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

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Abschnitt 4.2 , 4.2 Examples
Abschnitt 4.2
Thermal hazards from electric fault arc Guide to the selection of personal protective equipment for electrical work (bisher: BGI/GUV-I 5188 E)
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

Abschnitt 4.2 – 4.2 Examples

The following examples depict work being carried out at different work locations in a typical municipal low voltage supply system.

4.2.1 Work location 1: Low voltage distribution in a transformer station

Work tasks are frequently carried on the low voltage distribution system in a transformer station.

Fig. 8
Work on a low voltage distribution system

An increased degree of risk exists when performing such work because, in the event of a fault at the workplace, significant short-circuit power is generated directly behind the transformer. The transformer output, as well as the transformer fuses or power supply branch circuit breaker trip times, are decisive for the energy released in an electric arc. One important factor is influenced by the structure or the switching status of the low voltage network with relationship to the type of energy supply to the low voltage stations (station meshing or per station low voltage network supply). The short-circuit power and the prospective short-circuit current at the workplace depend on whether a unilateral or a multilateral supply exists. It is often practical with meshed low voltage networks to neutralize the meshing prior to working on live components in the low voltage distribution system and to establish a unilateral energy supply, as is the case in the example considered.

Fig. 7
Municipal low voltage supply system being considered

Step 1: Data for the workplace being considered
This example represents a municipal supply system (Fig. 8) where work location 1 is being considered. There are 20/0.4 kV transformers present at the network stations with rated capacities S rT of 630 kVA or 400 kVA and short-circuit voltages u K of 4%. The standard 1-kV aluminium cable cross-sections are 150 mm2 for the mains cables and 35 mm2 for the house installation cables. The drawing in Fig. 7 depicts the network separation points, which can be opened during work on live components in order to establish a unilateral energy supply to the respective network areas in question. Work location 1 is supplied by a 630 kVA transformer over a 630 kVA NH (low voltage, high performance) transformer fuse with operating class gTr AC 400 V. The fuse current-time curve is depicted in Fig. 10.

Fig. 9
Work location 1 equivalent circuit diagram

Step 2: Determination of I'' k3 , R/X
Using the short-circuit current calculation according to VDE 0102 (Short-circuit currents in three-phase a.c. systems - Part 0: Calculation of currents), with a unilateral energy supply switching status for the work location results in a prospective short-circuit current (initial short-circuit alternating current) I'' k3 of

I'' k3max = 23,1 kA(c = 1.05)
I'' k3max = 20,9 kA(c = 0.95)

The R/X ratio for network impedance in the fault circuit equates to 0.2.

Step 3: Determination of Electric arc current
The minimum fault current relevant for the NH fuse trip time with an electric arc short-circuit current results from the minimum prospective short-circuit current I'' k3min with the aid of limiting factor k B, which characterizes the current-limiting effects of the electric arc in the fault circuit. Because a low voltage system and a worst-case examination are being dealt with in the initial ansatz, a current limiting factor of k B = 0,5 will be assumed according to Section 3.3. For minimum fault current, it follows that

I' kLB = k B I'' k3min = 0,5 20,9 kA = 10,45 kA

The trip time for this current is taken from the protection characteristic curve in Fig. 10 is t = 0,1 s. This time equates to the short-circuit duration t k.

NOTE:
In practice, the characteristic curve for the overcurrent protection device in use should be applied.

Fig. 10
Mean time/current characteristic curves for the fgTr AC 400 V fuse in use

Step 4: Electric arc power at the workplace
Using the maximum prospective short-circuit current I'' k3max, it follows for short-circuit power at the workplace that

Under worst-case conditions, the maximum possible value for normalised arc power can be determined using the formula k Pmax = 0,29/(R/X) 0,17. This example results in the computation k P, max = 0,38.

From this results an electric arc energy W LB:

W LB = k P S'' k t k = 0,38 16,004 MVA 0,1 s = 608,2 kJ

This energy is the anticipated value for electric arc energy at workplace 1 in the event of a fault.

Step 5: Establish the working distance
A working distance of a = 300 mm is used for work on low voltage distribution systems. This corresponds to the minimum distance between a person's torso and the frontal area of the opened equipment.

