Arc flash versus arc resistant; different concerns

Transcription

1 Technical Collection Technical paper N PCIC Middle-East -78 Arc flash versus arc resistant; different concerns Didier FULCHIRON, Schneider Electric France. Abstract "Arc flash" and "arc resistant" terms became common in the industry when speaking about the possible consequences of an arcing fault on an electrical installation or within an electrical switchboard, and they are often mixed together. However, their relevancies differ significantly regarding the conditions of the event considered as well as the characterization which can be performed on any piece of equipment. Arc Flash protection is related to the protection of an operator from the direct thermal radiation from an electrical arc, and how such thermal radiation can be quantified, while Arc Resistant classification, or Internal Arc Classification, characterize the capability of a closed piece of equipment (switchboard) to contain the effect of an internal fault without creating a burn risk for individuals around. This paper aims at reminding the basis of these two concepts and at showing how they should complement each other for personal safety, according to the various environment and working conditions. It will also illustrate the scopes of the applicable reference documents, documents which are partly voluntary standards and partly regulation. Keywords: arc-flash, arc-resistant, switchgear, maintenance PCIC

2 Introduction Safety of workers in electrical installations has always been paramount. With the increase of the power, and of the fault currents, and the evolution in the technologies of switchgear and installations themselves, the risk level linked with any arcing fault shall be properly assessed, and managed. Among the available tools, arc flash calculations and internal arc classification are two different and complementary approaches. They have both some reference documents, and their own scope. It is valuable to review these differences in order to avoid possible misunderstandings. The analysis presented in this paper is focused on medium voltage scope. Similar concerns do exist with low voltage installations and equipment, but the reference documents are not all the same, and the internal arc classification does not exist for low voltage. Historical Background A Arc-resistant During the 60's and 70's, and linked with the large development of metal-enclosed switchgear for replacing the open type installations, attempts for characterization of the behaviour of switchgear under fault conditions have been introduced. The German laboratories association PEHLA proposed in 1969 a test method which has been used for several year as the only recognised performance demonstration. This "internal arc test" had still many open options to be discussed between stakeholders but, from the very beginning, it focused on the effects on an operator of the hot gases flowing out of the enclosure. End of the 70's (1978), the IEC (International Electrotechnical Commission) introduced such a test in the standard dedicated to metal-enclosed switchgear assemblies - the IEC (withdrawn). At that time, the test was optional and subjected to agreement "between manufacturer and user". Based on the IEC document, North America developed also a guidance document, initially the EEMAC G14-1 [1] applicable for the ANSI Metalclad switchgear. Several improvements led to the publication of the IEEE C [2] in 2001 "Guide for Testing Metal- Enclosed Switchgear Rated Up to 38 kv for Internal Arcing Faults". The term "arcresistant" has been introduced by these publications. Along the years, it became more and more often requested, and evolved towards some kind of optional performance. Acknowledging this situation, the IEC modified the relevant standard to propose a well defined test procedure, as well as codified performance levels. The Internal Arc Classification was then introduced in the new standard IEC [3] (replacing the 60268) first published in The IEEE document has been revised in 2008, but remained a Guide. The second edition of the IEC (2011) clarified even further the concept of Internal Arc Classification by defining it as a rating, with the relevant test as a type test. The concept of internal arc test has been extended to low voltage assemblies by the IEC Technical Report 1641, which is now IEC/TR Ed.3 (2014) [4]. B Arc flash Starting in the 1900's, the National Fire Protection Association, in the U.S.A., proposed electrical installation rules through publication NFPA 70 [5]. Beginning of the 70's, this document became regulation and NFPA prepared a complement dedicated to workplaces, the NFPA 70 E [6]. It took later on under consideration the accidents occurring with arcing faults. They focused on situations where workers face live conductors, most often conductors on which they are operating. Such situations are linked to live-working conditions, often used for Low Voltage installations, and also quite often for Medium Voltage switchgear in North America. This particular concern led to evolution of NFPA 70 E, starting The publication of the document NFPA 70, edition 2002, also known as National Electrical Code, introduced the arc flash concern. The NFPA 70 refers, through informative notes, to the document NFPA 70E which defines specifications for the Personal Protective Equipment (PPE) to be worn when working on electrical equipment. These specifications are drawn based on the radiated heat received by the worker in case he faces an open arc, with the goal of preventing self-inflammation of the garments, and burns of the worker. The NFPA 70 being enforced in many states within the U.S.A. as a regulation, that makes the rules mandatory. Is is also used outside the U.S.A. as a reference document PCIC Page 2

