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

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Abschnitt 3.3 , 3.3 Determination of system electric arc ene...
Abschnitt 3.3
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 3.3 – 3.3 Determination of system electric arc energy in the event of a fault

Fig. 3
Determination of electric arc energy

Electric arc energy W LB is determined by the electric arc power PLB and the duration of arcing, meaning the time t k until tripping by the protection device:

W LB = P LB t k

Electric arc power PLB is dependent upon the type of arc formation and the geometry of the live components at the fault location. It is determined with the help of the normalised arc power kp from the short-circuit power S''k.

Normalised arc power k P can be determined with consideration given to the effective electrode gap d (distance between system conductors), (e. g. according to the text in German, "Schau, H.; Halinka. A.; Winkler, W.: Elektrische Schutzeinrichtungen in Industrienetzen und -anlagen"). Reference values are specified in Annex A.3.3.4.

Worst-case examinations can be calculated using the maximum value k Pmax:

k Pmax =0,29  
(R/X) 0,17

Consequently, the following correlations can be drawn with respect to electric arc energy in the event of a fault:

The decisive short-circuit current I'' k3 is the prospective 3-pole short-circuit current at the workplace (fault location). It is the outcome of a short-circuit current calculation (see Annex A.3.3.2.).

The actual short-circuit current I kLB in the low voltage range is significantly lower than the calculated system short-circuit current I'' k3 (current limiting factor k B) due to the attenuating properties of the electric arc and cannot be determined in certainty. In principle, the applicable correlation is:

I kLB = 0,5 I'' k3min (see Annex A.3.3.2)

In the > 1 kV range, the limiting properties of the electric arc can be disregarded. The following applies: k B = 1.

The duration of arc combustion is determined by the protection device and generally can be taken from the protective equipment manufacturer's selectivity calculations and/or the trip characteristic curves (current-time curves).

The low voltage range is generally considered to be safe if one assumes a current limitation of 50% and uses this reduced current to ascertain the trip time from the protection characteristic curve. The current limiting factor then equates to k B = 0.5; it follows that

I kLB = 0,5 I'' k3min

The overcurrent protection device trip time should now be determined with the help of the characteristic curve and the ascertained electric arc short-circuit current I kLB (see A 2.3.3).

Fig. 4
Example for determining the overcurrent protection device trip time

NOTE:
At short-circuit durations longer than 1 s, it can be assumed that the person will be able to with draw 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).