Thanks to selectivity, a short-circuit current can be interrupted extremely rapidly at any level. This means less stress and strain on installation components, which therefore have a longer life.
If we are dealing with radial networks, the highest fault currents are more likely to occur on the supply side, near the transformer. Furthermore, the larger the transformer, the higher the rated load current. For this reason, the most suitable device for installing is an air circuit breaker.
ABB air circuit breakers are built to withstand high short-circuit currents for up to one second, thus making them suitable for guaranteeing selectivity among themselves and towards other downstream devices (for further in-formation, see Time Selectivity in the Selectivity Techniques section of this website).
Generally, when moving downstream in a radial network, the load currents decrease down to the rated current values of the individual loads; even the short-circuit currents decrease as well, because of the impedance in the cables.
However, this isn’t always true. Sometimes the loads are near to the transformers, or the cables are simply not long enough to limit the short-circuit current.
And what happens when we deal with distributed power generation? These types of networks feature localized power sources that generate power near to the loads and, in the event of a fault, may generate high short-circuit currents.
The circuit-breakers always protect the installation from the high currents, but selectivity can be difficult to achieve, especially if we have to rely on energy selectivity and we are dealing with circuit-breakers of similar size.
To cope with this particular situation, ABB has devised an innovative solution: the PR223EF electronic trip unit, which extends the advantages of Zone (Logic) selectivity to molded-case circuit breakers.
The example below shows how we can obtain selectivity in such complex situations.