fusing analog Inputs and outputs

Utilizing the Contrologix 1756 platform analog input/output modules, I don't see any recommended fusing requirements in the AB books even though I still use fusing anyway.

Am I wasting wiring time and money putting in fusing for these modules?

I would not fuse inputs individually, in most cases just one device is attached and the power consumption will be small. Just make sure there is a fuse for the input supply in case of a short.

The outputs are another thing...I guess it depends. I have used those terminal fuses that have the light that come on when the fuse blows.

I highly recommend that all outputs be fused individually, after the PLC Output.

There are several reasons:

Some PLCs have one "internal" fuse for several outputs. If the fuse blows, you don't know which output (field device) caused the problem.

Some PLCs have one "internal" fuse for each output. This is better...

However, in either case, and in most cases of PLCs with cards, if a fuse blows then you have to pull the output card to replace the fuse... the whole system has to be brought down.

This might be OK for small processes, however, in very large processes this can be quite a pain in the a$$. The entire process has to be stopped... just to change a fuse... doh!

Another pain in the a$$ is with the brick-type PLCs. They do not have any "internal" fuses that you can change. They might have surface-mounted fuses for protecting any number of solid-state outputs... but then, what are you gonna do when you blow one of those? Nothing.

If it happens to be that ALL of your outputs are used in the process then you have to replace the Module because there are no spare outputs to go to.

Sure, these days, the price of a "brick" has fallen tremendously...
...but do you have a spare on hand?
...if it happens to be the CPU Module, do you have someone on site that can re-load the code?

The BEST way to preserve your PLC outputs is to fuse each one, individually, AND to have that output feed an interposing relay... either an electro-mechanical relay with a KNOWN, very low current draw that is far below the rated output current of the output, or some sort of solid-state relay.

The absolutely safest way is to have the output feed to an Opto-Isolator which then controls your field-device.

However, the "Big-Boy" BA$TARDS are charging something on the order of $50.00 per output for Opto-Isolators.

And again, however, there is an "Almost Big-Boy" that is charging much, much less. That company is OPTO-22.

They are charging about $5.00 per output! You do have to cough-up for their module-card, but they are reasonable as well.

Your second most important goal, after developing a proper and working process, is to protect the processor, and all of the output modules.

The Opto-Isolators will provide for your second goal... however, they can't do anything in terms of your first goal... that is up to you.

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