The objective of system protection is to detect faults and to selectively isolate faulted parts of the system. It must also permit short clearance times to limit the fault power and the effect of arcing faults.
High power density, high individual power outputs, and the relatively short distances in industrial and building power systems mean that low voltage and medium voltage systems are closely linked.
Activities in the LV system (short circuits, starting currents) also have an effect on the MV system. If the situation is reversed, the control state of the MV system affects the selectivity criteria in the secondary power system. Electrical installations in a power system are protected either by protective equipment allocated to the installation components or by combinations of these protective elements.
Rated short-circuit breaking capacity
The rated short-circuit breaking capacity is the maximum value of the short circuit that the protective device is able to clear according to specifications. The protective device may be used in power systems for rated switching capacities up to this value.
If a short circuit, which is higher than the rated switching capacity of the protective device used, occurs at a particular point in the system, back-up protection must provide protection for the downstream installation component and for the protection device by means of an upstream protective device (grading).
Selectivity, in particular, has become a topic for discussion in the previous years. Partly, it has become a general requirement in tender specifications.
Due to the complexity of this issue, information about proper selection and application is often insufficient. These requirements as well as the effects of full or partial selectivity in power distribution systems within the context of the relevant standard, industry, country, system configuration or structure should be clarified in advance with the network planners, installation companies and system operators involved.
The system interconnection together with the 5 rules of circuit dimensioning must also be taken into account.
To maintain the supply safety of power distribution systems, full selectivity is increasingly demanded. A power system is considered fully selective, if only the protective device upstream of the fault location disconnects from supply, as seen in the direction of energy flow (from the infeed to the load).
In certain situations, partial selectivity (up to a particular short-circuit current) is sufficient. The probability of faults occurring and the effects of these on the load must then be considered for unfavorable scenarios.
Reference: Siemens – Application manual (Totaly Integrated Power)