DGUV Information 203-078 - Thermal hazards from electric fault arc Guide to the ...

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Annex 3 , Parameters and risk analysis of thermal hazards to...
Annex 3
Thermal hazards from electric fault arc Guide to the selection of personal protective equipment for electrical work (bisher: BGI/GUV-I 5188 E)

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Titel: Thermal hazards from electric fault arc Guide to the selection of personal protective equipment for electrical work (bisher: BGI/GUV-I 5188 E)
Normgeber: Bund
Amtliche Abkürzung: DGUV Information 203-078
Gliederungs-Nr.: [keine Angabe]
Normtyp: Satzung

Annex 3 Parameters and risk analysis of thermal hazards to persons related to electric arcing

  1. A 3.1

    Energy parameters for thermal hazards to persons related to electric arcing

    The electrical energy fed into an electric fault arc is almost completely converted therein and emitted or released back in various forms. The impact of electric fault arcing is, for this reason, determined primarily by the electric arc energy W LB. Electric arc energy clearly identifies the relationships associated with system short-circuit-related arcing. Different network and system conditions will result in different electric arc energies.

    The significant level of exposure or risk a person is subjected to due to the direct thermal impact of an electric arc is the energy density impacting the exposed surface of the skin. This is the incident energy E i that is present as direct incident energy E i0 with the thermal impact of a proximate electric arc. If the person is wearing PPE, then the incident energy should be considered as transmitted incident energy E iT. In the testing of PPE, a determination is made as to whether the transmitted incident energy will exceed the limits for the onset of a second-degree skin burn. A successful test will verifiy that the PPE is arc-resistant and provides protection up to the level of direct incident energy, as per the test settings.

    There is a complicated non-linear correlation between electric arc energy and direct incident energy, which is determined through the specific transmission and exposure relationships, including system configuration and the effective distance between the arc flash and the person (transfer relations). The transmission and exposure conditions related to the thermal effects can be very diverse. A risk analysis must include or cover all related cases and require a "worst-case" examination.

    The correlation between electric arc energy and direct incident energy is known for both protection classes for the PPE Box test (protective textile and clothing) according to VDE 0682-306-1-2. These are control parameters for the test settings and characterize the transfer relations for the test setup.

    The effects of radiation (including reflections) exist during the Box test, particularly as a result of arc flash directivity (gas flow) resulting from the small-scale box structure and through "worst-case" transfer conditions due to the influence of electrode materials. Comparable examinations using other configurations show that, with the same electric arc energy being fed into the Box test structure, the highest level of thermal incident energy results.

  2. A 3.2

    Process of risk analysis

    The electric arc energy W LB that is to be expected within the scope of application must be determined in the risk analysis. The maximum value of anticipated electric arc energy will be ascertained and is measured in kJ. Based on this, it must then be verified that the maximum occurrence of exposure (thermal impact) will not exceed the level of protection and strength afforded by the PPE. The related parameter is then the electric arc energy for the test category being examined in the Box test. The level of equivalent arc energy for the PPE test must cover this level. For specific applications, existing deviations from the distance, geometry and test transmission relationships can be taken into account when determining equivalent arc energy W Lbä.

    When selecting the PPE test or protection class, the relation to the anticipated electric arc energy value must be considered on the basis of the equivalent arc energy. The thermal hazards associated with electric arching are deemed covered if

    W LBW LBä

    applies.

    It should be explicitly pointed out that the test currents used for the Box test categories do not correspond to the usage limits of PPE with respect to the level of short-circuit current!

    The risk analysis is comprised of the following work steps:

    • Determination of the anticipated electric arc energy value,

    • Examination of the PPE electric arc protection level,

    • Consideration of divergent exposure conditions.

    The determinations below will be comprised in the work steps for the workstation or area being analysed:

    • Nominal or stipulated network voltage.

    • Prospective (metallic) short-circuit current.

    • R/X ratio for network or short-circuited electrical circuit impedance.

    • System geometry (electrode gaps and volume relationships at potential fault locations).

    • Working distance (potential electric fault arc onset and combustive locations, minimal effective distances to arc flashing).

    • Type, model, settings and characteristics of the protection device(s) (circuit breakers, fuses or other special protection devices upstream from the work area).

    • Protection level for the PPE test category.

