MB30-CS datasheet(99/105 Pages) MITSUBISHI

99 / 105 page

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) =

3 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 (ZL), the

transformer (ZT) and the motor (ZM).

Overall impedance of 3-phase supply

ZL + ZT + ZM

(ZL + ZT) • ZM

(Zs) =

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 (ZL)

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 (ZT)

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 (ZM)

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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