As a general rule, I make sure that all I/O wires go direct to terminal, then from there to whatever else in the panel.

So the question is, do most people do that or if the device is in the enclosure, wire direct from the card to the device, not to terminal?

well... i used to do the same but then cabinets used to get so huge... most of the new installations I work on
these days use some sort of distributed I/O. the idea behind bringing things to terminals is to have access for
troubleshooting, and sometimes to find right I/O easier. some neat freaks will want it that way no matter what
(I'm more flexible ).
i would still do it if there is no access to some connection (high density cards or devices that must be taken appart
to reach teminals etc.). for standard 16-pt cards with screw terminals - that's good enough so by default I would skip
the extra terminals (more material, more labour, more points of failure).
if customer likes everything on terminals, that ok, they'll just pay a bit more, they can even have relay on each
output and whatever else they want...

We wire all I/O to terminal blocks except for like the Flex I/O and fuse each output with glass fuses in a 1492-H4.

Hope this helps.

you mean each output individually?

Saving the extra terminal strip is one of the reasons that we use distibuted ET200S i/o blocks. The access to these is much much better than the terminals in classical rack/slot i/o modules. The 24VDC outputs are shortcircuit proof (no need for extra fuse), and the cost is much lower than the classical rack/slot type i/o modules.

If you HAVE to use high density classical rack/slot type i/o modules, then I recommend to use a separate terminal strip. I remember with horror how awful the access to for example PLC5 32-channel i/o modules was (still is).

Yes each output gets a 2A fuse.
I am new [3 months] at this company and that is the way they do. Maybe it has been requested by the customers in the past but it is how they have me do it.

Thats nuts, you would usually just fuse the common that supplies each set of outputs, thats the way i have always seen it done anyway

No it makes perfect sense with 1 fuser per output.
1. You minimize what is disabled by the blown fuse in that way. Chances are that the proces or machine can continue operating.
2. It greatly aids in finding the fault. The blown fuse points you directly to the shorted circuit. A godsend if the error is intermittent !

I agree if you have miles of field wiring then its a fair point but *EVERY* output is taking it to extremes, whats the point of fusing a contator or relay in your control panel ?

You are right. A fair compromise is every output that goes into the field.

No set rules that apply to all situations - we do have applications where every output is fused by necessity. A machine making beer bottles with literally thousands of outputs - when one solenoid shorts or a wire shorts - you do not want to shut down any more of the machine than necessary with the faulted output signal.

Individual fusing allows continuation of machine operation and as mentioned, ease of troubleshooting the offending circuit. I have designed and built panels for a variety of fortune 100 manufactures that write this into the specification - yes it is costly - but in the long run, 200 fuse blocks is cheaper than an hour of downtime on a system that runs 24x7 for 10 years before it is expected to be retired. Not crazy at all . . . in fact, not fusing is crazy in a lot of cases.

We built panels for several customers whose spec called for fusing every output, ex: Chrysler. Their reasoning was it made things easier to troubleshoot. Costly, yes; but I have to agree with them. Whatever the customer wants.

Also, regarding the original question about wiring from the I/O module to a terminal strip; that is how I like to see it. Over the last 20 years I've been on several projects where the PLC I/O was being upgraded and it made it easier if it was all wired to terminals.

Sorry for bringing this up again. I would almost always insist on taking the in and outputs to interface modules which is fused and has led indication (if your using high density cards). Would rather blow a fuse than a input or output. But then again if the customer is strapped for cash or there is no panel space you must do what you have to.

A contactor or relay is a much more likely point of failure than a solid state device (which most I/O cards are). Between normal failures, the lack of cycles (1 million switching cycles is usually the maximum), vulnerability to dirt, oil, and water (not that this should be in your panel), and the tendency to freeze from cold/heat, I avoid relays except when they are a necessary evil. Frequently you can get I/O cards by the way that are either mechanical relays, transistors (24VDC), OR triacs (120/240VAC), which eliminates a stage. Furthermore, why wouldn't you locate the relay in the MCC (starter panel) where it belongs? And bringing starters/contactors directly into your PLC & I/O panel is a huge issue with regards to SCCR among other things.

The only logical reason to have a relay or contactor is if there's a lack of drive power at the PLC output, or if you need isolation for some reason, that I can think of.

I have seen many, many I/O panels where they take a perfectly good PLC I/O block and wire relays to it essentially "just because". Guess what usually tends to be the problem when you troubleshoot it? Nope, haven't had too many I/O cards fail lately. But there's a reason that relays are so cheap...volume sales.

One of the huge improvements GM saw when they were involved in creating the first PLC's for "rapid" product changeovers is that reliability went up massively due to the fact that the vast majority of the relays were eliminated. Building an I/O panel full of unnecessary relays simply brings you back to square one again.

As to fusing every I/O point...this is easily done. Depending on your PLC, you can get the fuses built into the terminal block on the I/O card. I have seen (but haven't used) "electronic" fuses on some I/O cards. If not, you can also get modular IDC mounted terminal blocks with fuses and indicator LED's built-in. These are what I've been buying because it makes troubleshooting time (downtime is much more expensive than panel building or component time) so much faster.

If you've ever fought an intermittent fault that is "somewhere" that keeps blowing your single common power fuse, you'd understand why fusing every I/O point is a much better situation. The cost isn't that bad if you are already buying terminal strips and/or I/O. It just means that you increase the cost of the I/O panel by a few percent because you buy it with fuses integrated into it.

Hmmmm at a guess i would say you dont work for an OEM do you ?

This is the way to go. Forget fuses.
With "electronic" fuses, which are really shortcircuit proof i/o with diagnostics, you get not only the most simple and compact installation, but you get meaningful and instant diagnostics on your HMI.

I agree with that removing relays will increase the overall reliability. I/O with hw diagnostics brings the whole thing to the next level.

Of course it costs more than bare bones i/o modules, but it is definitely not more expensive than traditional i/o plus traditional terminal strips with fuses.

No. I have to live with the stuff I build. As in run it, troubleshoot it, and maintain it.

Didnt think so, if you did work for an OEM try telling your MD that you want to fuse every output at the cost of £xxx as it will be easier for the end user to fault find, he just sees this coming straight off his bottom line.

I can guess what the answer would be 99 times out of 100

I would also hazard a guess that the vast majority of OEM's would not do it unless its specified.