Settle a bet! INPUT devices connected to PLC input card


Thread Starter

Garrett Place

I have a question for anyone who has wired a panel/machine. How common is it to fuse individual or groups of INPUT devices before they are connected to the PLC input card. If this is common, how is it done and for what reason. Would it be done for AC or DC or both?

Thanks for the help. I hope this will settle a bet I have with a co-worker.


John Waalkes

> I have a question for anyone who has wired a panel/machine. How common is it to fuse individual or groups of INPUT devices before they are connected to the PLC input card. If this is common, how is it done and for what reason. Would it be done for AC or DC or both? <

Pretty rare I would say. We thought about doing it for limit switches that were shorting themselves out occasionally when the contacts would crossover internally.

You haven't lived until you tried to figure out which limit switch out of hundreds is the one that is blowing your fuse, during production of course :)

We never actually did this, since we are in the process of replacing most of the switches with proxes, and for those that we can't get rid of, going with an 8-pole base and adding a current limiting resistor in series with the neutral.

> Thanks for the help. I hope this will settle a bet I have with a co-worker. <

Good luck :)

Generally I have never fused inputs because it would only protect the input card and it would cost you more to fuse the inputs than to replace the card

Steve Myres, PE

I have seen the control common (hot) fused for each section of a machine and sometimes even for individual inputs. This is not based on protecting the circuit against excessive current consumption by the PLC input circuits. Rather, the thinking is that if a fault develops in the wiring either between the power supply and the input device or between the device and the PLC, or the device faults internally, the fuse will isolate the problem to this machine section or input point. Otherwise the overcurrent protection on the main control source will open and disable a lot more input (and maybe output) devices.
Not common at all. You will fuse the power supply feeding the input devices, but you do not need to fuse the PLC inputs themselves.

Doesn't matter if it's AC or DC.
I would the fuse the power to a group of inputs.
Why? Have you ever tried to find a shorted device or its wiring when all you have is a main fuse which has blown?
By fusing the inputs in groups you have provided a valuable troubelshooting tool that will reduce downtime.

I have never fused the inputs themsleves. I do fuse the sensor/switch power feed, and I usually group these either based on location, or by input card, that way a short can be traced to a group of devices. I also used center bar jumpers on my power source terminal strip, that way by removing the jumper bar a maintenance tech can very quickly identify with device is causing a short. But to individually fuse each of the inputs is a waste of money because it only protects the wire between the fuse and the input card.
Come on guys. If cost&space no object, you know its always better to fuse each input/output.

When you go work on a certain device, like a high level switch, you can pull the individual fuse (if you paid extra for the fused terminal block). That way you don't kill the power to like 7 other devices. If reducing downtime and faster diagnostics is important, the customer should fuse them all. How much of the paper plant do you want to take down by that 1 of 8 inputs that has the problem?

Go cheap and take the chance. Thats risk
I have seen both ways. Fused inputs cost more, but if a wire shorts out it is much easier to find. If downtime is very expensive, such as an
automotive assembly plant, then fused inputs might make sense.

Bill Sturm
I've wired panels for 10 years before I became an engineer and I have never heard of a fused input. I agree with Doug usaully you have a good sum of inputs and it would cost a bundle to fuse each individual or each card. No sense in it whether it being DC or AC

Fusing individual inputs is uncommon. Fusing per card is not so uncommon.

The wiring and devices in the input circuits need to be protected. Ground faults occur in field wiring. Whether the protection is done with other equipment, all inputs grouped, per card or per input depends on:

1. Whether sourcing or sinking input configurations are used.
2. Device groupings by similarity and by function.
3. Fault discrimination for economic wiring size selection.
4. Field device ratings.
5. Ease of fault finding particularly when there are a lot of inputs.
6. Isolation for maintenance. (Fuse/switch)
7. Isolation of a fault. i.e. Effects of faults on other inputs and system behaviour particularly on critical systems.
8. Division of functions within the PLC.
9. Probability of faults.
10. Consequences of faults.

It's a good idea sometimes to directly wire the supply to an input so the PLC can check the status of the fuse.

Vince Dooley
Not common at all, as most input modules are high impedance/resistance devices that require no current limitation.

Working in automotive paint robotics, we commonly see, due to hazardous area requirements, intrinsically safe barriers (fusible or diode) to prevent shorts from generating an ignition source (spark).

(Good with numbers,
sort of retarded.)
[email protected]


Donald Pittendrigh

Hi All

Fusing the supply side of an input, especially with the type of fuse holder with an LED to indicate fuse blown, is not a bad idea as it enables finding of an earth fault or short circuit on the field wiring which would otherwise
be a time consuming fault to find. Fusing of the return path, presumably to protect the field wiring from overload by the PLC input???? is dumb and a waste of time and money for previously mentioned reasons.

Donald Pittendrigh
Responding to Don Pittendrigh's comment:

I disagree. I can personally verify that an input fuse on the return is helpful, at least in two refinery SIS (ESD for the older, but wiser List
members). In both events, trip input circuits were inadvertently exposed to higher voltage. The only consequence from both incidents, was fuse
operatiom. And, in each cases the plant remained on line. Incidentally, input card damage was also avoided.

Phil Corso, PE
(Boca Raton, FL)
In our shop it is standard practice to fuse the hot wire of each block of inputs (either for each PLC card or sometimes each group within a card). Since we have a custom panel shop, and rarely do the field wiring, it makes commissioning troubleshooting a lot easier if something is mis-wired in the field (yes, it does happen) or if an end device fails. If a fuse goes, one only has to see that an entire block of inputs is off to know that something has happened to one of those devices, and have to trace 8 or 16 wires instead of perhaps hundreds.

Don Zunti, P. Eng.
Delco Automation Canada

Jeffrey Eggenberger

Cost and space are always the object. As an industrial electrician, it is common to change an input device while hot. Many are just plug connected anyways, and those that are not are easily changed under power (providing you do not have to put yourself into a compromising position, in which case you should lock out the equipment anyway).

There is no such thing as a natural-born pilot. Chuck Yeager

Jeff Eggenberger

Scott Fleming

It's easy to tell by the comments who are the engineers and who are the people that have to make things run.

There is no safety reason for individually fusing inputs.

HOWEVER, When joe bob electrician has a fuse blow at 3am that feeds 16 limit switches etc.. all on different parts of a machine he's in for a long night. If he is lucky a dead short caused the fuse to blow and it can be easily isolated, however this is not always the case. If a wiring or device short is intermittent (ie: maybe only shorts when something moves) trying to figure out which of the 16 devices is doing it can be fun. All of a sudden the couple bucks that was saved by the engineer on fuse blocks and a little bit bigger panel is irrelevent.

If a machine is designed for an environment where downtime doesn't matter then go ahead put all 500 inputs on one fuse just don't be surprised when an angry electrician throws a hand full o fuses at you in the morning.

By the way I am a "technician" who does both engineering and field work and speak from experience and now when I spec out a machine "all input field devices are to be individually fused" is included" is a requirement for any equiment that comes into my plant.