Standardizing on PLC I/O Fuse Sizes

[Scouseget]

Hi all, this is my first post so please forgive me is this is too wordy or confusing.

I just trolled through previous posts to see if there was anything definitive regarding fusing PLC I/O's, and found that there are lots of opinions about this but no concensus. This mirrors the situation at work; ask 10 designers and you'll get 10 different answers.

What I'm hoping to do is to come up with guidelines for our designers about fuse sizes for I/O's. What we do now, is to fuse every I/O individually as this is a client requirement as it "protects" the individual loop components, and allows isolating the field equipment during commissioning and trouble-shooting. The question is, what size of fuses?

My contention is that the fuse is really there to provide short circuit protection, not overload protection, and therefore it is not only not necessary, but poor practice, to "close" fuse. If I had my way, I'd use 2 amp fuses across the board, even for analog loops. That way, the maintenance guys wouldn't use whatever fuse is at hand whenever one blows, this probably being 10 or 15 amp fuse; standardizing on say 2amp fuses, and having a bunch of such spares in the cabinet, would ensure the right size fuse being used every time one is replaced.

Some guys here use 1/8amp fuses for analog loops, 1 amp fuses for D/I's, and various sizes for D/O's, which I really do not agree with. Do you see any problems using only 2 amp fuses for everything (unless a D/O requires a higher amperage fuse of course)? Any other suggestions will be appreciated.

 

[Ozpeter]

The way I see it is that there are 3 at least different current levels in play:

1) The max current your I/O handles in order to correctly work (including transients)
2) The max current your I/O can source/sink with damage to the module etc
3) The max current your I/O wiring can handle without damage to insulation etc

1 needs to be less than 2 and 3. I would think that 3>2. But as long as the fuse is rated between 1 and 2 then it shouldn't matter what the actual value is. But this would also imply that the fuse size is related to the class of I/O (analog vs 24vdc vs 120vac vs relay etc)

 

[califflash]

Trip curves also play a huge role. I typically use 1 amp bussman fast acting agc fuses. I will use up to 2 amp fast acting fuses. The reason being that the outputs on the Plc that I use on all my equipment is rated at 2 amps per point. I typically only use the 2 amps for something with a high inrush like a contactor.

I am against time delay fuses on solid state outputs, which I use almost exclusively.

If i need more service then I will use an isolation relay. Wago makes a nice terminal block style unit with a fuse, relay, and hand-off-auto switch built in. I would use something like that if I was truely worried about losing an output.

 

[ASF]

Possibly the most accurate statement you could have ever made - about just about anything -

"ask 10 designers and you'll get 10 different answers."

To be honest, no matter what you try and standardise on, no matter how simple or complex your solution, there will be advantages and disadvantages, and you will get people who think you're doing it right and doing it wrong.

At the end of the day, as a designer, if you can back up your position and have a solid argument as to why you want it done that way, then just roll with it and ignore anyone who doesn't like it. If you bend to make them happy you'll upset someone else.

 

[Mill_Control]

My contention is that the fuse is really there to provide short circuit protection, not overload protection,

A short is gonna overload something 2A is probably OK for most digital I/O but analog is usually expensive enough to justify fusing tight. Had a cable cut on a Temposonic yesterday. Glad that one wasn't fused to protect the wiring