Fuses and PLC Question
Fuses and PLC Question
(OP)
Hello,
In my facility, I have three PLC input cards connected next to each other (SLC 500 1746-ia16) controlling several field devices. On each input (16 total) of the cards, I wired a 100ma fuse to protect the inputs from blowing. After several months of the cards working, numerous inputs on each card blew; however, their fuses did not blow. I examined the inputs on each card and saw that the resistor got blown. Could it be possible for it to be a voltage issue? Some of the field devices are powered by different 120v sources. Do you guys have any suggestions on what the problem is?
Regards,
In my facility, I have three PLC input cards connected next to each other (SLC 500 1746-ia16) controlling several field devices. On each input (16 total) of the cards, I wired a 100ma fuse to protect the inputs from blowing. After several months of the cards working, numerous inputs on each card blew; however, their fuses did not blow. I examined the inputs on each card and saw that the resistor got blown. Could it be possible for it to be a voltage issue? Some of the field devices are powered by different 120v sources. Do you guys have any suggestions on what the problem is?
Regards,





RE: Fuses and PLC Question
Inputs are usually completely passive and optically isolated so it's very hard for them to ever 'draw too much'. I never fuse my inputs. You typically fuse the supply (excitation) power if it is sent to the field but not the individual inputs.
Yes, it was almost certainly an overvoltage that popped the inputs.
Lightning is a favorite cause if you have far ranging signals coming to the inputs. What sort of job is this PLC tasked with?
You can protect inputs with MOVs and some resistance between the field and the inputs.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Fuses and PLC Question
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Fuses and PLC Question
Sorry for the late reply. This PLC is in control of several push buttons to start our machines, lights, and to keep our motors running. Thanks for your advice on protecting the input. At the current moment, we are logging data on the voltage to see if that was the main problem. What we did was just replace the cards that got blown and its functioning now; however, we still want to find the root cause of this issue.
I am sorry, but I dont have any schematics available.
I will continue investigating and let you guys know on further updates!
RE: Fuses and PLC Question
It turned out that the the low current fuses were several thousand Ohms resistance. It's a long shot but the resistance of the fuses nay be having unforeseen consequences.
Bill
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"Why not the best?"
Jimmy Carter
RE: Fuses and PLC Question
I routinely use them for output channels but I've used them a few times to protect high density 64 channel cards where I don't want a careless mistake to wipe out an expensive input card.
RE: Fuses and PLC Question
MikeL.
RE: Fuses and PLC Question
Hope this makes sense what I'm trying explain, just my thoughts
RE: Fuses and PLC Question