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ICOUM 2014 11th International Conference on Live Maintenance 21-23 May 2014 Budapest, Hungary Function and use of arc fault protection systems in low-voltage switchgear installations Rainer Ziehmer, DEHN + SOHNE GmbH + Co. KG. Abstract: The thermal risk for electrotechnical switchgear installations resulting from arc faults is considered and assessed based on the “Guideline for the selection of personal protective equipment when exposed to the thermal effects of an electric fault arc“ published by the ISSA (International Social Security Association ). In this context, suitable “personal protective equipment“ (PPE) must be worn to protect workers from second-degree burns. However, the protective effect of this PPE is limited depending on the arc fault energy to be expected. Technical measures can also be implemented to additionally protect workers during live working. Arc fault protection systems are capable of reducing the arc energy to an acceptable level by limiting the persistence of the arc fault to some milliseconds. This paper describes the structure, mode of operation and implementation of these protection systems. Index Terms: arc fault protection systems; electric fault arc I. STATUTORY AND LEGAL FOUNDATIONS The German Occupational Health and Safety Act I requires employers to perform a risk analysis for the relevant workplace of their employees and to define and take adequate occupational health and safety measures. If this is not the case, not only employees are at risk. Deliberate or gross negligence may also result in fines or even imprisonment. In addition to the German Occupational Health and Safety Act, the German Ordinance on Industrial Safety and Health 2 forms the basis for safe and adequate work practices. Moreover, the ISSA guideline for the selection of personal protective equipment when exposed to the thermal effects of an electric fault arc 3 applies. When working on electrical installations, the EN 50 II 0- 1:2004 standard 4 must be observed and implemented. This standard includes different work methods and describes the associated procedures. Working in the vicinity of live parts or live working gains more and more importance in practice. According to accident statistics, these work methods do not necessarily increase the accident risk. As with all work, conscious action and suitable measures such as risk analysis, organisation, training programmes and the selection of adequate work equipment ensure quality and especially safety. In addition to the measures described above, the employer is obliged to provide suitable protective equipment. This paper describes arc fault protection systems which can be used for the work to be performed depending on the result of 978-1-4799-5993-8/14/$31.00 2014 IEEE the risk analysis to protect employees whilst working on electrical installations. II. FORMATION AND EFFECTS OF ARC FAULTS Due to the energy turnaround in Germany, live working on electrical installations will become increasingly important in the future. Do you always think of the possible risk of an electric arc when disconnecting an installation? Do you wear appropriate personal protective equipment? There are many sources of danger. The insulating clearance between live parts, for example, can be reduced over the years as a result of pollution in conjunction with air humidity or falling tools can cause a short-circuit and thus an electric arc. Moreover, loose contacts or animals in the installation can also lead to an arc fault. About 160 arc fault accidents resulting in serious injury or death occur in Germany each year. The extremely high temperatures cause skin burns which require comprehensive medical treatment such as skin transplantations. Full recovery is often not possible. Temperatures up to 20,000 C can occur in the core of the electric arc. Consequently, plastics and metals melt and evaporate. Inhaling the resulting toxic gases and the pressure wave caused by flying debris are also a serious risk. Considering all these factors, effective protection during work on electrical installations is imperative to ensure that you do not have to ask yourself: “What could have been done better?“ III. ARC FAULT PROTECTION BY MEANS OF TECHNICAL MEASURES The guideline for the selection of personal protective equipment when exposed to the thermal effects of an electric fault arc makes it easier to select adequate personal protective equipment. The representative selection of classified personal protective equipment in Fig. I shows its limitation of use depending on the transformer output and the tripping time of the protective equipment. R. Ziehmer Function and use of arc fault protection systems in low-voltage switchgear installations Rt!prsenldtivl! 1iOn with the following pa(arnlIS: Ultn 400 v U, 6% 300mm k,. 0.25 k, 1 (small cabinets) D(HNcarc“ AP(i,APJ,APr,AP( c:1a2 .ClilSS1 , , , , I rn _ rn _ _ Fig. 1. Representative selection of classified PPE But what if the arc energy to be expected is so high that thermal protection alone is no longer ensured by personal protective equipment? 400 VI 909 A 630 kVA gTr arc fault PEN PLB.“, 3.5 MW LV power distribution -_I . - I J“ l ,_“j“.2 ,. t“ RM$ f:fO$pachve o.lrrl.:nl I(A) WLB = 5ULB ILB tk = 5200V 10kA200ms = 693kJ Fig. 2. Tripping time of an NH fuse following an arc The sample calculation in Fig. 2 shows which energies are to be expected in case of a transformer power of 630 kV A and a tripping time of the upstream fuse of 200 ms. The resulting arc energy is much higher than the energy of 318 kJ tested in case of class 2. Is it possible to limit the arc energy? Possible solutions are described in the revised IEC 60364-4- 42:2010 standard 5 for low-voltage electrical installations. 