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9/11
Circuits supplied with DC current
Choosing a circuit breaker
Miniature circuit
breakers
Dimensions
Section 10
The choice of circuit
breaker type, for
protection of a DC
installation, depends
mainly on the following
criteria:
n
Nominal current which is used to choose current
rating
n
Nominal voltage which is used to determine the
number of serial-connected poles contributing to
breaking
n
The maximum short circuit current at the installation
point, used to define breaking capacity
n
The type of network (see below)
Type of networks
Earthed networks
Unearthed networks
Diagrams and
various possible faults
The source has an earthed
polarity
The source has an earthed
mid-point
Analysis of each
fault
fault
A
Maximum lsc only the positive
polarity is concerned
lsc close to maximum lsc
only the positive polarity is
concerned at half voltage U/2
No effect
fault
B
Maximum lsc both polarities are
concerned
Maximum lsc both polarities are
concerned
Maximum lsc both polarities are
concerned
fault
C
No effect
Same as fault A but this time the
negative polarity is concerned
No effect
The most unfavourable case
Fault A
Faults A and C
Fault B
Distribution of the
breaking poles
All the poles effectively
contributing to breaking are
serial-connected on the positive
polarity
(1) (2)
On each polarity provide the
number of poles required to break
maximum lsc at voltage U/2
Distribute the number of poles
required for breaking on each
polarity
(1) or negative if it is the positive polarity that is earthed
(2) provide an additional pole on teh earthed polarity if the aim is isolation
b
a
U
i
R
B
A
C
b
a
i
R
B
A
C
U/2
+
U/2
b
a
U
i
R
B
A
C
Short circuit current at the terminals of an
accumulator battery
When its terminals are short-circuited, an accumulator battery delivers a current
given by Ohm’s law:
lsc =
Vb = maximum discharge voltage (battery charged at 100 %).
Ri = internal resistance equivalent to all the cells (value normally given by the
manufacturer according to battery Ampere/hour capacity).
Example
What is the short circuit current at the terminals of a stationary battery with the
following characteristics:
n
Capacity: 500 Ah
n
Maximum discharge voltage: 240 V (110 x 2.2 V cells)
n
Discharge current: 300 A
n
Backup time: 1/2 hour
n
Internal resistance: 0.5 m
Ω
per cell
Answer
Ri = 110 x 0.5.10
-3
= 55.10
-3
lsc =
== 4.4KA
As shown in the calculation above, shortcircuit currents are relatively small.
Note: if internal resistance is not known, the following approximate formula can be
used:
lsc = kC where C is the battery capacity expressed in Ampere/hour and k is a
co-efficient close to 10 and in all cases always less than 20.
Vb
Ri
240
55.10
-3
Isc
240 V DC
300 A
500 Ah
Ri = 0.5 m
Ω
/cell