
Другой способ использования ИК-окон - подготовить все компоненты, которые должны быть проверены, чтобы они имели одинаковую излучательную способность (например, с помощью электрической ленты, излучательной краски, ярлыков IR-ID). В этом случае все контролируемые компоненты будут иметь одинаковые значения скорости передачи и коэффициента излучения; следовательно, полученные результаты будут намного проще сравнивать.
Может ли IR Windows переносить общий рейтинг дуги?
Электрическое распределительное устройство имеет много разных форм и размеров. Поверхности и объемные элементы шкафов различаются по каждой модели, типу и рейтингу. Каждый кабинет подчиняется испытаниям, которые устанавливаются органами сертификации, такими как UL, IEEE и т. Д. Этот тест завершается на узлах шкафа, а не на компонентах, составляющих сборку. Конструкции и размеры электрического шкафа бесконечны, и мы там, прежде чем НЕ МОЖЕМ или НЕ ДОЛЖНЫ использовать данные из одной конструкции корпуса для другой конструкции, если они не идентичны во всех отношениях. Вот почему компоненты никогда не носят общий номинальный рейтинг дуги и должны быть подчинены отраслевым стандартным тестам, чтобы подтвердить, что они соответствуют минимально требуемому уровню механической прочности и экологическим свойствам для электрических шкафов и сборок, которые они собираются вставить.
Conclusion
Из-за частого возникновения дуговой вспышки в промышленности чрезвычайно важно знать риски, связанные с проверкой высоковольтных распределительных устройств и связанных с ними предметов. Проблемы, связанные с безопасностью оператора из-за события дуговой вспышки, заставляют инспекторов принимать новые методы в соответствии с NFPA 70E, стандартом для безопасной электромонтажной практики. Ударные и флэш-анализы В большинстве случаев требуется анализ опасности. Также доступны рекомендации по индивидуальному защитному оборудованию. Одна новая общая практика безопасности связана с использованием инфракрасных прозрачных окон, которые устраняют многие риски, связанные с инспекциями в режиме реального времени, поскольку они позволяют инфракрасной камере иметь прямой вид живых электрических компонентов без необходимости открывать электрические шкафы.
