MV design guide - Schneider Electric

Standards
IEC - ANSI comparison
According to ANSI
c The circuit breaker must be able to break:
v the rated short circuit current at the rated maximum voltage
v K times the rated short-circuit current (maxi symmetrical interrupting
capability with K: voltage range factor) at the operating voltage (maximum
voltage/K)
v between the two currents obtained by the equation:
maxi symetrical current
rated short-circuit current
rated maxi voltage
= = K
rated voltage
We therefore have a constant breaking power (in MVA) over a given
voltage range. Moreover, the asymmetrical current will be a function of the
following table taking S = 1.1 for Merlin Gerin circuit breakers.
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
ratio S
Asymmetrical interrupting capability = S x symetrical interrupting capability.
Both at specified operating voltage
Symetrical interrupting capability at
specified operating voltage = 1.0
0 0.5 1 2 3 4
cycles
0 0.006 0.017 0.033 0.050 0.067 seconds
Example:
c Isc = 40 kA
c % asymmetry = 50%
c Iasym = 1.1 • 40 = 44 kA
44 44
c Isym = = = 36 kA
1 + 2(50%) 2 1,22
Sequence 6 will therefore be tested at
36 kA + 50% asymmetry,
this being 44 kA of total current.
c Rated short-circuit breaking capacity (kA)
Sequence n° current broken % aperiodic component
1 10 50 - 100
2 30 < 20
3 60 50 - 100
4 100 < 20
5 KI to V/K < 20
6 SI to V 50 - 100
7 KSI to V/K 50 - 100
8 electrical endurance
9/10 reclosing cycle at ASI and AKSI
11 C - 2 s - O at KI
12 rated Isc duration = KI for 3 s
13/14 single phase testing at KI and KSI (0.58 V)
Short-circuit breaking testing must comply with the
14 test sequences above, with:
I : symmetrical breaking capacity at maximum voltage
R : reclosing cycle coefficient
(Reclosing factor)
K : voltage range factor:
K = Vmax
Vmin
Iasym
S : asymmetrical factor: = 1.1
Isym
for Merlin Gerin circuit breakers
V : maximum rated voltage
76 Merlin Gerin MV Design Guide Schneider Electric