fuse sizing?

[mroyce09]

I'm a little over my head right now and am hoping someone can help me out. I'm working on a project that requires an AB SLC 5/05, 115V Input card (1746-IA16) and Analog Input card (1746-NI4). I was told that each inputs connection on the cards need be connected to a fused terminal block. Electricity is not my forte and I can't figure out what size fuses are needed. If someone could help me out that would be great!

 

[Bullzi]

Not sure why you would fuse protect the input? I would use a fuse to protect the power supply that you use for the inputs.

On the 110VAC inputs, where does the power come from for your field contacts? From your panel? If it does I would put a 1A fuse on each power leg to your contacts. If you have only a single run out to the field and jumper the power to all the devices then you might want to use a 5A fuse. Then inputs draw very little power so you don't need a big fuse. Just enough to protect the wiring and the supply in case of a short.

On the Analog, (I am assuming it is 4-20mA), I would put a 0.5A fuse on the power out to each transmitter.

If you have a drawing that would help you determine how to move forward.

Hope this helps.

 

[rootboy]

I'd have to agree with Bullzi, there's isn't much point in fusing your inputs at the card. Now the power line that goes out to the various switches, etc needs to be fused.

As for sizing that fuse, size it to protect the line feeding your devices. Should you have some device that has special needs, then fuse that device as necessary.

 

[rupej]

Fusing a foot or so of wire in the panel for each input? Are they really worried it's going to short there?

 

[nhatsen]

Read this other thread. I'm with post #12:

We fuse all our analogs individually and each di / do card supply individually. Most of our field devices are outside, in pits or tanks or other environments where cables can get damaged, water ingress happens from time to time, etc.

 

[mroyce09]

Thank you everyone for your responses. I think the thought is, if each input is fused, it would protect the card/devices in the event of a power spike or something like that. As far as the power supply is concerned, I thought that that cards themselves get the power from the SLC 500 power supply.

 

[redbarron55]

Thank you everyone for your responses. I think the thought is, if each input is fused, it would protect the card/devices in the event of a power spike or something like that. As far as the power supply is concerned, I thought that that cards themselves get the power from the SLC 500 power supply.

Not so much.
Lightning strikes usually are pretty fast and the voltage probably spikes faster than the fuse clears.
A MOV might crobar fast enough, though.
We installed lightening suppressors in our analog inputs from the field and they have protected the instruments and inputs so far and in each instance they have cleared the fuse each time they came into play.
(fused instrument lines ya know).
When the devices crowbar from the lightning impulse they also draw enough current either from the 24 volt supply or the lightning the fuse is history, but cheaper than input cards and instruments!