353 / 408
11
11/39
Dimensions
Section 12
Circuit breakers for direct
current applications (cont.)
24 V - 48 V direct current applications
Technical advice
Constraints related to "direct current" applications
In direct current, inductors and capacitors do not disturb the operation of the
installation in steady state. Capacitors are charged and inductors no longer oppose
changes in the current.
However, they create transient phenomena when the circuit opens or closes, during
which time the current varies. Actual loads have both characteristics and generate
oscillatory phenomena.
Type of load
Inductive load
An inductive load will tend to lengthen the current interrupt or establishment time,
because the inductance
L
then opposes the change in the current (
Ldi/dt
).
The transient phenomenon will mainly be characterised by a time constant imposed
by the load and whose value corresponds approximately to the interrupt or closing
time that the switchgear has to withstand. In addition, during the interrupt time, the
switchgear must be able to withstand the additional energy stored in the inductor in
steady state.
An inductive load therefore requires particular attention with respect to its time
constant.
A low value (typically < 5 ms) facilitates interruption.
Capacitive load
During a closing operation, a capacitive load will cause an inrush current due to the
load on the capacitor, virtually under short-circuit condition at the beginning of the
phenomenon.
On opening, it will tend to discharge. The time constant is generally very low (< 1 ms)
and its effect is secondary with respect to the inrush current. A capacitive load will
require particular attention to the inrush or discharge current surges.
DB124240
U
S
i
S L
R
E
Ri
U
L
= L di/dt
DB124241
E/R
τ
= L/R
i
t
Inductive load
DB404550
i
S
C
E
Req
E
S
U
L
Rsource + Rcables
DB124243
τ
=
i
i = E/Req
E
t
Rsource
+ Rcables
Iinrush =
Rsource C
Capacitive load