Step 6: Test level for the PPE
The test levels for PPE under standardised Box test conditions according to VDE 0682-306-1-2 are

Electric fault arc protection class 1: W LB P1 = 158 kJ
Electric fault arc protection class 2: W LB P2 = 318 kJ

Step 7: Transmission factor, equivalent arc energy
When working on low voltage distribution systems in transformer stations, it should be assumed that large-scale installations will be used with spatial limitations primarily due to a rear wall structure. A transmission factor of k T = 1,5 is assumed at this location. Using a working distance of a = 300 mm, it follows for equivalent arc energy that

W LBä = 237 kJ for Electric fault arc protection class 1
W LBä = 477 kJ for Electric fault arc protection class 2

Step 8: Selection of protection class
W LB = 608,2 kJ > W LBä, Kl2 = 477 kJ applies. Consequently, the system must be shutdown or measures must be taken according to Section 4.1 and a new calculation must be made.

Execution of the required work steps will yield the results below.

Step Determination Parameter Result for worst-case examination Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 60 mm60 mm
2Short-circuit current calculationI''k3pmax 23,1 kA23,1 kA
I''k3pmin 20,9 kA20,9 kA
R/X 0,20,2
3Current limitation k B 0,50,633
Minimum fault current I kLB 10,45 kA13,23 kA
NH fuse characteristic curve (Fig. 10) t k 0,1 s0,045 s
4Short-circuit powerS''k 16 MVA16 MVA
Normalised arc power k P 0,380,338
Electric arc power P LB 6,1 MW5,4 MW
Electric arc energy (anticipated value) W LB 608,2 kJ243,4 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: small-scale system k T 1,51,5
Equivalent arc energy (protection level) W LbäKl1 237 kJ237 kJ
W LbäKl2 477 kJ477 kJ
8Comparison: W LBW LBä?608,2 kJ > 477 kJ243,4 kJ < 477 kJ
PPE Electric fault arc protection class Take other measures or isolate Class 2

Table 1 Example summary: Work on the low voltage distribution system of a (630 kVA) transformer station; Work location 1

In the case of a station with a 400 kVA transformer (short-circuit voltage 4%; NH fuse 400 kVAgTrAC 400 V), the prospective short-circuit current - under otherwise similar conditions as above - will fall within the range I''k3 = 12,7 to 14,1 kA.

The R/X ratio equates to 0,2. The characteristic curve for the NH fuse (Fig. 10) for k B = 0,5 and I kLB = 6,4 kA reveals a short-circuit duration of t k = 0,045 s. Short-circuit power equates to S'' k= 9,769 MVA.

A normalised arc power of k P = 0,38 results in an electric arc power of P LB = 37 MW and an anticipated electric arc energy value of W LB = 167,6 kJ. The same working distance a = 300 mm and the same transmission relationships (k T = 1,5) as before means that PPE protection class 1 will be required.

Step Determination Parameter Result Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 60 mm60 mm
2Short-circuit current calculationI''k3pmax 14,1 kA14,1 kA
I''k3pmin 12,7 kA12,7 kA
R/X 0,20,2
3Current limitation k B 0,50,633
Minimum fault current I kLB 6,4 kA8 kA
NH fuse characteristic curve (Fig. 10) t k 0,045 s0,04 s
4Short-circuit power S'' k 9,8 MVA9,8 MVA
Normalised arc power k P 0,380,338
Electric arc power P LB 3,7 MW3,3 MW
Electric arc energy (anticipated value) W LB 167,6 kJ132,1 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: small-scale system k T 1,51,5
Equivalent arc energy (protection level) W LbäKl1 237 kJ237 kJ
W LbäKl2 477 kJ477 kJ
8Comparison: W LBW LBä?167,6 kJ < 237 kJ132,1 kJ < 237 kJ
PPE Electric fault arc protection class Class 1 Class 1

Table 2 Example summary: Work on the low voltage distribution system of a (400 kVA) transformer station; Work location 1

4.2.2 Work location 2: Low voltage cabling

Work is frequently carried out on cable joints in the cable network (see Fig. 11). Work location 2 in this example (T-joint at the end of approx. 100 m network cabling) is depicted in Fig. 7. The level of fault current and electric arc energy is greatly dependent on the distance between the work location and the network supply station (transformer) and, for this reason, on the length of the corresponding network cable.

Fig. 11
Work on a cable sleeve

 

Fig. 12
Mean time/current characteristic curves for the NH gL/gG AC 400 V line fuse being considered

In this example, the work location is being fed through a network cable from a 630 kVA transformer station. The NH fuse in the supplying station's cable branch is decisive for breaking the electric fault arc. In this context, an NH 250 A full-range line fuse is used with operating class gG or gL AC 400 V. The characteristic curve is depicted in Fig. 12.

Execution of the required work steps will yield the results below.