3 about safety of electrical installations, and associated working procedure. This application leads to a request for characterisation of the installations, and of pieces of switchgear, regarding the "arc flash" situations. Such characterisation does not have, to date, any global reference methodology. Conditions of operation When conducting a risk analysis, one shall identify the various situations which have to be considered, according to how the installation is designed, constructed and operated. The key point of personnel safety occurs if, and when, personnel is present. Therefore, the following categories could be investigated: 1. individual in the vicinity without any technical reason (e.g. public area); 2. individual in the vicinity with no interaction with the electrical installation (e.g. visitors, general cleaning); 3. individual interacting with the electrical installation, for normal operation; 4. individual interacting with the electrical installation for maintenance purpose. When considering public area, as in first category, it should be understood as any area which could be accessed by individuals who are not specifically aware of the electrical risk. It covers the general public, but also for instance the workers in a workshop where a switchboard is installed (without a dedicated closed room). These area are normally accessible only when the switchgear is under normal operating conditions, and any work on it will be done after delimitation of a safety perimeter. The second category involves individuals who are aware of the presence of electrical switchgear, and have been informed of the basic safety rules. These rules could imply a defined PPE adapted to the location (close vicinity or dedicated room for instance). For electrical operators, as considered in categories 3 and 4, it is usual (and generally mandatory) to wear PPE, the class of which to be adapted for the intended work. Such PPE generally includes glasses, helmet, gloves, safety shoes and work clothes. The intended work should also be described in a proper procedure which should define any special safety precaution to be implemented to conduct the work; example could be the creation of a safety perimeter as quoted above. Figure 1 Cotton indicators arrangement around switchboard for IAC test Arc resistant approach The concept of "arc-resistant", or of "internal arc classification" addresses the protection of persons in case of an arcing fault event within a metal-enclosed or insulationenclosed switchboard under normal operating conditions. That means typically that there is no open door or panel. Such a condition is relevant when considering categories 1, 2, and 3 as proposed above. A difference could be made between authorised individuals and public, because any authorised individual can be requested to wear working clothes which have a minimum arc resistance (strong cotton clothes are already significantly better than any polyester fabric); that is the idea of the types of accessibility A and B used in the standards. Assessing the behaviour with a closed enclosure means there is no heat radiation impacting directly the individual. The possible heat transfer from the fault to the individual is considered to be through the flow of hot gases generated by the fault. The acceptance criterion covering this effect of heat transfer is defined as the noninflammation of cotton indicators simulating the presence of the individual. PCIC Page 3

4 The basic feature of the enclosure is then either to ensure the containment of the hot gases, letting only mild temperature gases to vent out, or the exhaust of the hot gases through a dedicated opening in a direction considered as acceptable; it could be on a side which is classified as non accessible, or through a duct leading the gases in some secured area. Other safety related acceptance criteria are also part of the internal arc classification for verifying the absence of risk of mechanical injuries for the individual; they include the fact that no door nor panel open and that no dangerous solid part is projected. In the IEC classification, clear precedence is given to safety of individuals around the switchboard, and therefore no criterion is provided about the resulting damage level inside the whole enclosure. In the IEEE document, an additional suffix "C" is defined, which introduces an acceptance criterion between compartments; the added value of this suffix is not completely clear as the document states: "It does not imply that the equipment may be operated with doors, covers, or panels opened or removed and maintain its intended degree of protection". Attention should be paid also to the fact that the test procedure, with the cotton indicators, is focused on the clothes for someone close to the switchboard, and checks that these clothes will not be ignited; it could be considered as inadequate for protecting the hands and arms of an operator actually touching the enclosure or some control on it. That is a good reason to enforce the use of gloves as part of the PPE when operating a switchboard. Arc resistant approach The concept of arc-flash protection addresses the heat radiated by an arc, and received by an individual. Such a concern only arises when the individual could be in direct view of the electrical arc, and that means he is also in direct view of the active conductors; most of these situations are in the category 4 of the list proposed above. However, some designs of switchgear imply that the enclosure is open prior to perform normal switching operations; with such design, situations of category 3 come under the scope of the arc flash approach. Such designs, for instance requiring opening a door to operate or withdraw a circuit-breaker, should be phased out. The situation of an exposure to the arc after the accidental opening of an enclosure, e.g. by the blast of an internal fault, is never considered. Based on the radiated power and the distance, various thresholds of incident energy could be defined associated with the withstand of defined PPE, and ultimately a distance beyond which no special protection is required ("arc flash boundary"). These thresholds are based on the hypothesis of "survivable burn injuries" for the individual in case of arc flash. The various thresholds are expressed in cal/cm². The wording of NFPA 70E is "Protective Clothing and Personal Protective Equipment for Application with a Flash Hazard Analysis" and five levels of exposure are listed in the chapter "Personal and Other Protective Equipment": 5. Normal working equipment 6. Clothing arc-rated for 4 cal/cm² 7. Clothing arc-rated for 8 cal/cm² 8. Clothing arc-rated for 25 cal/cm² 9. Clothing arc-rated for 40 cal/cm² The first level means the worker is outside the arc flash boundary, and that normal basic PPE provides already the defined level of protection against arc radiated energy. In the Annex H, some simplified methods are proposed for severity assessment, and some other threshold values are also used, as in Table H3 (1.2 and 12 cal/cm²). It shall be noticed that all these thresholds being expressed as energy, and not power, the duration of the arc flash event is also an influencing parameter, and then the protection devices and protection settings influence the working distances covered by the three levels. PCIC Page 4