    NOTE:
    It should be pointed out that the different switching states of the distribution network or energy supply system can lead to different short-circuit power readings and energy levels. For this reason, it may be necessary to analyse a number of such cases in a system.

    Analysis of the energy supply system must take place for all work areas, generally meaning from the feed point of the affected network up to the user outlet.

  3. A 3.3

    Work steps

    1. A.3.3.1

      Ascertaining the general operating conditions

      The starting point is to consider the general operating conditions. A list should first be compiled, including network voltage levels, network equipment types and locations, as well as work tasks.

      NOTE:
      In so doing, it must be considered that, for different network switching states and upstream supply systems, different prospective short-circuit current readings can result. Short-circuit current is greatest when the network junction (switchgear bus bar or distributor) is supplied through multiple feeders or transformers. Differing short-circuit current values under different switching states in the same system must nevertheless be taken into account, because electric arc energy at lower short-circuit current levels due to the longer protection device trip times may by all means be greater than that at the higher current levels.

      With respect to (electrotechnical) work activities, all tasks performed on open electrical equipment or where a system must be opened (work in the vicinity of live components, live working) play a role.

      NOTE:
      In the case of type tested switchgear for which the test validation of arc resistance is available (medium voltage: electric arc testing according to VDE 0671-200, low voltage: electric arc testing criteria 1-5 according to EN 60439-1, Supplemental sheet 2), personal protection can always be assumed when operating or performing work tasks on a closed system; this does not need to be incorporated into the further analysis. On non-tested systems, it must not be assumed that the system will remain closed in the event of an electric arc fault and/or that the effects of inadmissible electric arcing will not occur outside the system (e. g. due to escaping hot gases, bursting parts, etc.); this situation must be treated as in the case of an opened system.

    2. A.3.3.2

      Calculation of short-circuit current at the workstations being considered

      A prerequisite for the risk analysis and the selection of PPE is to be knowledgeable about the prospective short-circuit currents or short-circuit powers associated with the equipment (or network junction) potentially being worked on.

      NOTE:
      As a rule, the risk analysis should be undertaken for different workstations in a network or supply system. In larger systems, it is often advisable to develop and observe identical structures and parameters or similar basic electrical configurations (circuitry).

      Short-circuit current calculations are to be performed according to standardised procedures (VDE 0102). Calculation software is usually available for this process. Maximum and minimum prospective 3-phase initial short-circuit AC currents

      I'' k3max

      and

      I'' k3min

      are to be determined for each workstation/system area for the possible/relevant network switching states. Standard determinations of these currents are made for metallic, zero impedance short-circuits (impedance at the fault location is zero).

      Information regarding short-circuit current or short-circuit power can also be obtained through the power supply network operator. It is important to ensure that the fault location short-circuit currents apply to the work location being considered.

      NOTE:
      If the power supply network operator can only provide short-circuit current (or short-circuit power) at the supplying step down transformer for the low voltage network, then a calculation must be made of the short-circuit current for work locations (fault locations) located remotely from the low voltage network transformer, based on the technical data of the supplying medium voltage to low voltage transformer with consideration given to the type and length of low voltage cable used. If applicable, a multi-source feed to the fault location should be taken into consideration.

      In the event of an actual short-circuit (with arc flashing), a reduced current, the electric arc short-circuit current or fault current with an electric arc short-circuit, will flow as a result of the electric fault arc (fault location impedances). If software is available that can be used for determining the short-circuit current associated with an electric arc short-circuit I kLB, then this current should also be determined for the relevant switching states.

      Electric arc short-circuit current can be calculated based on I'' k3min as well as with the help of a current limiting factor k B 5. The following applies

      I kLB = k B I'' k3pmin

      Factor k B is determined on the basis of the arc voltage U B dependent on the network nominal voltage, the R/X ratio of the short-circuited electrical circuit impedance and the electrode gap d (distance between neighboring conductors in the electrical system) 5.

      NOTE:
      The reduction or limitation of the fault current resulting from an electric arc at the fault location plays a practical role only in low voltage systems. In practice, current limitations for medium or low voltage networks can be ignored (k B = 1).

    3. A.3.3.3

      Determination of short-circuit duration (duration of arcing)

      The arc flash or short-circuit duration t k is a significant parameter and will be required for the risk analysis. It is determined by the protection device and generally can be taken from the protective equipment manufacturer's selectivity calculations and/or trip characteristic curves (current-time curves).

