Fusing Inputs

Is it a good idea to fuse the power side of input wires going to the field? For example from my control cabinet L1 going out to a limit switch in the field and then that limit switch N.O comes back as a PLC input.

I am building a panel and considering fusing the power side of all inputs in that are located in the field. Is this a common practice? Is it a bad idea in any way?

I planned to use terminal block fuse holders with maybe a 1/4 fuse in general? What do you think?

I would normally ALWAYS put some sort of circuit protection on inputs. For the main reason that if there is a short on one of the input wires, you can easily figure out which wire is the offending wire. Field shorts certainly occur more often than we'd like, IMHO. Having an entire machine fail when there is one short - and having to try and figure out where the short is amongst maybe a hundred inputs - is really ugly.

At a minimum, you need circuit protection on your inputs to ensure the power supply doesn't crowbar from a field short or an unsafe condition doesn't occur from a field short. This safety discussion is assuming you are using sourcing inputs: sinking inputs only have ground potential so safety is not as big a concern. But I rarely use sinking inputs: too confusing for most troubleshooters.

So with troubleshooting being the main reason to fuse groups of inputs, it is common to gang a few inputs together on one fuse (maybe 8 on the same fuse). Then if there is a short, you can at least narrow it down to a group of 8 wires with the short - then start chasing them down one at a time.

Another alternative to fuses in a DC system is to use PTC 'fuses' that heal themselves once the short is removed. Very convenient for the operators: fix the problem and there is no fuse to replace. With the PTC, it is also possible to build a little companion circuit that lights an LED if a short is detected and therefore makes it real easy for the operators to figure out which bank of eight inputs is having the problem. If I recall, Weidmuller sells some pretty nifty circuit boards that make it easy to circuit protect your inputs in groups.

I've never seen this done, as long as the power is fused after the transformer it will be protected - and shut down all inputs.

If you fuse each input separately the PLC won't know if the sensor is in the OFF state or the fuse is blown - which could cause unwanted, and unsafe, operation.

I fuse any supply leaving the cabinet individually, not the input, but the supply to it

fuse the power output and have 1 input detecting when it breaks. please ground all 0Volt
if you fuse every input you can not see if the input is working.
fuse the outputs separate and same detection.

I normally have a separately fused hot wire for each input module. this makes troubleshooting easier.

We are working on a project now where the specs want a separately fused hot wire for each input. So if there are 4 PB's in the same operator station then that's eight wires to pull

That is a lot of extra wires to pull and seems like overkill, but that's what they want.

If you do not ground the power supply and let it float there is no way a short in the field can happen as there is no ground reference.

BobB said:

If you do not ground the power supply and let it float there is no way a short in the field can happen as there is no ground reference.

Click to expand...

True enough, but its always handy to be able to test to ground...

please refer your Local Electrical Regulation/Law

Tim Ganz said:

Is it a good idea to fuse the power side of input wires going to the field? For example from my control cabinet L1 going out to a limit switch in the field and then that limit switch N.O comes back as a PLC input.

I am building a panel and considering fusing the power side of all inputs in that are located in the field. Is this a common practice? Is it a bad idea in any way?

I planned to use terminal block fuse holders with maybe a 1/4 fuse in general? What do you think?

Click to expand...

I do something similar but I fuse per input module and from that fuse to an input on a separate module. This way you don't have a ton of extra wires going into the field and if you clear a fuse you get an input on another module which in my site always goes to an HMI.

Depending on how you layout your IO you can setup your logic so that if all inputs on a module are off for a certain period of time set an alarm that there is an issue which could be a blown fuse for the inputs on that module or other things.

One other thing that I do is all supply side wires for each input are on a knife disconnect terminal block so if I have a short I will only clear one fuse plus I get an input indicating that and to find the fault you just open the knife disconnect blocks for that module and find your short.

I would always protect the DC supply to the inputs and other.
depending on machine requirements I would usually have either the Stop or the monitor of an E/Stop circuit int a PLC input.
if that is off the Program will stop \If the Circuit breaker is tripped then the information will also stop the program.
If you wanted to know that the Power was faulty you would then independently feed an power normal condition to the input.

Scale of the panel comes into this as well. If your panel has a couple of external devices supplied off the DC, then the implication of a common DC supply without any individual fusing is minimal. However if it has a truckload of devices, the serviceman who has to find what is blowing the input DC fuse @ 2:00 am isn't going to thank you...

thats what 3" nails are for

dogleg this is wrong practice, as when a fuse blows the PB stops working.
another way is to use some resistors, that way you can fuse every PB apart, but no extra wires are needed.