2. OBJECTIVE
• Discuss the application in Short-Circuit Calculation of
ANSI/IEEE and the IEC Standards
• Demonstrate the Short-Circuit results in calculating a
common Low-Voltage network using both methods
separately
• Discuss the applications of the result of each method in
low voltage protective device selection and rating
evaluation
4. ANSI c37
I int,rms,symm – bolted 3-phase short-
circuit current at half-
cycle
I”k – bolted 3-phase short-circuit
current at half-cycle
IEC 60909
Zeq - is the sub-transient impedance
network at half cycle utilizing
the machine base impedance
V pre-fault – is the applied voltage just
right before the inception
of fault
C - factor used to adjust the pre-fault
voltage; C=1.1 for Un < 1000V
Un - is the applied voltage just right
before the inception of fault
Zk - is the adjusted sub-transient
impedance network at half cycle
utilizing the machine base
impedance
5. Compare to the LVCB interrupting
kA rating
Adjusted Interrupting Current, RMS Peak Short-Circuit Current
Adjusted Braking Current, RMS
Compare to the LVCB “making” kA
capability
Compare to the LVCB “breaking”
kA capability
ANSI c37 IEC 60909
20. The calculated peak current (ip) exceeds the device capacity
though the breaking current (Ib ) requirement has been
satisfied
21. Reminder
• It is not the intention of this discussion to fully show in
detail the methods used by both standards
• It is highly recommended to directly refer to either of the
standards applicable to your power system network
• The importance of having short-circuit calculation can
not be over emphasized. It is the responsibility of a
practicing electrical engineer to support his design with
calculations that are acceptable in the industry