      It must be considered that, in current-time dependent protection devices, the trip time will be influenced by the level of the actual short-circuit current and, thereby, from the current limitation through the electric fault arc, itself. The actual short-circuit current in the low voltage range does not correspond to the prospective short-circuit current, but to the electric arc short-circuit current I kLB and can be significantly limited. Determination of the actual short-circuit current I kLB, with consideration given to a number of influencing variables, can only be done by approximation 5 and is subject to a degree of uncertainty (see A.3.3.4).

      One is generally considered to be in a safe zone if a current limitation of 50% is assumed and this reduced current is used to establish the trip time as taken from the protection characteristic curve. The current limiting factor then equates to k B = 0,5; it follows that

      I kLB = 0,5 I'' k3pmin

      When using scatter range information for the current-time curve for a protection device (e. g. fuse), the value from the upper range limit should be used for short-circuit duration.

      NOTE:
      A protection device is considered to be a device positioned upstream from the respective work area, or a separate protection device installed or activated especially in connection with a work task. With a multi-source feed to a fault location, the protection device with the longest trip time should be used to determine short-circuit duration.

      NOTE:
      When using software tools (selectivity calculations), it must be ensured that the calculation is made based on the limited electric arc short-circuit current I kLB.

      Regarding protection devices, their protection boundaries and selectivity levels should be taken into account. With non-current-limiting fuses and circuit breakers with direct actuation, the short-circuit duration can be taken directly from the current-time curve or the selective tripping schedule. With circuit breakers, the setting of time delay levels or selective tripping times must be taken into account where applicable. The following reference values are considered to be typical for circuit breakers trip times without a time delay:

      Circuit breaker Undelayed trip time
      Low voltage (< 1.000 V)60 ms
      Medium voltage (1 to 35 kV)100 ms
      High voltage (> 35 kV)150 ms

      Table 9 Typical circuit breaker trip times

      Information provided by the manufacturer will provide more specific related data.

      Current limiting fuses feature a short-circuit duration of less than 10 ms. The fuse current-time curves exhibit the virtual melting times, meaning the actual trip times will not necessarily coincide. For safety reasons, fuses used in current limiting situations should feature a short-circuit duration of t k = 10 ms. This value is considered to be on the safe side.

      NOTE:
      At short-circuit durations longer than 1 s, it can be assumed that the person will be able to withdraw from the immediate danger area, if applicable. For this reason, longer periods will not need to be considered. This does not apply, however, if withdrawal of the person from the working environment is precluded or restricted (e. g. work in tight cable trenches or canals, narrow work corridors, work from ladders or lifting mechanisms).

    4. A.3.3.4

      Determination of the anticipated electric arc energy value

      The determination to be made is the maximum value of electric arc energy that can be anticipated at the respective fault location or within the scope of application being considered.

      Electric arc energy is dependent on network conditions, meaning from the network short-circuit power S'' k at the potential fault location and the short-circuit duration t k, as determined by the electric protection devices (trip times for circuit breakers and fuses, as well as separate protection devices if applicable) as taken from the protection characteristic curves:

      W LB = P LB tLB = kP S'' t k
       = k P √(3) U Nn I'' k3pmax t k

      Network short-circuit power at the fault location is the result of the nominal or stipulated network voltage U n and the maximum prospective 3-phase short-circuit current I'' k3max for the relevant network switching states.

      NOTE:
      With a multi-source feed to a fault location, overall short-circuit current I'' k3max will be composed of the respective partial currents. That share of the short-circuit current emanating from motors that could be fed back to the fault location must be taken into account, if applicable.

      In the case of a fault located within the switchgear or distribution system, the line impedance between the energy supply source (usually a transformer) and the system must generally be taken into account.

      Furthermore, electric arc energy is dependent on system conditions characterised by factor k P, which accounts for the type of arc formation and the electrode geometry at the fault location. This factor can be determined by approximation with the aid of arc voltage 5. For arc voltages, there are empirical conditional equations, which - aside from electrical circuit parameters - require knowledge of system conductor wire spacing. The 50% arc voltage value determination can be assumed.