5 Risk analysis and specification General In all cases, specification of any protection level or protection mean should be made only after a relevant risk analysis. The deliverable of risk analysis is expected to be detailed enough in order to satisfy the following concerns: - an acceptable safety level for the individuals in all situations; - the satisfaction of relevant regulations; - a proper differentiation of the various situations for specification of adapted solutions; - the avoidance of over-specification of protections, which would be both costly and cumbersome; - the adapted procedures where applicable. The reference documents provide some hints for the points which should be investigated and which could contribute to the global safety. No single device, or performance level, could be deemed sufficient to ensure safety. It is well known that accidents are always the result of several mistakes or anomalies, and possibly a protective measure could be implemented for each of these steps. While conducting the risk analysis, one should be aware that arc-resistant and arc flash approaches basically deal with the thermal effects of a possible arcing fault. Arc-resistant performance covers also some possible mechanical risks and, as reminded in the IEC, other kinds of risks could be linked with arcing faults. It is reminded that any performance linked with arcing faults should be the last stage of risk reduction process, because avoiding the presence of personal in front of energised switchgear could be achieved for many usual situations, if it has been considered early in the design of the facility; already civil work could participate to reduce the occurrence, and then remote control and operation or similar solutions. About arc resistance Considering the arc-resistant performance, a table is provided in the application guide part of the standards with a list of items which could reduce the probability of the arcing fault event itself; avoiding the event is obviously a step to be considered before trying to manage its negative effects. However, specifying the behaviour in case of fault could be relevant if the results of the risk analysis show an occurrence and/or a severity which are unacceptable otherwise. The fact that arc-resistant performance does apply on many situations calls for a very large scope of analysis, possibly restricted if the installation can be accessed under strict rules only. Specification of arc-resistant switchgear shall consider the electrical data of the installation, typically as the fault current level and the upstream protections, but also the constraints about installation conditions. Switchgear is delivered with installation instructions the respect of which is necessary to get the expected performance; they deal with the room size, possibly the evacuation of gases and so on. It could be wise to review the risk analysis after the definition of the switchgear and its installation for checking the proper achievement of the risk reduction. About arc flash When dealing with arc flash, the situation is rather complex. Preventive actions can be implemented to limit the risk of occurrence of arc flash, by design of the equipment and by proper working procedures. The risk is linked to either malfunction of the equipment or mistake by the operator. As it exists only in the presence of exposed live conductors, a first point is to ensure that only informed individuals could face such a risk, and that is the primary purpose of many labels. Designs could also incorporate insulated conductors to some extend, and provided that the insulation is rated against inadvertent contacts, it reduces the risk associated to mistakes. The insulation "grade PA" defined in IEC [7] satisfies such requirement. The insulation covering specified for conductors in IEEE Metal Clad switchgear (IEEE C [8]) is also satisfying the same requirement, expressed as "... prevent the development of bus faults that would result if foreign objects momentarily contacted bare bus." However, the risk analysis will probably never conclude that arc flash event can be disregarded. The next step is then to characterise the event in order to adopt the relevant PPE. The NFPA 70E acknowledge the fact that such characterisation involves many data PCIC Page 5