Step Determination Parameter Result Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 45 mm45 mm
2Short-circuit current calculationI''k3pmax 6,3 kA6,3 kA
I''k3pmin 5,7 kA5,7 kA
R/X 1,01,0
3Current limitation k B 0,50,59
Minimum fault current I kLB 2,85 kA4,25 kA
NH fuse characteristic curve (Fig. 12) t k 0,15 s0,09 s
4Short-circuit power S'' k 4,365 MVA4,365 MVA
Normalised arc power k P 0,290,24
Electric arc power P LB 1,266 MW1,047 MW
Electric arc energy (anticipated value) W LB 189,9 kJ94,2 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: large-scale system k T 1,91,9
Equivalent arc energy (protection level) W LbäKl1 300 kJ300 kJ
W LbäKl2 604,2 kJ604,2 kJ
8Comparison: W LBW LBä?189,9 kJ < 300 kJ94,2 kJ < 300 kJ
  PPE Electric fault arc protection class Class 1 Class 1

Table 3 Example summary: Work on a cable network joint; Work location 2

The work being performed at Work location 2 (cable sleeves) under consideration requires PPE in the Electric fault arc protection class 1 according to the appraisal in Section 3 and with precise calculations.

4.2.3 Work location 3: House junction box

The replacement of a house junction box is often associated with work on live equipment (Fig. 13 (inside/outside)). Such an example in Work location 3 is considered in Fig. 7. Energy is once again supplied to the work location from an upstream network station with a 630 kVA transformer. In contrast to Example 2, the short-circuit current is significantly less because the house connection cables have only comparatively small cross-sections. The house connection cable in the example has a length of approx. 15 m.

The branch fuse in the upstream cable distribution cabinet is decisive for breaking the short-circuit; in this case, an NH 250 A fuse is used with operating class gG AC 400 V.

Fig. 13
Work on a house junction box

Execution of the required work steps will yield the results below.

Step Determination Parameter Result Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 45 mm45 mm
2Short-circuit current calculationI''k3pmax 6,3 kA6,3 kA
I''k3pmin 5,7 kA5,7 kA
R/X 1,01,0
3Current limitation k B 0,50,59
Minimum fault current I kLB 2,85 kA4,25 kA
NH 250 A fuse characteristic curve (Fig. 12):
t k = 2,5 s *
t k 0,15 s0,09 s
4Short-circuit power S'' k 4,365 MVA4,365 MVA
Normalised arc power k P 0,290,24
Electric arc power P LB 1,266 MW1,047 MW
Electric arc energy (anticipated value) W LB 189,9 kJ94,2 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: small-scale system k T 1,91,9
Equivalent arc energy (protection level) W LbäKl1 300 kJ300 kJ
W LbäKl2 604,2 kJ604,2 kJ
8Comparison: W LBW LBä?189,9 kJ < 300 kJ94,2 kJ < 300 kJ
PPE Electric fault arc protection class Take other measures or isolate

Table 4 Example summary: Work on a opened house junction box; Work location 3

*

Referencing the characteristic curve (Fig. 12), a trip time of t > 1 s results, so that it can be assumed that the maximum time relevant to the exposure equates to t k = 1 s. (also refer to the note at the end of Section 3.3).

It can be seen from the results in the example that PPE in the Electric fault arc protection class 2 is not adequate for work on a house junction box. The high anticipated value of electric arc energy is brought about by a long short-circuit duration, from which a long exposure duration emerges.

In order to facilitate work in this case,

  • protection devices guaranteeing defined and sufficiently rapid breaking characteristics must be used or

  • compliance with an adequate minimum distance must be required or

  • PPE tested for greater Incident energy levels must be used.

The option mentioned at first will be singled out for consideration below. For this, it must be ensured that the NH 250 A gG branch fuse present in the network supply station's cable branch is replaced with a safe-work fuse with a low rated current and/or with fast-acting or super-fast-acting operating characteristics for the duration of the work task. This means that prior to beginning and subsequent to completing the work task a fuse replacement will be necessary. If an NH 160 A safe-work fuse is used with an operating class aR (fast-acting: üf2, veryfast-acting: üf1, super-fast-acting: üf01, hyper-fast-acting: üf02) is used, a current-limiting break will occur in any case. Regarding the calculations in this context, a short-circuit duration of t k = 0,01 s is to be applied.

An NH 160 A aR/690 V - üf01 fuse is used for this example, whereby a trip time of 6.87 ms results.

The performance of work tasks using PPE in the Electric fault arc protection class 1 is now made possible through the use of the safe-work fuse.