      For a very rough estimation without considering the system geometry, the theoretical maxima of the parameter k P can be used, which can be determined according to

      kPmax =0,29  
      (R/X) 0,17

      this equation. R is the active component thereby, while X is the reactive component of impedance in the short-circuited electrical circuit 5.

      Furthermore, it was determined that the following specified range of values k P are typical for conventional system configurations, in practice, and can be used as reference values:

      U n d R/X k P
      400 V30 mm0,20,229
      0,50,215
      1,00,199
      > = 2,00,181
      45 mm0,20,289
      0,50,263
      1,00,240
      > = 2,00,222
      60 mm0,20,338
      0,50,299
      1,00,270
      > = 2,00,253
      10 to 20 kV120 to 2400,10,04 to 0,08

      Table 10 Reference values for normalised arc power

      NOTE:
      When using the maximum value or the reference value, the determination of geometric parameters is circumvented at the cost of precision. Particularly with the ansatz for maximum value, a significant safe distance can emerge under certain circumstances.

    5. A.3.3.5

      Determination of working distance

      Working distance a is the distance between the electric arc and the person's body (torso) that is effective during the work activity or must be maintained in the working environment being considered. Where different tasks are being carried out in the working environment, the shortest distance emerging should be applied. The configuration of the potential electric arc-related electrodes in the system (conductor arrangement) is decisive in determining the fault location (location of the electric arc flash).

      Those electrical systems at which persons perform work tasks (repairs, service, maintenance, assembly, inspection, measurement, etc.) are considered integral to the working environment and workstations. A work task is considered to be any activity performed in the vicinity of live components or live working.

      Typical working distances resulting from the work positions and the characteristic design or geometry and dimensions of the electrical equipment are:

      Equipment type Typical working distances
      Low voltage distribution/house junction box, main control cabinet300 to 450 mm
      Low voltage switchgear300 to 600 mm
      > 1 kVaccording to
      DIN VDE 0105-100

      Table 11 Typical working distances

      Distance relationships should be determined as accurately as possible so that a determination of the working distance can be established. Yet, it can generally be assumed that the distance to the person's torso will not fall short of a = 300 mm while working and, particularly in the low voltage range, that this can be applied as a reference value.

      NOTE:
      Personal protection can always be assumed when working on closed systems that have passed a type test for arc resistance; consequently, a working distance does not need to be determined (see 4.3.1). In the case of non-tested systems, however, the potential for electric arcing and related effects outside the system must be expected (e. g. when opening doors). The working distance that must then be provided for will be composed of the distance to the system encasement and the typical working distances referenced above (values taken from the lower limits).

      Establishing a working distance that the worker must not fall short of represents a possible measure for facilitating work activities with PPE at a specific level of protection (test category or protection class).

    6. A.3.3.6

      PPE electric arc protection level

      It must be ensured during test setup for the Box test that the thermal transfer relations (including the effectiveness of the electrode material) correspond with "worst case" conditions according to VDE 0682-306-1-2. The electric arc energies W LBP in the test setup corresponding to the respective incident energies E i0P in the test can be used to establish utilisation limits for PPE:

      Box test Statistical mean value
      VDE 0682-306-1-2Electric arc
      energy
      W LBP
      Direct incident
      energy
      E i0P 1
      Class 1158 kJ135 kJ/m2
      Class 2318 kJ423 kJ/m2

      Table 12 Box test parameters

      NOTE:
      The specified direct incident energy values E i0 , which identify the Box test categories, do not correspond to the ATPV values, which are determined in tests according to VDE 0682-306-1-1 or in their subsequent procedures according to NFPA 70E and IEEE 1584; neither do they compare with the established transmission and exposure conditions, nor are analytical conversions or mathematical transfers possible in these values.

      At an effective distance of a = 300 mm (corresponding to the test setup), the electric arc energy values W LBP lead to the applicable incident energies. Electric arc energy W LBP, which identifies the test category in the Box test, is used as a comparative parameter W LBä for the ascertained electric arc energy W LB within the scope of application.

      At the same time, it is presupposed that the use of PPE is foreseen for working distances of a = 300 mm and for systems that are small-scale and limited by side, rear and partition walls, analogous to the Box test setup (with a volume of around V = 1,6 10-3 m3). Corrections are possible with divergent conditions.