6 and hypothesis, and proposes also some simplified method for determination of the required protection, when dealing with a given installation. A more comprehensive analysis could be performed using the document IEEE 1584 [9], and the application of this document requires much data about the installation and the equipment considered; several conflicting points could be identified: - detailed analysis could avoid over-specification; over-specifying PPE makes the job more difficult; - detailed analysis leads to differentiate each piece of equipment with its own arc flash data; - detailed analysis avoids possible mis-use of the simplification rules - need for installation data requires to do the labelling when the location of each piece of equipment is known; - differentiating PPE, according to which part of a switchboard is accessed, could generate mistakes; - performing a detailed analysis requires skilled people, and could need involving a subcontractor. Figure 2 Example of label for arc flash information Both the NFPA and the IEEE documents provide much more detailed information on low voltage installations and equipment than on medium voltage. That comes partly from history. The LV installations were earlier impacted by regulation of such topic as there are more exposure, and workers are less reluctant to perform live work, sometimes under-estimating the risk level. Some technical issues are also introducing differences according to the voltage level. Among these issues is the fact that arcing faults within MV switchgear are less stable than in LV installations, with possibly many restrike locations and varying arc length, thus more difficult to characterise. When coming to specification of any switchgear, there is clearly a lack of reference document to support writing some verifiable requirement. The fact that the actual arc flash severity is dependant upon the system brings an additional difficulty. The approximation rules proposed in the IEEE 1584 cannot be used as a basis for contracting. An alternative could be, after selection of the switchgear, to order an arc flash study to be performed by skilled provider. The recently published IEEE [10] defines what could be the specification of such a study. Associated protection means PPE PPE, standing for Personal Protective Equipment, covers a wide range of garments and accessories intended to be used by workers facing potential dangerous situations. They are often defined and required by regulation as part of the overall "safety at work" issues. It is widely accepted that all electrical workers operating or maintaining equipment shall wear basic PPE including hard hat, protective glasses, insulating gloves, safety shoes and clothes at least similar to untreated thick cotton fabric. The later point is considered by the internal arc classification, as the access category A for authorized personal uses indicators made of 150 g/m² cotton fabric (the category B uses lighter fabric). The result is that for operating an arc-resistant switchboard, no additional PPE is specified beyond the usual work clothing and equipment. Any task requesting the opening of a door or cover shall be considered as cancelling the arc-resistant performance. PCIC Page 6

7 It is understood that the same kind of basic PPE also complies with the required protection for an individual staying beyond the arc flash boundary. However, in case of exposure to arc flash risk, even beyond the boundary, it is recommended to use a face shield ensuring that no part of skin could be directly exposed. For potential exposure to an incident energy level higher than 4 cal/cm², the NFPA requires dedicated PPE which are "arc-rated" according to the maximum exposure level which could be anticipated. To implement such a requirement, equipment which could be open and provide access to live conductors have to be labelled with the maximum possible incident energy level (see example on Figure 2), and the work procedure for each kind of envisioned task shall remind the pieces and ratings of PPE to be used. Figure 3 Illustration of basic PPE level The higher the rating, and the more the worker is hampered for the task he has to perform. Such draw-back could generate new risks as possible mis-handling of tools. The proper use of the PPE relies on the worker himself, his awareness of the situation and his training to perform the assigned task. The human factor remains a key issue in the global efficiency of such protection. Reducing the fault duration For all arcing fault situation, there is some cumulative effects which should push towards reducing the fault duration. Advantages could be identified in the use of solutions for shorter fault duration, whichever the approach considered: - reduction of heat and gases in the switching room, and this particular point is also linked with personal safety as a general concern; - reduction of the damage within the faulty equipment (possible repair); - reduction of the dust pollution on pieces of equipment in the same room (LV, communication...). Nevertheless, the impact of such duration could be seen as different according to which kind of approach, arc-resistant or arc flash, is considered. For any arc-resistant switchgear, the performance has been established for a rated duration, most often one second. It shall be checked during the installation design stage that the implemented protection plan actually provides a fault clearing within such duration. With modern digital relays, grading between protection stages could be achieved with less than 300 ms, and it is rather easy to comply with the one second limit. However, these limits are sometimes overlooked and feed-back shows that situations do exist where they are not respected. When considering the arc flash approach, the incident energy is directly proportional to the fault duration; no hypothesis about evolving faults is considered, and the duration is always too short for introducing any cooling effect of the surfaces subjected to the heat radiation. Any reduction of duration is beneficial, and all the technical solutions could be investigated. These solutions are not clearly the same as during an internal arcing fault event; typically some effects, like the pressure, could be very different when a door is open and any pressure-based tripping system could become ineffective against arc flash. A first step should be reviewing the protection plan, and all the grading between the various stages, to cancel any useless time delay while keeping coordination. An PCIC Page 7

