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Fuses and PLC Question

Fuses and PLC Question

Fuses and PLC Question


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?


RE: Fuses and PLC Question

It's really hard for fuses to protect PLC cards. Fuses have a thermal delay that is usually greater than the PLC electronic's delay in blowing. There are semiconductor fuses for that but they cost $$$$ like many dollars a piece.

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

Are there any coils (contactor, valve, brake etcetera coils) involved? Can you show us a typical part of the schematics, with inputs and to what devices they are connected?

Gunnar Englund
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 was a long time ago and I forget the specifics, but I had issues with electronic circuits fused for very low currents.
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.

"Why not the best?"
Jimmy Carter

RE: Fuses and PLC Question

Brute force solution - use interface relays between the nasty outside world and the PLC, on both inputs and outputs. Over here in IEC-land we have several manufacturers incorporating relays within DIN-rail terminals so they don't take up much space and they aren't expensive - likely not much more than an MOV and fuse. Here's an example:

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

I like 'brute force", I routinely use the interface relay solution for the systems I do, especially if there are multiple power sources involved. DIN rail interface relays are usually the same width of most DIN rail mount terminal blocks and don't need much more room, especially the top entrance cage clamp relays like in the post above.


RE: Fuses and PLC Question

Hello I'm new here. I have ran into a similar situation myself. I would lean towards it being an over voltage issue blowing the input card. These cards state they have a pretty wide range of 85...132V AC @ 47...63 Hz. However, you stated that there are multiple 120vac supplies. Are those coming from separate 120 secondary transformers on multiple machines or sections of the machine? Check phasing of the transformers to each other. If your rotation is off between 2 transformers you could be getting a potential 240vac to ground if the 2 transformers are on separate legs of your 3 phase power. This may not be the case but if you are getting different voltages fed to the control transformer it could be intermittently spiking above the 132vac limit and popping your inputs. I would check if you can voltage difference and if there is any significant amount from your primaries on one transformer to the next. (L1 on one machine to L1 on the other, L2 - L2, etc.)

Hope this makes sense what I'm trying explain, just my thoughts

RE: Fuses and PLC Question

If the inputs are run in the same cable as outputs to solenoids the spike generated when the solenoids are de-energized can be several thousand Volts sometimes causing the inputs to turn on even though the switch is open. Providing MOVs across the coil usually fixes that, I would avoid using fuses, you want the machine to work not shut down.

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