Poe/ip

I'm looking for facts and opinions on running 24VDC through an ethernet cable to Ethernet/IP remote I/O modules via Passive Injector/Splitter such as this. I've looked up the POE specs so I know that this won't be "legit" POE because it doesn't conform to the voltage spec, but I've also done the math and I think the physics are sound, so long as the Remote I/O power draw is carefully examined. I've attached an example application, powering an Acromag XT1112 16-channel input module and (16) GX3-AP-2E photo sensors through a 100ft Cat 6 copper cable with only 1.2V drop across the cable.

So my question is, would you do it? Why/why not?

strantor said:

I'm looking for facts and opinions on running 24VDC through an ethernet cable to Ethernet/IP remote I/O modules via Passive Injector/Splitter.

I've looked up the POE specs so I know that this won't be "legit" POE because it doesn't conform to the voltage spec, but I've also done the math and I think the physics are sound, so long as the Remote I/O power draw is carefully examined.

So my question is, would you do it? Why/why not?

Click to expand...

This does not conform to POE, and I think it will confuse people if you refer to it that way. You're just using the cable that you happen to have - Cat5 cable - to power a sensor 100 feet away.

I am not sure that I would parallel the pairs. It looks like a single pair can deal with the load, and the voltage drop is OK (not great, but OK). As far as I am aware, under 50VDC you don't need to worry about electrical code. So the voltage drop is not a problem. That much wattage dissipated by the conductors in the cable should be fine. Paralleling conductors is sort of frowned upon by electricians. I've seen it done with fire alarms for driving alarm horns, and maybe for an Air conditioning system .. but not much else.

I would drive the load from a 24 VDC power supply (a real power supply, not an injector), with the power distributed to one pair on the cable. Use separate fuses or terminal blocks with some separation method so it is easier to isolate a problem if the power supply trips or a fuse blows or (insert problem here).

We have temporary installations (running for 10 years) similar to this. We didn't have enough instrument cable run, but we had a spare ethernet cable run. The ethernet cable has 2 pairs used to supply 24VDC to 2 different 4-wire instruments. The signals come back on regular shielded twisted pair. The power goes through normal instrument terminals with removable pins, so isolation is easy if required.

My understanding about electricians' (NEC's, really) aversion to parallel current carrying conductors is that the more you add in parallel, the less you can guarantee the conductors will share the load evenly (ex: one conductor out of 5 in parallel might be exceeding its rating). IIRC the NEC allows two parallel conductors without derating, but any more than two requires the use of a derating table.

That being said, using only one pair as a power pair would leave the other pair open to be used in a Kelvin configuration with a power supply that has Sense terminals, to ensure a consistent 24V in the field, regardless of loading/voltage drop. I haven't ran the numbers so I'm not sure anything would be gained, as most 24V devices are actually 10-30V devices or something like that. But it is definitely something that deserves a closer look.

I totally agree with your fusing recommendation. Thank you for your input!

P.S. I've just left the subsea/ROV industry in January, and the things they will do there are... eye opening. They would not think twice about running 3,600V/40A/3ph power through a 4.5km umbilical to a ROV, utilizing multiple conductors per phase, and dissipating so much power in the umbilical that it requires water cooling on the spool.

In my opinion, its not allowed.
you are powering a sensor thru an injector back to the plc which will have no reference to the injector voltage.

on top of that, you now have an additional source of power for the sensors and maintenance will not have a clue what is going on UNLESS they pay close attention.

Please see nfpa79 electrical standard for industrial machinery.

james

I've seen some devices that use "hybrid" ethernet connectors, carrying both power and ethernet, but they were designed specifically for that. Access points, things like that. I think this was before the days when proper standards based PoE was common. I wouldn't recommend it for random devices, especially sensors.

I can't see what you gain from not doing it properly.

With the amount of time and effort that you have obviously put into this, you could have just installed the 24v supply properly.

That being said, it looks like your thinking is sound and you "could" do it and it will probably work just fine. But later on, 1 year, 5 years, someone is going to come along and need to work on this and that poor schmuck is going to spend hours just figuring out what you did, before he can start fixing the problem that he came for.

James Mcquade said:

In my opinion, its not allowed.
you are powering a sensor thru an injector back to the plc which will have no reference to the injector voltage.

on top of that, you now have an additional source of power for the sensors and maintenance will not have a clue what is going on UNLESS they pay close attention.

Please see nfpa79 electrical standard for industrial machinery.

james

Click to expand...

I'm not talking about adding a new power supply (active injector). I'm talking about using the panel's own 24VDC power supply through the ethernet cable via a passive injector. It will be clear what I mean if you click on the first link I posted.

The sensor will be referenced to the common 24V.

mk42 said:

I've seen some devices that use "hybrid" ethernet connectors, carrying both power and ethernet, but they were designed specifically for that. Access points, things like that. I think this was before the days when proper standards based PoE was common. I wouldn't recommend it for random devices, especially sensors.

Click to expand...

I have found a few examples of industrial devices on the market currently offering POE-Ethernet/IP. None of them seem to have a special connector, but also none of them are performing the way that I describe; they are true POE devices, utilizing 48V or similar.

AMCI NR25 resolver

Imperx EIPVR process camera


Keyence NU-EP1

ICPDAS Modbus gateway

janner_10 said:

I can't see what you gain from not doing it properly.

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You would save the cost of an additional power cable and the cost of paying a man to run said cable. The passive POE injector/splitter cost <$7. If there are many stations to wire, there could be an appreciable savings. However, each station would need to be carefully evaluated for power budget, which is Engineering time, which might actually cost more than just paying someone $15/hr to run a separate cable. It might be a wash, or worse.

Lynx777 said:

With the amount of time and effort that you have obviously put into this, you could have just installed the 24v supply properly.

That being said, it looks like your thinking is sound and you "could" do it and it will probably work just fine. But later on, 1 year, 5 years, someone is going to come along and need to work on this and that poor schmuck is going to spend hours just figuring out what you did, before he can start fixing the problem that he came for.

Click to expand...

Good points, thank you.

FYI I think I've decided that I'm going to not do this, as the time required to analyze it is better spent on other things. That being said, I am still interested to hear from anyone who has done it, and any failure/success stories.

There is no reason it shouldn't work. My only problem would be that the injectors are not industrial products and you would end up with ghetto wiring termination trying to incorporate those injectors into a wiring system.

strantor said:

You would save the cost of an additional power cable and the cost of paying a man to run said cable. The passive POE injector/splitter cost <$7. If there are many stations to wire, there could be an appreciable savings. However, each station would need to be carefully evaluated for power budget, which is Engineering time, which might actually cost more than just paying someone $15/hr to run a separate cable. It might be a wash, or worse.

Click to expand...

Penny pinching then.