Fused and non-fused terminal blocks ampacity

[lpena]

This may seem like a very basic question, but I just need to hear from other users about the correct way to use terminal blocks. Basically you select terminal blocks based on the voltage rating and amps. Well i have 24VDC circuits that are derived from a 24VDC power supply (see attached). The power supply has a max. output of 40 Amps or 960 Watts. I would like to fuse each 24VDC circuit/loads. I plan to use fuse terminal blocks such as Weidmuller WSI 6 LD. But looking at the datasheet of the terminal block, it says that it has a current rating of 6.3 Amps. The load is about 2 Amps. So that should be no problem, but should i also consider that the power supply has a max current of 40 Amps? Please advise. Thanks

 

[GoldDigger]

I would say that as long as the individual circuits are protected by fuses at less than 6.3A, the higher capacity of the power supply would not matter.
But if, for example, you had ten loads fused at 3A each, then you could not use some poles of that terminal block as a distribution block for the 30A from the power supply.

 

[lpena]

I would say that as long as the individual circuits are protected by fuses at less than 6.3A, the higher capacity of the power supply would not matter.
But if, for example, you had ten loads fused at 3A each, then you could not use some poles of that terminal block as a distribution block for the 30A from the power supply.

i am not sure what you mean with "some poles". And why can i use the terminal blocks as a distribution block.

 

[GoldDigger]

i am not sure what you mean with "some poles". And why can i use the terminal blocks as a distribution block.

What I am saying is that, using my example of ten circuits, you could land ten wires from the power supply on ten fuse terminals of the block.
Or you could join those ten input terminals with a suitably sized wire and land a single wire from the power supply to one of those terminals. (Although some would see that as potentially overloading that screw terminal somehow.)
What I would not do, for example, is run one wire from the PS to one screw terminal and, with or without a fuse, distribute from the far side of an interconnected terminal pair to the ten individual circuits.
Nor would I try to use their accessory cross connect strips to distribute to multiple poles unless the manufacturer approves them for a higher current.

I wonder whether the disconnects are rated for load breaking at 6.3A or just for carrying that current.

 

[lpena]

GoldDigger. Thanks for your replies. So you think it is OK to use the fused terminal blocks i specified earlier? Thanks. The fuses would be 3 Amps.

 

[Jraef]

The devil is going to be in the details here. If your PSU can put out 40A, that wire coming from it would have to be #8, and I seriously doubt any fuse block rated fopr 3A is going to accept #8 conductors. But there are some types of terminal blocks that have a "comb" feeder system that might be rated for 40A, so that would work to feed the line sides of the blocks if you want to do it that way.

Another issue is how the PSU responds to short circuits and overloads. Many have a "crow bar" circuit that shuts down the entire PSU, and it generally acts faster than a fuse, but of course kills everything. To avoid that, a few PSU suppliers offer Electronic Circuit Protection (ECP) units to go down stream. They distribute the output of the PSU into smaller circuits and have electronic "fusing" that trips faster than fuses, but is resettable and gives indication.

An important aspect of this is that sometimes you must have a "Class 2" circuit for a device that needs power. Class 2 means the power source is limited to 100VA, so on a 24VDC PSU, that's just 4A. Some of the ECPs allow you to have Class 2 circuits on the output side even though the source is larger than 100VA.

Most fuse blocks that have that disconnecting feature are specifically NOT rated for "switching duty" at all by the way.