Rather basic, but I can't figure out what exactly backup fuses are! I was studying a Weidmuller PRO ECO 120W 24V 5A power supply's datasheet (accessible via the catalogue page). It has recommended three types of backup fuse for input, but I can't find out what backup fuses are and why to connect them. I searched for it but it didn't help me that much!
The instruction manual states:
This unit is equipped with an internal fuse, so no additional external device protec tion is required. The recommended backup fuse is listed in the Technical Specifications.
Notice: When the internal fuse is triggered, the probable cause is an internal malfunction. The device must then be inspected in the factory!
I don't see any limit on the fault current capability of mains supply to which the product is suitable to be connected. However the standards that apply may state limits for such products and require backup protection with higher interrupting capacity if the product is connected to a supply that is capable of supplying a higher fault current. Local electrical codes may also require external protection under some circumstances.
The datasheet states that the PSU has an internal fuse. The backup fuse is the external fuse that you supply.
Figure 1. Extract from Weidmuller PRO ECO 120W 24V 5A datasheet.
The datasheet gives you three options. Choose one to suit - perhaps to match fuse or circuit breakers in the rest of you control panel.
Redundant fuses or breakers would be a convenience for additional fault protection if there is frequent power cycling. Normally PSU's in large companies such as Weidmuller are tested for reliability with 10k power cycles but the surge currents can cause stress on components. If your line voltage is not well regulated and turn on occurs during peak voltage this would cause the maximum surge current such as power restore in a building with imbalanced phases for startup with surge drop on one phase and rise in another phase.
Inrush current is 40A max.
Power cycling is a reliability test and the design of soft start would reduce the need for redundant tighter threshold of surge currents , if the protection inside is a "slo-blow" type fuse.
Consult with OEM tech support for additional reasons for choices and to confirm my above assumptions if you have doubts on power cycling or slow blow internal fusing.
Internal faults can be externally caused from unfiltered lines and transient conditions, so every situation is different, but I would expect at least minimal line filter is included inside this design.
If you plan on buying in large quantities, I would suggest a rated load power cycling test plan to confirm reliability.