1. ELECTRICAL PLANT DIMENSIONING
CIRCUIT BREAKER SELECTION

2. Protection of transformers
Generalities
As to be able to protect LV/MV transformers LV side, we must mainly take into account:
rated current of the protected transformer, LV side, from which the rated current of the CB and the setting depend (In);
the max estimated short circuit current in the installation point which defines the minimal breaking power of the protection circuit breaker (Isc).

3. Protection of transformers
MT-BT switchboard with one single transformer Sn In
Isc
U20

4. Protection of transformers
The rated current of the transformers LV side is defined by the following expression
where
Sn = rated power of the transformer [kVA]
U20 = rated secondary voltage (no load) of the transformer [V]
ln = rated current of the transformer, LV side [A]
In =
Sn x 103
3 x U20

5. Protection of transformers
The full voltage three-phase short circuit current immediately after the LV side of the transformer can be expressed by the following relation once we suppose infinite power at the primary:
where
Ucc % = short circuit voltage of the transformer [%]
ln = rated current, LV side, [A]
lsc = three-phase rated short circuit current, LV side, [A]
Isc =
In x 100
Ucc %

6. Protection of transformers
The short circuit current is normally lesser than the preceding deduced values if the CB is installed at a certain distance by means of a cable or bar connection, according to the connection impedance.

7. Protection of transformers
Choice of the circuit breaker
The following table shows some possible choices within the SACE Emax ACB range according to the characteristics of the CB to protect.
Attention
Those indications are valid at the conditions that we declare in the table; different conditions will lead us to repeat calculations and modify the choices.

8. Protection of transformers
Sn [kVA]
Ucc (1) % ,25 6,25 6,25 6,25
In (2) [A]
Isc (2) [kA]
SACE Emax E1B E1B12 E1B E2B E2B20 E3B E3B32  E4S E6H50
(1) For values of the percent short circuit voltage U’cc% different from the Ucc% values as per table, the rated three-phase short circuit current I’cn becomes:
(2) The calculated values refer to a U20 voltage of 400 V. for different U’20 values, do multiply In and Isc the following k times:
I’sc =
Ucc %
U’cc %
Isc
U’20 [V] k

9. Protection of lines
The choice of the circuit breakers for switching and protection of the lines means the perfect knowledge of:
rated operating line current lB
max admissable cable current lZ
presumed short circuit current in the point of installation of the circuit breaker Icc

10. Protection of lines
The correct circuit breaker must be apt to satisfy the following conditions:
it must own short circuit breaking power (lcu or eventually lcs) greater or equal to the short circuit current lcc
it must use a protection release so that its overload setting current ln (l1) satisfies the relation lB < ln < lZ
the let through energy (l2t) that flows through the circuit breaker must be lesser or equal to the maximal one allowed by the cable (K²S²)

11. Protection of lines
As far as the verification required by the IEC 364 standard, according to which the overload protection must have an intervention current lf that assures the operation for a value lesser than 1,45 lz (lf < 1,45 lz), we must state that it is always verified for SACE CBs since those are IEC standard compliant and the value is less than 1,3 ln.

12. Primary and secondary distribution
Selective protection A B C

13. Primary and secondary distribution
The example emphasizes the need of co-ordination of the intervention between the two A and B circuit breakers so that, in case of fault in C, only the B circuit breaker trips, thus leaving complete continuity to the rest of the plant supplied by the circuit breaker A.

14. Primary and secondary distribution
Selectivity might be total or partial:
total selectivity: only the circuit breaker B trips for every current value lesser or equal to the max short circuit current foreseen in C;
partial selectivity: the circuit breaker B opens only according to fault current lower than a certain value; values that are equal or greater than this will give the intervention of the two A and B circuit breakers

15. Primary and secondary distribution
Amperometric selectivity is obtained by setting on different values the instantaneous tripping currents of the circuit breakers’ chain (greater values for upstream circuit breakers)

16. ImA is the selectivity limit !
Primary and secondary distribution A B
ImB
ImA
ImA is the selectivity limit !

17. Primary and secondary distribution
Chronometric selectivity is obtained by introducing intentionally always greater delays in the intervention tripping timings of the upstream circuit breakers in the chain.

18. Primary and secondary distribution
Total selectivity

19. Primary and secondary distribution
Switchboard A
Switchboard B
400V
2500 kVA
(fault current 57,5 kA)
E2N20 MS (disconnector)
E4S40
with PR112
E2N20
with PR111
T5H 630
with PR222

20. Primary and secondary distribution

23. Primary and secondary distribution
Back-up protection A B C

24. Primary and secondary distribution
In the figure, the circuit breaker B, downwards in respect with A, might have a short circuit breaking capacity lesser than the presumed short circuit current in case of fault in C if the circuit breaker A satisfies at all the two following conditions:
it own correct short circuit power (greater or equal to the presumed short circuit current in its installation point and obviously greater than the short circuit current in C)
in case of fault in C with short circuit values greater than the short circuit breaking capacity of circuit breaker B, the circuit breaker A must limit the let through energy by tapering it to a correct value than can be stood by the circuit breaker B and by the protected lines

25. Primary and secondary distribution
The back-up protection is to be used in electric plants where operation continuity is not a main need: that is the tripping of an upstream circuit breaker will include in the black-out also those parts of the plant that are not interested in the faulty current. This co-ordination solution is used by those who need to contain the plant costs by reducing the general performance in case of fault.

26. Primary and secondary distribution
Co-ordination table for back-up protection
Upstream circuit breaker Short circuit breaking capacity
E2L - E3L 130 [kA] 380/415 V)
Downstream circuit breaker Breaker capacity on
back-upped loads
T4N 65 [kA]
T4S - T5N - E1B - E2B 85 [kA]
T4H - T5S - T5H - S7H - E2N 100 [kA]

27. Back up protection application example
T4L 250
100 kA !!!
T1N 160
T1N 160
T1N 160
* T1N 160 is 36 kA only mccb

29. Solution for selectivity
S6L 800
100 kA
T4L 250
T4L 250
T4L 250

30. Selectivity Table