What is the selectivity?
On economical grounds and for the reason of reliability of the service, it is not always ideal to interrupt the supply to the installation in the case of a fault as fast as possible. That’s why we have the selectivity between protective devices.
So what is the selectivity? The whole point of the selectivity is that the protective device immediately upstream of the fault must respond at first. Only the faulty part of the installation should be isolated. All the other switching and protective devices connected to the system should remain operative.
In a distribution network, the ratings of the distribution circuit breakers will be smaller and smaller as we go downstream from the transformer to the load. Similarly, the settings of the short circuit magnetic releases will be also lower and lower. At the same time, the magnitude of the short circuit current which may occur will be also progressively lower.
This results in a sort of natural selectivity depending on the magnitude of the short circuit current.
The principle of current selectivity is applied mostly for distribution feeders at the extremity of the system, with appreciable reduction of the short circuit current due to the long length of the leads.
The prospective short circuit current at the location of installation of the circuit breaker must be known.
Two circuit breakers are mutually selective if the short circuit current flowing through the downstream breaker is lower than the (adjustable) threshold of tripping of the magnetic release of the unit connected upstream. This value is considered as the limit of selectivity.
There must be a definite spacing between the two characteristics, depending on the permissible tolerance band of the releases of the breakers.
Although the method of comparison of the time-current characteristics is exact, it is also time consuming. The published tables of the manufacturers, indicating the selectiveness of the circuit breakers among one another makes the selection easier.
As far as overload is concerned, the thermally delayed bimetallic overload releases of the circuit breakers with different rated currents are always selective to one another. The tripping times of the different ratings of the circuit breakers for the same overload currents are automatically different (as for example the 100 A version and the 6.3 A version).
If current selectivity can not be achieved, as for example between two quick acting circuit breakers having practically the same reaction time, selectivity is to be realised via the adjustable delay time of the breakers.
The time selectivity in the case of large circuit breakers for the protection of installations is realised by delaying the magnetic tripping time by a few half cycles. The total clearing time of the downstream circuit breaker must be shorter than the minimum necessary duration of the command time of the circuit breaker connected immediately upstream.
In other words, for mutually selective circuit breakers acting in time staggered sequence – The delay time of the upstream circuit breaker must be longer than the total clearing time of the circuit breaker connected downstream.
The minimum delay time that can be realised between the time staggered circuit breakers are 60 or 100ms. The tripping characteristic of the delayed breaker is shifted upwards on the published time-current characteristic diagram.
The time selectivity between circuit breakers reacting in a time staggered sequence is achieved by not allowing the contacts or the magnetic release to react directly with the short circuit current. A mechanically delaying mechanism or an electronic circuit delays the action of the circuit breaker.
For the upstream circuit breaker, one can no longer speak about a quick acting, current limiting interruption. More than one half cycles of the actual short circuit current flows through the delayed acting protective device and also through the installation. This, obviously, is to be designed accordingly to withstand this stress.
Reference // Basics of circuit breakers by Rockwell Automation