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Combining the best of both worlds Copyright Material IEEE Paper No. ESW-2008-22 Lt.Cdr. F.G. Marx, M.Sc. Netherlands Defense Academy & Quercus Technical Services Baileystraat 3 8013 RV, Zwolle The Netherlands [email protected] [email protected] Abstract – US and Canadian electrical safety standards are state of the art. A wide variety of possible electrical hazards, such as arc-flashes , are covered. European standards have not developed this way (yet). However, elements from these European standards might be of interest for the US electrical safety community. Three of these elements are considered in this article. This article explains possible switching strategies that could reduce the risk of an electrical shock during indirect contact with live parts, in the three basic grounding schemes. It covers an important strategic difference between European and US electrical safety standards that enables a wider variety of installation owners to be compliant with the electrical safety standards. Finally, a certification strategy that increased the common electrical safety awareness in the Netherlands is described voltage installations. Chapter three compares two differences between the NFPA 70E and the general Index Terms — electrical shock, safety training, touch potential, certification schemes, grounding schemes. I. European workspace safety standard, the EN-50110: one can be found in the scope, the second in practical working procedures. Chapter four covers a popular Dutch certification strategy that certifies workers based on the EN-50110. Certification op personal has dramatically improved safety awareness in the Netherlands. Chapter five concludes this article. INTRODUCTION Risk is commonly regarded (and accepted) as the product of “possibility” times “effect”. US standards cover more possible effects of potential electrical hazards than equivalent European standards. The biggest gap between the US and Canadian (US&C) electrical safety standard (such as the NFPA 70E [1]) and the equivalent European standard (the EN-50110 [2]) is the absence of any reference to arc flashes in the European standard. European standards have not developed in this direction (yet). Despite of this gap, there are some interesting features from European standards that might be of interest for the US electrical safety community. Some of these elements will be discussed in this paper. Figure 1: touch potentials versus switch off time [6] A sub-goal of this article is to show the role of a document called IEC 60479 [7] in European standards. This document explains the way the electrical impedance of a human body changes as a function of the “touch potential”, the degree of moisture of the skin and the current path. From the data presented in this document it is possible to derive a graph that indicates the maximum amount of current a human body can handle given a certain amount of time (figure 6), as well as its equivalent voltage with respect to time (figure 1). Example: figure 1 explains that a human body can handle a touch potential of 100 V (50 or 60 Hz) during 0,45 seconds. While an electric potential of 100 V direct current (ripple free) can be touched safely for an infinite amount of time. This information can be used to determine switch off times during The second chapter of this article describes the way “indirect contact” is handled by European documents equivalent to the US National Electrical Code (NEC) [3]. In this case the Dutch NEN 1010 [4] and NEN 1041 [5] have been selected. The NEN 1010 regulates the design of lowvoltage installations, the NEN 1041 the design of high- 1 internal faults in subsystems as will be explained in the following chapter. II. covered by means of a typical European system: Transformer T1 is the power source in figure 3 Generator T1 is the power source in figure 3 and 4. Phase voltage of 230 V Line voltage of 400 V Frequency of 50 Hz. COMPARING NEN 1010 AND NEN 1041 WITH NEC This chapter covers protection against indirect contact in the three principal grounding systems: IT, TN and TT. A. Touching a metal enclosure that carries an electrical potential as a result of an internal fault is called “indirect contact” (see figure 2). European standards , such as the NEN 1010 or NEN 1041 (equivalent to the NEC) state that figure 1 is to be consulted in order to protect personnel in case of these internal faults. Touch potential and switch off time in a TN-system. Figure 3 shows a TN -system with an internal, fully developed earth fault. Explanation of the data found in this figure: L1 phase 1 . L2 phase 2. L3 phase 3. N Neutral PE Protective Eearh Uf Touch potential RB grounding resistance of the supply source gL type of fuse. IF Fault current Figure 2: indirect contact [8] First some definitions: European standards define three different (basic) grounding systems based on three letters: T Ground (French: Terre) N Neutral I Insulated The three possible combinations are that define systems are: TN, TT and IT. Figure 3: a fully developed earth fault in a “TN”-system [9] The first letter indicates the way the supply s ource is grounded. The second one indicates the way metal enclosures are grounded (table 1). TABLE I European standards state that the maximum touch potential Uf has to be calculated. A fully developed