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9.1 Purpose
Japanese and international standards require, in sum-
mary, that an overcurrent protector must be capable
of interrupting the short-circuit current that may flow
at the location of the protector. Thus it is necessary to
establish practical methods for calculating short-cir-
cuit currents for various circuit configurations in low-
voltage systems.
9.2 Definitions
1. % Impedance
The voltage drop resulting from the reference current,
as a percentage of the reference voltage (used for
short-circuit current calculations by the % impedance
method).
reference voltage
voltage drop at capacity load
% impedance =
x 100 (%)
(Reference voltage:3-phase – phase voltage)
2. Reference Capacity
The capacity determined from the rated current and
voltage used for computing the % impedance (nor-
mally 1000kVA is used).
3. Per-Unit Impedance
The % impedance expressed as a decimal (used for
short-circuit current calculations by the per-unit
method).
4. Power Supply Short-Circuit Capacity
3-phase supply (MVA) = kl3 x rated voltage (kV) x
short circuit current (kA)
5. Power Supply Impedance
Impedance computed from the short-circuit capacity
of the supply (normally indicated by the electric power
company; if not known, it is defined, together with the
X/R ratio, as 1000MVA and X/R=25 for a 3-phase
supply (from NEMA.AB1).
6. Motor contribution Current
While a motor is rotating it acts as generator; in the
event of a short circuit it contributes to increase the
total short-circuit current. (Motor current contribution
must be included when measuring 3-phase circuit
short-circuit current).
7. Motor Impedance
The internal impedance of a contributing motor. (A
contributing motor equal to the capacity of the trans-
former is assumed to be in the same position as the
transformer, and its % impedance and X/R value are
assumed as 25% and 6 (from NEMA.AB1).
8. Power Supply Overall Impedance
The impedance vector sum of the supply (Z
L
), the
transformer (Z
T
) and the motor (Z
M
).
Overall impedance of 3-phase supply
Z
L
+ Z
T
+ Z
M
(Z
L
+ Z
T
) • Z
M
(Z
s
) = (%Ω)
9. Short-Circuit Current Measurement Locations
In determining the interruption capacity required of
the MCCB, generally, the short-circuit current is cal-
culated from the impedance on the supply side of the
breaker.
Fig. 9.1 represents a summary of Japanese standards.
9.3 Impedances and Equivalent Circuits of
Circuit Components
In computing low-voltage short-circuit current, all im-
pedances from the generator (motor) to the short-cir-
cuit point must be included; also, the current contrib-
uted by the motor operating as a load. The method is
outlined below.
9.3.1 Impedances
1. Power Supply Impedance (Z
L
)
The impedance from the power supply to the trans-
former-primary terminals can be calculated from the
short-circuit capacity specified by the power company,
if known.
Otherwise it should be defined, together with X/R, as
1000MVA and X/R=25 for a 3-phase supply. Note that
it can be ignored completely if significantly smaller
than the remaining circuit impedance.
2. Transformer Impedance (Z
T
)
Together with the line impedance, this is the largest
factor in determining the short-circuit current magni-
tude. Transformer impedance is designated as a per-
centage for the transformer capacity; thus it must be
converted into a reference-capacity value (or if using
Ohm’s law, into an ohmic value).
Tables 9.1 show typical impedance values for trans-
formers, which can be used when the transformer
impedance is not known.
3. Motor Contribution Current and Impedance (Z
M
)
The additional current contributed by one or more
motors must be included, in considering the total 3-
phase short-circuit current. Motor impedance depends
on the type and capacity, etc.; however, for typical
induction motors, % impedance can be taken as 25%
and X/R as 6. The short-circuit current will thus in-
crease according to the motor capacity, and the im-
pedance up to the short-circuit point. The following
assumptions can normally be made.
a. The total current contribution can be considered
as a single motor, positioned at the transformer
location.
b. The total input (VA) of motor contribution can be
considered as equal to the capacity of the trans-
former (even though in practice it is usually larger).
Also, both the power factor and efficiency can be
assumed to be 0.9; thus the resultant motor contri-
bution output is approximately 80% of the trans-
former capacity.
c. The % impedance of the single motor can be con-
sidered as 25% and the X/R as 6.
9. SHORT-CIRCUIT CURRENT CALCULATIONS
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