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The basics of selectivity (discrimination) between circuit breakerselectrical-engineering-portal.com/selectivity-between-circuit-breakers

The basics of selectivity (discrimination) between circuit breakers (photo credit: elstarelectric.pl)

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 tothe installation in the case of a fault as fast as possible . That’s why we have the selectivity between protectivedevices.

So what is the selectivity? The whole point of the selectivity is that the protective device immediately upstream of thefault must respond at first. Only the faulty part of the installation should be isolated. All the other switching andprotective devices connected to the system should remain operative.

Selectivity reduces the duration of a fault and limits its possible damaging effect only to a part of the installation. Theservice interruption is reduced to a minimum.

Let’s discuss now about two types of selectivity between circuit breakers //

Current selectivity

Time selectivity

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Figure 1 – Time-current characteristics of two current-selectivecircuit breakers

Current selectivity

In a distribution network, the ratings of the distribution circuit breakers will be smaller and smaller as we godownstream from the transformer to the load. Similarly, the settings of the short circuit magnetic releases will be alsolower and lower. At the same time, the magnitude of the short circuit current which may occur will be also progressivelylower.

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 thesystem, 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 lowerthan the (adjustable) threshold of tripping of the magnetic release of the unit connected upstream. This value isconsidered as the limit of selectivity.

Whether two circuit breaker are really mutually selective is checked by comparing the time-current characteristicsof the breakers. The tripping characteristics of the two breakers may not touch or intersect each other up to themaximum value of the permissible fault level.

There must be a definite spacing between the two characteristics , depending on the permissible tolerance bandof the releases of the breakers.

Although the method of comparison of the time-currentcharacteristics is exact, it is also time consuming. Thepublished tables of the manufacturers, indicating theselectiveness of the circuit breakers among one anothermakes the selection easier.

As far as overload is concerned, the thermally delayedbimetallic overload releases of the circuit breakers withdifferent rated currents are always selective to one another.The tripping times of the different ratings of the circuit breakersfor the same overload currents are automatically different (asfor example the 100 A version and the 6.3 A version).

Go back to Selectivity Types ↑

Time selectivity

If current selectivity can not be achieved, as for examplebetween two quick acting circuit breakers having practicallythe same reaction time, selectivity is to be realised via theadjustable delay time of the breakers.

The time selectivity in the case of large circuit breakers for the

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Figure 2 – Time-current characteristics of two time-selective circuitbreakers

protection of installations is realised by delaying the magnetictripping time by a few half cycles. The total clearing time of thedownstream circuit breaker must be shorter than the minimumnecessary duration of the command time of the circuit breakerconnected immediately upstream.

In other words, for mutually selective circuitbreakers acting in time staggered sequence – Thedelay time of the upstream circuit breaker must belonger than the total clearing time of the circuit breakerconnected downstream.

The minimum delay time that can be realised between the timestaggered circuit breakers are 60 or 100ms . The trippingcharacteristic of the delayed breaker is shifted upwards on thepublished time-current characteristic diagram.

The time selectivity between circuit breakers reacting in a timestaggered sequence is achieved by not allowing thecontacts or the magnetic release to react directly with theshort circuit current. A mechanically delaying mechanism oran electronic circuit delays the action of the circuit breaker.

For the upstream circuit breaker, one can no longer speakabout a quick acting, current limiting interruption. More thanone half cycles of the actual short circuit current flowsthrough the delayed acting protective device and alsothrough the installation. This, obviously, is to be designedaccordingly to withstand this stress.

Go back to Selectivity Types ↑

Reference // Basics of circuit breakers by RockwellAutomation

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