fusing inputs

I'm having an argument with a coworker. He claims that inputs should be fused. That makes no sense to me. He says that a fuse would prevent a voltage spike from the input to the plc. I say that the plc supplies the current to begin with, so why bother? Does anyone have anything to say about this?

I don't nor have I ever seen anyone else do it. Not saying it hasn't been done.The power to the input device like a sensor should be fused.

I fuse all power to each and every input device. Each device gets its own fuse. I don't want any one device to take down the entire panel.

I don't nor have I ever seen anyone else do it. Not saying it hasn't been done.The power to the input device like a sensor should be fused.

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Smoke the above statement confuses me you say you never do it never seen it but then you say it should be done??

We build panels, get orders from customers so see a lot of variety. Some customer specs are for all inputs & outputs fused. It drives the cost up, but no question its better from an isolation standpoint.

A middle ground is to provide a fuse or breaker for all devices outside of the control panel. A step up is to fuse or breaker individual machine groups.

Mickey, I think Smoke means the "POWER Supply Circuit" as opposed to the "switched leads" such as a four wire photo eye.

I personally don't fuse inputs, unless the customer specifically requires it and as far as outputs go, (on machines) I like one breaker or fuse to take out the whole works if there is a short. I want the whole machine to stop, not just one portion. There are cases where a partially operating machine can cause UNBELIEVABLE chaos........

Fuses actually do a rather poor job of protecting solid state inputs from "foreign" surges. If you select a low enough value fuse to save the input, you'll invariably be plagued by "nuisance" blows. If you go high enough to eliminate the nuisance blows, the current required to blow the fuse will likely actually be CAUSED by a failure of the input due to the surge you are trying to protect it from...... If you want to REALLY protect your inputs.... use isolation relays........ IMHO

Stationmaster

This is my opinion
Are you referring to fuse A , B , C , or all or some

Gil47 said:

This is my opinion
Are you referring to fuse A , B , C , or all or some

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I'm referring to "B", I would use a breaker for "A".
I see no reason to have "C".

in my opinion, using fuse in input or output PLC only useful if in AC input not in DC input. because the fuse in I/O only use for protect from noise. so if u already using noise filter in incoming wiring no need fuse to add in your system.

but,it's different if site have bad grounding or far away sensor. for that situasion, i prefer using fuse (one for several input) or external relay ( to each I/O) to protect PLC.


thanks.,,.

I think in evry system which divided foe section should the common be fused.shortcut might down the system and you will spend hours to find it.
Small systems can live without it.

irv_rizaldi

There is no connection between NOISE and FUSE.
What I wrote above is true for DC&AC.
I agree thats it better to use intermediate relay if you have inputs and outputs far from the PLC.

We (in the heavily washed down food industry) individually fuse all outputs, and usually fuse the inputs in groups of 8 or sixteen. Often, each fused input group will have one PLC input that is directly wired to the fuse so the PLC can detect and indicate a blown fuse for that group.

As for the OP remark, fusing the inputs doesn't necessarily protect the PLC from voltage spikes. The input impedance is so high for most digital inputs that it would take one heckuva spike in voltage to blow a fuse.

With outputs, fuses definitely can protect the output points and even though there is an initial cost, the first time one of them saves a whole output card, they are paid for.

Fusing the inputs in groups will someteimes make it possible to continue running part of a process while working on another part, and reduce the number of devices to check when the fuse blows.

Paul

Most of the time when we build panels, we have a single fuse each input card on the plc(similar to Fuse A in Diagram posted by Gil47) and then fuse all outputs individually post interposing relays(if required). It makes it easy to replace valves and with testing p2p, etc.

The ones I've worked on generally have a fuse in the B position, both digital and analog, with a breaker at the supply. This is more for maintenance than safety, if you defuse the circuit prior to working on the sensor/transmitter you don't run the risk of sending false signals to the PLC in case the wires touch.

Maybe OT:

I have an analog card that started blowing channels. I thought it might be a level sensor but I am not sure. It seems to be happening to several channels on 2 cards(same slot). The card is a 1756- IF16. The inputs are fused as a group, One fuse for all input on the card.

Not trying to steal yout thread but seems to be in the same ball park.

Kevin

Smoke the above statement confuses me you say you never do it never seen it but then you say it should be done??

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Brown, Blue, Black,... Brown fused (Hot to sensor), Black not (Return to PLC). Any +VDC short to the input will be seen by the sensor input power. Voltage drop by a common short is a different story. I have not seen the actual black wire fused in the example I provided. I fuse all power to the input devices not to the PLC input unless its going through a dry contact.

I am confused about fusing a group? Is the fuse small enough that 1 short will take out the group or does all the group have to short? Sorta like shutting down conveyors. All or none.