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 circuits 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 either be the short-circuit RMS value or the current limitation value.

Example SITOP modular 10 (Article number: 6EP1334-3BA10)

• 10 A rated current
• Current limitation typ. 12 A
• Short-circuit current rms value approximately 12 A

The technical specifications usually specify typical values, maximum 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 dimensioning.

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 technical 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 selected fusing according to this assumption.

List of ordering data and tripping characteristics of single-pole circuit-breakers 5SY4...

acc. to IEC 60898 / EN 60898, for use up to 60 V DC (250 V AC, switching capacity 10000 A)

Miniature circuit breakers acc. to EN 60898 (DIN VDE 0641-11) in 24 V DC circuits, which are powered by SITOP modular or SITOP smart power supplies

I_{out rated}: Rated output current

I_{out dyn}: Dynamic overcurrent with short-circuit during operation

?: instantaneous tripping, as dynamic overcurrent on short-circuit > limit current of electromagnetic tripping.

•: instantaneous tripping probable, as dynamic overcurrent on short-circuit at least 50% within tolerance range of the tripping characteristic.

X: no instantaneous tripping.

¹⁾ The selection of miniature circuit breakers that can be tripped is based on the consideration of the maximum possible short-circuit current of the power supply and the respective tripping characteristic at +20 °C. Further parameters that may be relevant in practice, such as self-heating, increased ambient temperature, line impedance and possibly currents flowing in parallel feeders, have not been taken into consideration.