What are the different types of short circuit currents?
elec calc™ can calculate different types of short-circuit. These data are relevant since, depending on installation type, earthing system and the point in which a short-circuit may happen, we do not exactly know which one of them will be the most harmful. Protections of the system, then, must be sized to operate against any kind of failure to occur.
The purpose of this article is to describe the different types of failures to better understand the calculated values:
To highlight the specific case of IT earthing system, the short circuit value represents a second failure short-circuit, since after first failure the current circulation is very low or almost zero (although is a failure that must be fixed as soon as possible).
The value of the short-circuit currents
Apart from the different natures of short-circuit, the values of currents are not always the same. This is due to the connection grid is constantly in variation, in terms of voltage, load level, etc., that will cause that, in case of failure, the short-circuit current will fluctuate inside a range of maximum and minimum values.
Maximum short-circuit currents
They correspond to the grid behaving with more power in the connection point, in this case the suitable short-circuit currents will have the higher values and then the damages will be more important. Then, the protections, apart from normal operation, will have to be capable to withstand these values to avoid permanent interruptions or damage in other equipment, these values define the minimum requirement for devices breaking capacity.
Minimum short-circuit currents
In these cases the grid is behaving with lower power (but inside acceptable range, obviously). Short-circuit currents will be smaller, even in some cases their order of magnitude will be comparable to the overloads. In any case, a short circuit, even of minimum value, is still an installation failure that must be fixed immediately. Protections then must operate against these kinds of failures, contrary the short-circuit effect will be spread to other installation points causing several damages.
Short circuit calculations with elec calc™
elec calc™ calculates all these kinds of short-circuits according to symmetrical components method, included in IEC-60909 standards family, applying all factors inside them as well as other particularities that other local standards may require. Every short-circuit calculation are updated in real time, while installation and components are verified for the correct sizing of protections, checking existing ones, and consideration of parameters to select the appropriate references from the multi-manufacturer catalogue.
Article written by Ricardo Martín Sobrino, Trace Software International Calculation Engineer