Medium Voltage Application Guide

13720.A
Percentage DC component
SWITCHGEAR
Rated peak withstand current (kA)
This is the maximum peak fault current level which the device is able to close (make) on. This rating must be
greater than the expected peak let-through fault current (I p ) at the point of installation.
Rated peak withstand current ≥ I p
I p = 2.5 x I s (for a 50 Hz supply with a
45 ms DC time constant)
I p = 2.6 x I s (for a 60 Hz supply with a
45 ms DC time constant)
Source: IEC 62271-1, IEC 62771-100
Rated short circuit breaking capacity, Isc (kA)
Where:
I p = asymmetrical peak let-through fault current, from the first fault
loop (kA)
I s = rms symmetrical fault current level, with no DC component
(kA)
This is the highest level of rms current which the circuit breaker can successfully open (break) on a fault, at its rated
voltage. When a short circuit occurs in a 3-phase system, the initial fault current is asymmetrical and is made up of
an AC symmetrical component and a decaying DC component. The rated short circuit breaking capacity must be
greater than the expected asymmetrical fault current level when the circuit breaker poles are opened.
Where:
I I sc = rated short circuit breaking capacity of circuit breaker (kA)
SC I a
I a = asymmetrical fault current level when circuit breaker poles are
Ia
= Is
+ Id
I s
opened (kA)
= AC symmetrical fault current component (kA)
I d = DC fault current component (kA)
Standard values for I sc = 6.3, 8, 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63 kA (source: IEC 62271-100)
Percentage DC component in fault current
100
90
4 = 120 ms
80
3 = 75 ms
70
2 = 60 ms
60
50
40
30
20
10
1 = 45 ms
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Time interval from initiation of short circuit current (ms)
The graph illustrates the percentage DC component of a fault, over a period of time, for systems with various time
constants. Most systems use the standard time constant ( 1 ) of 45 ms. The total opening time of the circuit
breaker is the pole opening time plus 10 ms for relay sensing, and this figure can be used to determine the
percentage DC component of a fault at the instant of breaking.
Page 92 Medium Voltage Application Guide 710-12280-00A