earth fault will create this condition. Therefore: DEFINITION OF THE DIFFERENT GROUNDING SCHEMES IF ⋅ Z s ≤ U0 Where: Zs m inimum impedance (in Ω), equal to the wire impedance of the phase L3 (ZL3) plus the impedance of the protective earth (ZPE). Uo Phase voltage Covering the details of these systems is not part of this article’s scope. This section focuses on a speci fic element: protection against indirect contact as regulated in the NEN 1010 and NEN 1041 and the role document IEC 60479 is playing. The three different systems (TN, TT and IT) are To calculate the fault current (If) and touch potential (Uf). IF = 2 Uo Uo = Z S (ΣZ L 3 + ΣZ PE ) Entering values: Therefore IF = resistance Ra high, hard to predict and might vary (depending on the soil). The touch potential U F is equal to the voltage across Ra plus the voltage across the protective earth. So, the (high) resistance Ra determines the (therefore high) touch potential. As this “touch potential” is high and hard to predict this system is not popular. In fact, it is hard to meet the design criteria stated in the NEN 1010 or NEN 1041. Typically, when this system is used, extra protective connections to earth are added. This should keep the touch potential within limits as it reduces Ra due to extra paralleled connections. Despite of these drawbacks, this system has a couple of principle advantages: a high resistance in the current path means a relatively low fault current. 230V = 1150A (0,1Ω + 0,1Ω ) Uf = ΣZ PE ⋅ I F = 0,1 ⋅ 1150 = 115 V By inspection of figure 1 the conclusion can be drawn that a touch potential of 115 V has to be switched off within 0,4 seconds. This means that the gL-fuses has to disconnect this 1150 A of fault current within 0,4 seconds. If a fuse cannot meet this requirement it has to be replaced by an other fuse or switch. The same design principles with respect to indirect contact are valid for “IT”-systems as well as “TT”-systems. A “TN”-system was used in this article as an example to calculate the touch potential as it is much more straightforward compared with an “IT”-system or “TT”system. The following section covers details of the touch potential in “TT”-systems and “IT”-systems, although no calculations are included. B. The characteristics in terms of touch potential of an ITsystem are covered in the next section. C. Touch potential and switch off time in an IT-system. The best system, in terms of touch potential is an “IT”system (figure 5) or ungrounded system. In an ungrounded system there is no direct connection between the starpoint of, in this case, generator G1 and ground. Touch potential and switch off time in a TT-system. As a result of the absence of this connection between starpoint and earth, the fault current is usually very low. This situation can be compared with the case of touching a battery on one side. Touching the plus only will not result in a current path as there is none. However, in a three phase energy distribution system things are a little more complicated. As a result of cable capacitance a (usually) small current will flow. A “TT”-system (figure 4) is not used frequently in Europe. The difference between this system and a TN system is the absence of a Neutral. However, as the second letter indicates , one or more extra grounding leads are conne cted to metal enclosures. Figure 5: a fully developed earth fault in an “IT”-system [11] G1 Where: Ra grounding resistance of the metal enclosures Id Capacitive short circuit current The magnitude of this current depends on the length of the cables, type of cable insulation and the system voltage. Dutch design rules and regulations state that the current in a one phase to ground fault in an ungrounded system should not exceed 100 A. As a result of this fairly low “short circuit” current, a touch potential lower than 50 Volts is usually easily met. Figure 5 shows an IT-system . Generally the gentlemen will keep smiling while touching a metal enclose with an internal fault. The capacitors drawn in this picture close the return path. As the impedance of these capacitors are high the current Id referred to as the capacitive short circuit current, is low. And therefore it is relatively easy to comply with the IEC 60479 requirements stated in figure 1. Due to the low fault current the “touch potential” will normally be lower than 50 VAC (generally accepted safe touch potential). Figure 4: a fully developed earth fault in a “TT”-system [10] Where: Ra grounding resistance of the metal enclosures Ia Fault current G1 Generator 1 A big disadvantage of this TT system is the fact that 3 G1 As a result of all this, the EN-50110: “it is recommended that persons responsible for such installations (such as communication systems) should use this standard as a guide to the aims to be achieved in setting out their rules and procedures”. Some Dutch companies that are involved in communication systems have embraced the EN-50110. This concludes the second chapter. The third one compares the EN-50110 with the NFPA 70E. III. COMPARING THE EN-50110 WITH THE NFPA 70E This chapter covers differences between the EN-50110 and the NFPA 70E in terms of the scope and working procedures. An interesting conclusion based on the IEC 60479 is included in the scope of the EN -50110. Several companies involved in radar systems, communication equipment and ships (excluded from the scope of both documents) used the EN -50110 to set up their working codes. Secondly, a difference in working procedure is covered. A. Scope of the EN-50110 versus NFPA 70E They succeeded in setting up a mode of operation that is compliant with the European version of the NFPA 70E. The companies benefit is the fact that there is no need of setting up a safety standard from scratch. But these people are not the only ones benefiting form this conclusion derived from “figure 6”. Converters contain several types of capacitors. These capacitors are present to protect switching components (snubbers). These capacitors might not harm anyone when directly touched. So why not exclude them from the stringent parts of the NFPA 70E, but leave a company free to use the NFPA 70E as a “guide”. The document IEC 60479 plays a second important role, next to system design. It also determines the type of installations in which Europeans are allowed to work live. According to the EN-50110 it is allowed to work live (without PPE) in any installation with a touch potential lower than 50 Volt AC (50 or 60 Hz) or 120 Volt DC (there are some energy limitations), as can be concluded from figure 1. But this conclusion is not really that exciting. Figure 6: body current with respect to time [12]. A more exciting conclusion can be derived from figure 6. Figure 6 explains in detail the amount of current with respect to time a human body can handle. Several companies have used figure 6 by claiming: if the amount of current that can be drawn from any electrical component cannot harm any person (region 1 and 2 in figure 6) because the energy storied is low: working live is allowed, regardless of the voltage (!). This comment concludes the first section of this chapter. The second part covers a significant detail in working procedures. B. Working procedures The EN-50110 describes the way employees and employers have to be trained as well as the level of training. It defines three levels of “qualified personnel”. The definitions used in this article may look strange but these are directly cited from the English version of the EN-50110 [13]: This conclusion is an important one for those who work with communication systems (excluded in the scope of the NFPA 70E and EN-50110). These people usually work with high voltage, low energy components. These companies used to refuse the (so called) energy distribution standards because the assumption in the energy distribution world is: high voltage means high energy. These people claim: nobody steps away 3 ft from a 138 kV capacitor in a television for a good reason: the maximum current that can be drawn from this capacitor might (only) be 0,1 mA (not dangerous at all, according to figure 6). 1) Nominated person in control of an electrical installation That person who has been nominated to be the person with direct management responsibility for the electrical installation 4 2) Nominated person in control of a work activity That person who has been nominated to be the person with direct management responsibility for the work activity. The Dutch government supported th e idea of certification schemes. During this period the government wanted to establish a situation where the “market” created their own set of rules based on European standards . Government support is an important step when certifying procedures or persons. Especially when companies want to prove that the electrical safety program is compliant with the relevant standards. The government supported the basic idea that if a person is certified, his knowledge, attitude and skills are sufficient and at such a (predetermined) level that he’ll be able to perform all his tasks in a safe way. A foundation called “STIPEL” [15] was therefore created. In this foundation the government, employers and employees as well as training centers and certification companies united. The mission of this foundation is: to create certification schemes in order to certify employers and employees. These certification schemes are actual, reflect a modern level of engineering skills and are redefined on a regular base. 3) Skilled person A person with relevant education and experience to enable him or her to avoid dangers which electricity may create. The first person is usually the manager, the second one is the experienced chief of the work floor. A skilled person could be the employee trained to perform certain tasks. The EN-50110 as well as national standards describe working procedures and the responsibilities in a lot of detail. This doesn’t seem to be that spectacular, but some details might be of interest. An interesting example from the EN50110 [14]: When the nominated person in control of the work activity is satisfied that the electrical installation is ready to be re-energized, notification shall be made to the nominated person in control of the electrical installation, stating that the work is finished and the electrical installation is available for reconnection. This is (roughly) how it works: Employers and employees are responsible for the content of the certification schemes. Certification companies check exam procedures, the quality of the exams and hand out certificates after passing exams. The government accepts these certificates as an official document with all its consequences in case of