2 On the one hand, the electric arc could be passed into arc resistant sections of the switchgear installation and could be safely enclosed there. For safety reasons, this cannot be implemented or can only insufficiently be implemented for work on open installations. This measure also negatively affects the availability of the installation since the affected parts must be cleaned or exchanged. Moreover, passive systems completely insulated with arc-fault-resistant materials which are supposed to prevent flashover between live and earthed parts of the switchgear installation are available. Since all connections and busbars must be integrated, this is often difficult in case of complex installations and can only be implemented with great effort . According to (1), the magnitude of the arc energy to be expected depends on the arc voltage, the arc current and the time until the current is disconnected. Since the arc current is limited due to its impedance, this also affects the disconnection time of the protective device. The lower the disconnection time of the protective device, the lower is the arc energy to be expected. Therefore, solutions are preferred which use quick acting fuses or change the tripping characteristic of the circuit breaker to minimise the energy for the duration of live working in case of an electric arc. To this end, e.g. the upstream fuse must be exchanged, meaning a much higher additional effort. A risk analysis must be performed based on the tripping time and adequate PPE must be selected. Moreover, arc fault protection systems are described both in the ISSA guideline and in the IEC 60364-4-42:20 I 0 standard. While the focus of the ISSA guideline is on personal protection, the standard mainly deals with systems that increase the availability of the installation. These arc fault protection systems have an identical mode of operation. After the electric arc has been detected, the short-circuiting cartridges situated between the phases cause a metallic short circuit. In this process, the electric arc and the temperature rise are limited due to the extremely short arc duration so that second-degree burns are prevented. At the same time, the transformer fuse or the circuit breaker is tripped. While the metallic short-circuit is caused in the short-circuiting cartridge, not only the thermal effects, but also the toxic effects and the pressure are limited. In comparison to Fig. 2, Fig. 3 shows that the energy of the three-pole electric arc is considerably reduced if an arc fault protection system is used. In the example, WLB is reduced by a factor of 40 since the disconnection time is exactly reduced by this factor. Nevertheless, PPE must always be worn even if an arc fault protection system is used. ICOUM 2014 11th International Conference on Live Maintenance 21-23 May 2014 Budapest, Hungary 400V/ 909A 630kVA gIr L1 L2 3 PEN DEHN:u: short-:ircuiter DEHNarc connol unit arc fault PLB“, 3.5MW LV power dis.tribution Fig. 3. Limitation of the arc energy by means of arc fault protection systems In addition to fixed systems, there is also a mobile arc fault protection system which can be used for live work on open switch racks up to 630 kV A. 3 In addition to visual arc detection by means of sensors or fibre optic cables installed in the installation, a current detector connected to them can detect the electric arc / arc current Fig. 4. Mobile components of the arc fault protection system within some milliseconds, thus activating the short-circuiting unit. The resulting metallic short-circuit extinguishes the electric arc and disconnects the protective device. The IEC 60364-4-42:20 I 0 standard describes arc fault protection systems which work according to this principle and must extinguish the electric arc after at least 5 ms. The ISSA guideline also recommends to use this mobile arc fault protection system to ensure personal protection. The difference between mobile and fixed systems is that the mobile components (Fig. 4) - Control unit with three light sensors, power and control line sockets - Disconnecting blade - Two short-circuiting cartridges - Junction piece can be integrated in open switch racks within a few minutes, are removed after the work has been completed and can be used for the next installation again. This minimises investment costs since the system must be acquired only once. The light sensor supports, NH in-line fuse switch disconnector and the retaining device for the control unit are firmly mounted in the switch rack and are used to mount the control unit, sensors, short-circuiting cartridges, disconnecting blade and junction piece. The NH in-line fuse switch disconnector designed for this purpose, which are rated for short-circuit currents up to 25 kA, can also be used during normal operation, for example as a spare outgoing strip. After the sensors and all other mobile components have been fixed and interconnected via the control lines, an internal routine tests all components so that maximum arc fault protection is ensured during live working (Fig. 5). The sensors are arranged in such a way that they always detect an arc fault. Since this system is used for open switch racks in closed rooms, false tripping by extraneous light such as the flashlight of a digital camera is excluded. Even if a fuse is pulled under load, the system is not tripped. R. Ziehmer Function and use of arc fault protection systems in low-voltage switchgear installations Fig. 5. Working on an open switchgear installation (mobile arc fault protection system installed) IV. SUMMARY The “TOP principle“ is used to define the priority of the measures for risk analysis. In this context, technical solutions have priority over organisational measures and PPE. According to clause 4, section 2, of the German Occupational Health and Safety Act, hazards shall be eliminated at their source. The arc fault protection system described in this paper is used to reduce the thermal, toxic, dynamic and radiation effects to a minImum. A two-year trial phase has shown that the DEHNarc system provides reliable arc fault protection and is suitable for everyday use. The fixed components can be installed in the relevant secondary substations during maintenance work and are not removed so that they can be used at any time. The DEHNarc system (sensors and control unit) only has to be purchased once and is used in all pre-equipped secondary substations during live