The use of this fuse will yield the following results:

Step Determination Parameter Result Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 45 mm45 mm
2Short-circuit current calculationI''k3pmax 3,4 kA3,4 kA
I''k3pmin 3,0 kA3,0 kA
R/X 2,02,0
3Current limitation k B 0,50,554
Minimum fault current I kLB 1,5 kA1,66 kA
NH fuse characteristic curve (Fig. 12) t k 0,01 s0,01 s
4Short-circuit power S'' k 2,353 MVA2,353 MVA
Normalised arc power k P 0,260,222
Electric arc power P LB 0,61 MW0,5 MW
Electric arc energy (anticipated value) W LB 6,1 kJ5,2 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: small-scale system k T 11
Equivalent arc energy (protection level) W LbäKl1 158 kJ158 kJ
W LbäKl2 318 kJ318 kJ
8Comparison: W LBW LBä?6,1 kJ < 158 kJ5,2 kJ < 158 kJ
  PPE Electric fault arc protection class Class 1

Table 5 Example summary: Work on an opened house junction box while using a safe-work fuse; Work location 3

4.2.4 Work location 4: Electrical installation behind a house junction box

As a rule, when working on live equipment or in the vicinity of live components in the house electrical installation, basic protection, meaning PPE in the Electric fault arc protection class 1, is sufficient. The following example depicts the calculation for a typical configuration behind an NH 63 A gL fuse.

Fig. 14
Work behind the house supply systemx

Step Determination Parameter Result Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 25 mm25 mm
2Short-circuit current calculationI''k3pmax 3,4 kA3,4 kA
I''k3pmin 3,0 kA3,0 kA
R/X 2,02,0
3Current limitation k B 0,50,554
Minimum fault current I kLB 1,5 kA1,66 kA
NH 63 AgLfuse characteristic curve (Fig. 12) t k 0,04 s0,04 s
4Short-circuit power S'' k 2,353 MVA2,353 MVA
Normalised arc power k P 0,260,25
Electric arc power P LB 0,61 MW0,56 MW
Electric arc energy (anticipated value) W LB 24,5 kJ22,6 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: small-scale system k T 11
Equivalent arc energy (protection level) W LbaKl1 158 kJ158 kJ
W LbäKl2 318 kJ318 kJ
8Comparison: W LBW LBä?24,5 kJ < 158 kJ22,6 kJ < 158 kJ
  PPE Electric fault arc protection class Class 1

Table 6 Example summary: Work on an electrical installation behind a house junction box; Work location 4

4.2.5 Work location 5: Low voltage distribution system for industry

The following example depicts the calculation for a typical configuration behind an NH 315 A gG fuse. Various tasks are carried out behind the NH fuse on the installation in this example. This ranges from simple adjustments on protection devices and equipment to replacement of the equipment itself.

The work location is on the electrotechnical equipment fora cooling unit.

Fig. 15
Industrial plant system overview

 

Fig. 16
Work on an industrial low voltage system (cooling unit control cabinet)

Step Determination Parameter Result Result for precise calculation according to 5)
1Network parameter: Nominal network voltage U Nn 400 V400 V
Equipment geometry: Distance between conductors d 20 mm20 mm
2Short-circuit current calculationI''k3pmax 15,34 kA15,34 kA
I''k3pmin 12,52 kA12,52 kA
R/X 0,870,87
3Current limitation k B 0,50,731
Minimum fault current I kLB 6,26 kA9,15 kA
NH fuse characteristic curve (Fig. 12) t k 0,022 s0,001 s
4Short-circuit power S'' k 10,63 MVA10,63 MVA
Normalised arc power k P 0,2970,149
Electric arc power P LB 3,16 MW1,59 MW
Electric arc energy (anticipated value) W LB 69,43 kJ15,86 kJ
5Working distance a 300 mm300 mm
6Standardised PPE test level W LBPKl1 158 kJ158 kJ
W LBPKl2 318 kJ318 kJ
7Transmission factor: small-scale system k T 1,51,5
Equivalent arc energy (protection level) W LbaKl1 237 kJ237 kJ
W LbäKl2 477 kJ477 kJ
8Comparison: W LBW LBä?69,43 kJ < 158 kJ15,86 kJ < 158 kJ
  PPE Electric fault arc protection class Class 1

Table 7 Example summary: Work on an industrial low voltage system

As a rule, when working on live equipment or in the vicinity of live components in an Industrial plant electrical installation, basic protection, meaning PPE in the Electric fault arc protection class 1, is sufficient.

5)

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