RS-485 - 2-wire

[Beeshive]

I hope that I'm not the only one that gets driven around the bend by the different RS-485 implementations of RS-485 2-Wire.

I have switchgear that consists of MasterPact breakers with Micrologic 5.0A / Micrologic 6.0P and PM800 power meters.

All of the devices are set up for 2-Wire 9600-8-N-1.

Almost all of the other RS-485 networks for Modbus and Bacnet in this facility are wired up using Belden 3106A 1.5 pair twisted wire with shield. On these networks, which is mainly ABB and Schneider VFDs and Belimo actuators, the twisted pair is the Data A- & B+ with the single conductor as the common and the shield is only grounded at one end of the loop.
The RS-485 network for the Micrologics and the PM800 is using the 1.5 pair twisted with shielded. The twisted pair is used for data, the single conductor is not used for common, and the shield connects to all of the devices and is earthed at both ends of the run.

I am currently having issues with a group of PM800s that are at the end of a 300' loop that has the monitoring front end in one building and the PM800s are in an other building.
Am I asking for trouble with the shield earthed at both ends? Yes the buildings share a common earth bond, but still. And why do the Micrologic 5.0A and 6.0P manuals call for the shield to be used as the common?

[Charles_Murison]

Hello @Beeshive 

I cannot speak to the Micrologic as I have no experience with those devices. For the PM800 meters, shield earthed at both ends could be a source of grounding loops if the earth of the shield is connected to the earth of the control power. I do not have access to a meter right now to test but you could try measure the resistance between control power ground the RS-485 shield connectors.

PM800 meters are fairly old at this point. Have you looked into the PM5000 or PM8000 as replacements?

Regards,

Charles