• Breaking capacity :
The maximum short circuit current which can be safely stop by C.B. is called Breaking capacity.
C.B. : Protection from overload and short circuit.
Breaking capacity :
MCB MCCB ACB Fuse
6KA 25KA 50KA 125KA
10KA 36KA 65KA
15KA 50KA 70KA
65KA 95KA
70KA
• Example :
SLD
- S1 :
% z is not ohm's value we have convert
% value to Ω value.
→Z(Ω) = 10%Z × (kv)2/ KVA
= 10×6×(0.433)×(0.433)/ 1000
Z(Ω) = 0.01124
Isc = V÷√3×Z(Ω)
= 433÷√3×0.01124
= 433÷ 0.01946
= 22,250.77 A
22,250.77÷1000=22.250KA
Peak s/c = 2.5×Initial value
= 2.5× 22.50
= 55.62 KA
Therefore, the breaking capacity is 65KA.
• S2 :
We have to convert % × to (Ω)value
X(Ω) = 10× % × (xv)2/ KVA
= 10× 20× (0.433)2/ 1000
= 37.4978/ 1000
X(Ω) = 0.03749Ω
For LT D.G : R(Ω) = 0.15× (Ω)
= 0.15× 0.03749
R(Ω) = 0.0056235
Z = √Rsq.+ Xsq.
= √ (0.00562)+(0.0374)
Z = 0.04302
Isc = V / √3 × Z(Ω)
= 433/ 1.733× 0.04302
= 433/ 0.07451
= 5811.300 A
= 5811.300/1000= 5.8113A
Peak s/c = 2.5 × Initial value
= 2.5 × 5.8113
= 14.528 KA
Therefore, the breaking capacity is 25 KA.
• S3 :
500kw
950A - 400sq.mm- 0.10Ω/km
No. Of cable is 3
R(Ω) = length (mts) × Î©/km/ 1000× no.of cable
= 50 × 0.10/ 1000×3
= 5/3000
= 0.00166Ω
Isc = V / √3×Z(t/f) + Z(cable)
= 433 / √3 × (0.01124+0.00166)
= 433 / 1.732+0.0129
= 433 / 0.0223
= 1941.704 A
1941.704/1000 = 19.417 KA
Peak S/c current = 2.5 × 19.417
= 48.54KA
Therefore, the breaking capacity is 50KA
• S4 :
500 kw length-50mts
500 × 1.9= 950 A
400sq.mm - 0.10Ω/km
(3 no.of cable)
R(Ω) = length (mts) × Î©/km ÷ 1000× no.of cable
= 50 × 0.10 / 1000×3
= 5 / 3000 = 0.0016
Isc = V/ √3×Z(t/f)+Z(cable)
= 433 / 1.733× (0.01124+0.0016)
= 433 / 1.733× 0.01284
= 433 / 0.02223
= 19,478.182A
→ 19,478.182/ 1000
= 19.478 KA
Therefore, breaking capacity is 25KA.
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