2 What is an fault arc - Die BG ETEM

Annex 6
Arc risk calculation algorithm for the selection of PPE tested according to
the box test
A6.1 General advices
Arcing processes as well as arc parameters generally escape exact calculations.
Very complex non-linear relationships, mutual interdependences and influences
being strongly changing with time and cannot be defined, prevent exact evaluation
practically. The external behavior of an electric fault arc is stochastic and subject of
very strong statistical dispersion. The determination of arc parameters is generally
reduced to empirical consideration with more or less well approaching to reality. With
respect to practicability simplest empirical calculation basics are used in the following.
Nevertheless such approaches require a number of basic data of the electric
power system, equipment and protective devices. In the algorithm 3 steps of consideration
are offered to the user for determining energy values. These steps are based
on different input data and, thus, require various efforts for analyzing, but also result
in a differentiated accuracy:
• EV – the use of extreme values for worst case consideration abstracting from
equipment specifics and covering all possible influences, and result, under
circumstances, in a distinct safety margin in the case under study
• GV – the use of guide values sparing detailed analysis and covering a larger
number of practical cases
• DV – the use of detailed values taking into account equipment data for an
exacter estimation with the consequence of higher expenditures in calculation.
The need of input data and calculation expenses increases with higher requirements
to accuracy and abstention of safety margins. Worst case considerations are helpful
for users without detailed information or knowledge, or if rough estimations of safety
needs or necessary measures are aimed at. No detailed knowledge and experiences
in arc risk estimation are necessary. Because of the likely poor accuracy these considerations
can lead, under circumstances, to the result that no practical solutions for
protection by only the PPE can be found. Regarding the use of the guide values no
accuracy and probability statement of the results is possible.
Experiences and additional knowledge regarding arcing faults and risks are essential
for the detailed consideration. It is the most exact alternative. It can be recommendable,
under circumstances, to include engineers skilled and experienced in this field
in the analysis work, or make demands upon the support of experts for arc risk
assessment.
57