SIEMENS
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Siemens KT 10.1 · 2016
15
Technical information and configuration
Fusing of the output circuit 24 V DC, selectivity
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Overview
Fusing of 24 V power supply circuits and selectivity
With non-stabilized rectifiers (power transformer equipped with
rectifier) the output usually had to be protected with a suitable
fuse so that its rectifier diodes would not fail in the event of an
overload or a short-circuit (this would destroy the DC loads due
to the resulting alternating voltage and lead to serious damage
in most cases).
On the other hand, the stabilized SITOP power supplies are
provided with integral electronic short-circuit protection that
automatically protects both the power supply and the supplied
24 V DC circuits against an excess current in the event of an
overload/short-circuit. A distinction must be made between the
following three cases with respect to fusing on the secondary
side:
Example 1: No fusing
Fusing the secondary side (24 V DC) for protecting the load cir-
cuits and lines is not required if the respective cross-sections are
selected for the maximum possible output current RMS value.
Depending on the event (short-circuit or overload) this may ei-
ther be the short-circuit RMS value or the current limitation value.
Example SITOP modular 10 (article number: 6EP1334-3BA10)
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10 A rated current
•
Current limitation typ. 12 A
•
Short-circuit current rms value approximately 12 A
The technical specifications usually specify typical values, max-
imum values are approximately 2 A above the typical value. In
the example here, a maximum possible output current rms value
of approximately 14 A must therefore be used for line dimension-
ing.
Example 2: Reduced conductor cross-sections
If smaller conductor cross-sections are used than are specified
in the relevant standards (e.g. EN 60204-1), the affected 24 V
load infeed cables must be protected with a suitable line
protection.
It is then unimportant whether the power supply enters current
limiting mode (overload) or delivers the maximum short-circuit
current (low-resistance short-circuit). The load supply is in any
case protected against an overload by the line protection
matched to the conductor cross-section.
Example 3: Selectivity
In cases where a load which has failed (e.g. because of a short-
circuit) has to be rapidly detected or where it is essential to
selectively switch it off before the power supply enters current
limiting mode (with current limiting mode, the voltage would also
fall for all remaining 24 V DC loads), there are two possibilities for
the secondary side connection:
•
Use of a SITOP PSE200U selectivity module or the SITOP
select diagnostics module for distributing the 24 V DC supply
among up to 4 load feeders.
Each output can be set between 0.5 A and 3 A
(Article number: 6EP1961-2BA11, -2BA31) or 3 A and 10 A
(Article number: 6EP1961-2BA21, -2BA41) or 2 A and 10 A
(Article number: 6EP1961-2BA00).
•
Series connection of appropriate 24 V DC fuses or miniature
circuit breakers
The basis for selection of the 24 V DC fuse or miniature circuit
breaker is the short-circuit current above the rated current which
the SITOP power supplies deliver in the event of a short-circuit
during operation (values are specified in the respective techni-
cal specifications under "Output, dynamic V/I on short-circuit
during operation").
It is not easy to calculate the amount of the short-circuit current
flowing into the usually not ideal "short-circuit" and the amount
flowing into the remaining loads. This depends on the type of
overload (high-resistance or low-resistance short-circuit) and
the type of load connected (resistive, inductive and capacitive/
electronic loads).
However, it can be assumed with a first approximation in the
average case encountered in practice that the difference of
dynamic overcurrent minus 50 % SITOP rated output current is
available for the immediate tripping of a circuit breaker within a
typical time of 12 ms (with 14 times the rated DC with a circuit
breaker characteristic C acc. to IEC 60898, or with 7 times the
rated DC with a circuit breaker characteristic B or with 5 times
the rated DC with a circuit breaker characteristic A). Please refer
to the following tables for circuit-breakers appropriate for se-
lected fusing according to this assumption.
© Siemens AG 2016