    7. A.3.3.7

      Consideration of divergent exposure relationships

      Equivalent arc energy W LBä can be determined for any working distance a by using an experimentally verified reverse squared distance proportionality from the electric arc energy of the test category W LBP. It represents that level where protection provided by the PPE for a respective distance a is still maintained. Moreover, the system configuration can be taken into consideration. The following is generally valid for the Box test

      The transmission factor for electric arc energy k T for Box test conditions equals k T = 1. For divergent combustion and transmission conditions, a coefficient can also be used with the following values:

      Type of system Transmission
      factor for electric arc
      energy k T
      (Very) small-scale systems with side, rear and partition walls1
      Large-scale systems, spatial limitations primarily due to rear wall structure1,5 to 1,9
      Open systems without significant limitations in the electrode chamber2,4

      Table 13 Transmission factor

    8. A.3.3.8

      Using the analysis results for risk assessment

      In the risk assessment or when selecting the PPE test category or protection class (Box test), the relation to the expected value for electric arc energy is to be considered based on the equivalent arc energy. The thermal hazards associated with electric arching are deemed covered if

      W LBW LBä applies.

      Starting with this relation together with the above mentioned determinant parameters and equations, the limits for PPE applicability in a chosen test category or protection class can be determined with respect to short-circuit current range, permissible short-circuit duration or protection device trip time (and therewith the protective system itself) and permissible working distance.

  4. A 3.4

    Alternative test methods

    The procedures described herein are not applicable for alternative test methods to the Box test method. It is then necessary to determine the correlation between electric energy and direct incident energy (transmission function) generally valid for the affected test setup or to ascertain the direct incident energy that can be expected in the event of a fault, and then to compare these with the incident energy level from the PPE test.

    In addition to the Box test, one test method is also used in accordance with VDE 0682-306-1-1 (ATPV test or Arc-Man test). As opposed to the Box test method, in which a directed test arc is generated, similar to an arc that might be expected in an accident when working on a control cabinet or distribution system, the electric arc generated in the Arc-Man method is open and non-directional, meaning it is generated in a quasi free field. The two methods can not be compared directly and are not transferrable or convertible among themselves. On the one hand, this is due to the type of electric fault arc, whose length and propagation are predetermined by the test setup, the electrode materials used and many other physical-technical differences. The heat transfer that takes place in the Arc-Man test is primarily due to radiation.

    On the other hand, Arc-Man test results lead to the so-called "Arc Thermal Performance Value", or ATPV. In this context, the incident energy is determined according to a statistical methodology, by which a 50% probability exists of suffering second-degree skin burns behind the PPE. Even if an electric fault accident is relatively improbable, the EU directive regarding PPE allows no interpretation of PPE that would tolerate such injury. For this reason, as a matter of principle, such test methods should not be used within the EU.

    ATPV is the direct incident energy that emerges with the special transfer relations existing in the test. It should be noted that ATPV does not correspond to the levels of direct incident energy associated with the test categories. The incident energy levels generated in the Box test method are not ATPV values or limits of the ATPV range.

    Products available on the international marketplace have been tested under certain circumstances according to both methods, meaning the Box test and Arc-Man test methods. Even if the test results are not directly comparable, they can nevertheless help in the selection of suitable PPE, particularly when the maximum anticipated electric arc energy lies above the electric arc energy for the electric fault arc protection class WLBP or the equivalent arc energy WLBä described in A.4.3.

    For this reason, a manufacturer who tests its products according to both methods can specify the ATPV realised, even in the EU marketplace, in order to provide the user with further selection criterion to help the selection of suitable PPE.

    When using ATPV for selection of PPE, however, a risk analysis must be undertaken in which the anticipated incident energy is ascertained. For this, NFPA 70E (Standard for Electrical Safety in the Workplace) and IEEE 1584 (Guide for performing arc-flash hazard calculations), among others, provide relevant algorithms.

    It must be noted, however, that ATPV-based testing and PPE selection are bound by the limitations of the methodology.

1)

Correlation:
1 cal/cm2 = 41,868 kJ/m2, 1 kJ/m2 = 0,023 885 cal/cm2.

5)

Schau, H.; Halinka. A.; Winkler, W.: Elektrische Schutzeinrichtungen in Industrienetzen und -anlagen.