DI line length

[CerealKiller]

I have a medium sized facilty with three storage buildings. Two of them are roughly 250' away and the other is about 500' away. All I have to do in those three buildings is to monitor the status of an exhaust fan in each. Electrical sub routed underground conduit.

In my limited experience, the controllers have been close to the equipment. Can I run the DIs back to the main building to one controller or is the line length too long? It'd be a pain to put the controllers in the buildings and configure a multi-port router.

Can someone also explain how a controller recognizes the change of state (i.e. change in resistance?)

Thanks.

[DaveCR]

Interesting question. I have not considered line length for a DI point for many years. It seems current hardware is pretty decent. At 250 feet I'd guess that wiring the point directly won't be an issue. At 500 feet, try it out. If there are any issues, wire in a transformer to drive a relay and monitor said contacts.

[osiyo]

Interesting question. I have not considered line length for a DI point for many years. It seems current hardware is pretty decent. At 250 feet I'd guess that wiring the point directly won't be an issue. At 500 feet, try it out. If there are any issues, wire in a transformer to drive a relay and monitor said contacts.

Yep, interesting question.

The OP asked about how some particular controller determines a change of state. And the answer is that it depends.

I know that in the case of one make of controller we commonly use, it checks every so many milliseconds by placing a 5 VDC potential on one wire, then checking to see if that potential is present or absent on the other. No? Then contacts are presumed open. Yes? Then contacts are presumed closed.

Yet another does this same type thing, but using 24 VDC.

Typically, in electronic circuits of the type employed with BAS controllers, in the case of the 5 VDC type device. it'd consider anything less than 2.5 VDC sensed on the second (return) wire as open, and anything above 3.5 VDC as closed. That's to negate possible influence by induced voltage.

However, I've seen cases where induced voltages over long runs have met or exceeded 5 volts.

A quick and dirty voltage drop calculation indicates that with a .25 amp current drawn by a relay coil, at 24 volts over a 500 ft distance using 18 gage copper wire, yah should only get a 2 volt drop. Leaving 22 volts. So a 24 volt coil relay should work. A .5 amp draw would take it down to 20 volts. Iffy as to whether it'd work or not.

But the RIB relays have some very low current demands, I've driven those with a 10 VDC analog output before.

That's only with the RIB series of relays (RUBU1C by Functional Devices). Coil draw for them is only 33 ma at 10 VDC, 46 ma at 24 V. Wouldn't work with the Veris work-a-like relays such as the V100's. For whatever reason of internal design, they required more power to pull in their contacts.

Just to be safe and sure, I'd probably go with the relay solution.

[CerealKiller]

Thanks for the replies.

Will try wiring the closer buildings direct and go with the relay solution on the farthest.

Osiyo, thanks for the controller operation explanation.

[s2sam]

Good day CerealKiller,

In addition to what was said, ensure you have adequate over-voltage protection on the inputs... Long lines between building, even buried in conduit, can experience high voltage surges from Lightning Strikes. Secondly, depending upon how your DI is configured (i.e. voltage, current, etc), lightning strikes could also cause ground line differentials which can also fry your inputs. Ideally, some form of galvanic isolation (opto, magnetic, etc) between the remote location and your DI would ensure high reliability and longevity.

Cheers,

Sam

[CerealKiller]

Thanks for clarifying that Sam. I know Echelon recommends that on their network, but really didn't occur to me about doing the same for DIs.

Our vendor recommended Citel DLA.