120V control circuit & comm cable between facilities

[AJElectric]

Two separate buildings one with a 600A 277/480V service and other with a 800A 277/480 service. Each has a 120/208 transformer & panel for lighting/convenience/etc. circuits.

A 120V-fed PLC control panel and operator interface in building "A" controls equipment in both buildings. Remote I/O and operator interface in building "B" via ethernet comm. Provisions are in place for both an ethernet run and a 120V control circuit from main PLC over to remote panel in building B with the expectation that both control panels (in separate buildings) utilize the same 120V circuit. Automation contractor suggested this, but isn't necessarily "specifying" it.

I don't get a warm fuzzy doing this. Mostly because the neutral bonded at both services, my EGC for 120V circuit grounded at both ends, would create a parallel path for the neutral from one main service to the other (both also having neutral connection to POCO transf.) Any input on how to do this correctly, if it's even possible?

A second question is about the ethernet comm. between facilities. Are ethernet ports typically electrically isolated from power circuits of electronic/PLC/hub/etc? How can it be that this cable wouldn't be a current path from one building to another? Does TELCO usually have some type of isolator at each building it serves and would we need to implement something like that here?

 

[zbang]

A second question is about the ethernet comm. between facilities. Are ethernet ports typically electrically isolated from power circuits of electronic/PLC/hub/etc?

Per the spec, twisted-pair Ethernet is transformer-isolated at each end; there is no metallic path between the cable pairs and the interface chips. OTOH if you want to the sure, use a fiber connection and media converters; I'd probably do that as a matter of course if the buildings are more than maybe 20' apart (and you have a 330' limit on TP Ethernet, anyway).

You can read a discussion of this at http://electronics.stackexchange.co...re-ethernet-rj45-sockets-magnetically-coupled

I also don't think I'd extend the 120v circuit, either. Put each set of equipment on a battery/UPS and ride off into the sunset.

 

[tom baker]

I agree with suggestion on using fiber optic, for that distance multimode will work just fine, multimode media convertors are much less expensive than single mode. I have purchased fiber optic cable that is preterminated with a pulling grip, pull in plug it and you are done.
The fiber cable comes from Cable Exchange, you can't buy direct but your wholesale house should be able to order

 

[Fnewman]

Fiber, for sure. With respect to the 120v supply, although the distances were not given by the OP, I would use local power in both cases. Likely, the cost would be about the same as running a feeder between the buildings anyway.

 

[AJElectric]

We went ahead with regular ethernet between the buildings - less than 100ft pull so I don't expect issues there - although the fiber idea is good as I have seen long runs cause issues in industrial settings. Perhaps we'll have to resort to that if we have network issues but for now this is low-cost and simple.

I discussed with the automation contractor the "shared" 120V circuit - they will have a computer-operator-interface at the remote building too and want this on a UPS. The rationale was to save cost of second UPS unit for the remote building (since they already have one at main computer). Since in my judgement that seems more of a liability than an asset (multiple power sources to one building) we'll forego it and have a second UPS.