8 additional possibility sometimes implemented is to switch the protection settings to "instantaneous" during any presence of personal in the vicinity of the switchgear, accepting the risk of loosing coordination in case of fault during the job. The use of upstream current-limiting devices (e.g. fuses) is a very efficient tool, but the applicability of fuses is limited in medium voltage installations (limited current ratings, and difficulties to achieve proper grading). Some other current-limiting devices are available on the market place which are tripped as a very fast protection. Switchboards could be fitted with dedicated protection schemes, like differential protection, or dedicated fast short-circuiting devices tripped by various means (overcurrent, pressure, light, etc.). The capability to trip the higher protection stage, to cover fault on the incoming cables, should be investigated also. Reducing the fault level The total power of an arc is related to the system voltage, as it influences the distance between conductors then the length of the arc, and is considered to be proportional to the prospective current (bolted fault current); the resulting approximate magnitudes are given in the table below. Table 1 Maximum Arc Power (3- phase, MW) The arc power has a direct effect on the consequences of the fault and reducing it will reduce the overall damages. The advantages listed for reducing the fault duration are also valid for reducing the fault current. However, when considering the arc-resistant feature, the safety concern is limited to the availability - and price - of a switchboard with the relevant rating; once the switchboard is installed, reducing further the fault current does not bring decisive improvement in the expected behaviour. As a result of the internal arc classification, the individual benefits already of a protection level consistent with his personnel usual equipment. When arc flash is at stake, reducing the fault level could lead to a reduction of the incident energy significant enough to change down the category, provided the tripping time of the upstream protection does not increase as a result of the lower current value. Reducing incident energy would be an improvement, either allowing the use of lighter PPE or reducing further the criticity of burns which could be experienced. Fault current level is mainly an issue of design of the network, but it could be also an issue of performance of the network. With low short-circuit level, industrial processes could experience perturbations, for instance due to starting currents of large motors, and consequences of the resulting drop of voltage on theother pieces or equipment. Having separate networks for the different categories of loads could be a way addressing this kind of situation; it could provide a safer, but more expensive, global installation. The global cost is worth to be considered, because all or part of the equipment would be specified with lower short-circuit performances, lower IAC ratings, and the over-price is not obvious. In addition, facilitating the operation by reducing the risk level has also an economical advantage. Operation procedures of the network could also help in that matter: as an example, is it permanently needed to keep a tie-breaker closed and operate transformers in parallel? Could presence of operator near the switchboard been allowed only after opening the tiebreaker? And, by the way, if there is a tie breaker, why not locating the two halves of the switchboard in separate rooms? But that addresses another parameter of safety by design... PCIC Page 8

9 Conclusion It has been highlighted that the two concepts of arcresistant and arc flash cover different situations. In addition, arc flash analysis is required by some regulations, depending on where is the installation, while arc-resistant feature is not referred by any regulation to date. The key points of differentiation are summarized in the table below. It appears that both approaches are contributive to the safety. Protection associated to the arc flash could also cover the conditions for electrical workers during normal operation, but that implies that the access to the location is restricted to electrical workers; in such case, perhaps normal operation could be performed remotely. Considering specification, arc-resistant performance is clearly defined and demonstrated as a rating, while arc flash categorization is location dependant, thus under another responsibility. References [1] EEMAC G14-1, 1987, "Procedure For Testing The Resistance Of Metal Clad Switchgear Under Conditions Of Arcing Due To An Internal Fault" [2] IEEE C , 2008, " Guide for Testing Metal- Enclosed Switchgear Rated Up to 38 kv for Internal Arcing Faults" [3] IEC , 2011, "AC metal-enclosed switchgear and controlgear for rated voltages above 1 kv and up to and including 52 kv" [4] IEC/TR 61641, 2014, "Enclosed low-voltage switchgear and controlgear assemblies Guide for testing under conditions of arcing due to internal fault" [5] NFPA 70, 2014, "National Electrical Code" [6] NFPA 70E,, "Standard for Electrical Safety in the Workplace" [7] IEC , 2014, "AC solid-insulation enclosed switchgear and controlgear for rated voltages above 1 kv and up to and including 52 kv" [8] IEEE C , 1999, "IEEE Standard for Metal-Clad Switchgear" [9] IEEE Std , "Guide for Performing Arc-Flash Hazard Calculations" [10] IEEE Std "Guide for the Specification of Scope and Deliverable Requirements for an Arc Flash Hazard Calculation Study in Accordance with IEEE Std 1584" Schneider Electric. All rights reserved. NRJED315651EN PCIC Page 9