accidents. Training centers have created education programs that are fully compliant with the certification schemes. - This means in reality that the “chief” (normally not around during work performed by a skilled person) has to step in his car and drive to the “skilled person” to check whether it is safe to re-energise. The responsibility of the process of reenergising has been appointed to the most experienced person. This, according to the EN-50110, should reduce the risk of accidents from happening. Notice that the EN-50110 acknowledges the existence of an installation manager. This has to be a person with an engineering degree. This person has got direct management responsibilities. He, therefore, is able to shut down the plant as he feels it is not safe. The installation manager can increase the safety awareness of the direction board. While developing these certification schemes it turned out that the manager and chief required more safety training (in terms of time). As a result of: the level of the training and the responsibilities the chief and manager stay at school for (usually) ten days before they are fully qualified. While the “skilled person” usually requires a seven days training period at school in order to pass the exam. The certificates are valid for a period of three years. Extending this period for another three years means that one has to pass exams again. Certification has increased the level of safety awareness drastically in the Netherlands (this is not a European effort). It might be a useful system in any country in the world. There are other examples or articles that might be or interest, but this arti cle covers two of them. Readers who like to read more can always order a copy of the EN-50100 via the internet. This article continues with certification schemes. IV. CERTIFICATION SCHEMES V. Fifteen years ago the Dutch energy distribution market (employers and employees) decided that they wanted to set a generally accepted standard for the education of employers and employees. At that moment different companies interpreted OSHA-equivalent laws, European standards and National standards in a different way. As a result of this the competency and electrical safety awareness of employees was different, depending on the company one was working in. CONCLUSIONS This article explained several elements that could be used to increase the already impressive electrical safety standards in the US and Canada. These elements are derived from Dutch designs standards, such as the NEN 1010 and NEN 1041 as well as the European safety standard EN-50110. Certification is not directly stated in thes e standards , but it has improved the national safety 5 awareness in the Netherlands dramatically. Switch off times related to touch potentials might be a good method to prevent personal accidents in case of a shock. The scope of the NFPA 70E could be modified in order to increase the number of companies embracing the NFPA 70E as a baseline document. Defining an “ins tallation manager” could be useful to increase the safety awareness of the managing board of directors. The responsibility of, for example reenergizing, has been appointed to experienced persons and not the skilled person. In a globalizing world benchmarking is a powerful tool to increase the level of standards. This article has covered interesting options for future improvement of the US and Canadian standards. But on the other hand, the author is pushing European and Dutch safety standards to include form s of arc flash analysis. We all learn from each other! [7] IEC 60479-1 Edition 3.0: B. 1994: Effects of current on human beings and livestock – Part 1: General aspects. [8] Quercus TTS, high voltage safety training, the installation manager. [9] Quercus TTS, low voltage safety training, skilled person [10] Quercus TTS, high voltage safety training, skilled person [11] Quercus TTS, low voltage safety training, the installation manager [12] from the IEC 60479-1 Edition 3.0: B. 1994: Effects of current on human beings and livestock [13] EN-50110, article 3.2.1, 3.2.2 and 3.2.3 (1998 version) [14] EN-50110, article 6.2.7.(1998 version) [15] homepage: www.stipel.nl VI. V. VITA REFERENCES Ivo Marx is currently working as a senior lecturer at the Royal Netherlands Defense Academy. He has sailed on different types of warships at different levels (on missions such as “enduring freedom” and “counter drugs”operations). Previous occupations were project engineer and head of the electrical safety section at the Navy’s engineering school. As a results of this position he has been heavily involved in the area of electrical safety in the Netherlands.. In 2000 he received his Masters in Marine Engineering (electrical option) at the University College of London. As a result of personal inte rests he got involved with Quercus TTS and the IEEE ESW workshops. [1] Standard for Electrical Safety in the Workplace, NFPA 70E, 2004 [2] EN-50110-1: 1998: Operation of electrical installations [3] National Electrical Code, NFPA 70, 2005 [4] (Dutch standard): NEN 1010: veiligheidsbepalingen voor laagspanningsinstallaties (safety regulations for low voltage installations) [5] (Dutch standard): NEN 1041: veiligheidsbepalingen voor hoogspanningsinstallaties : (safety regulations for high voltage installations) [6] NEN 1010, figure 41Z (2003 edition) 6