Related RS-485 Ground/Noise Question

[amatsu42]

I am searching for an answer to my grounding question and this topic is fairly close to my issue. If I should create a new topic instead, please let me know!

The requirements of Chapter 5 and the confusion of RS-485 and what section governs is giving me a headache.

I am installing a Wayne dispenser for CNG fuel and I need to run RS-485 for Modbus and payment communication (card reader in display). There is also a 24VDC DI signal going to the dispenser for the methane sensor dry contacts.

We ran PVC conduit following exception No. 2 of 514.8, but I need to run an EGC. Belden 89842 (our chosen cable) does not include a ground wire, but it does include shielding.

Question: If I only have (2) RS-485 cables (Belden 89842) do I need to run a separate EGC or is one of the conductors in the cable capable of being the EGC? If not, what section governs the minimum EGC size? Table 250.122?

Question 2: If I run the 24VDC DI signal (#14s) through the same RS-485 conduit, do I just run a #14 GND?

 

[jumper]

I made a new thread for you.

 

[GoldDigger]

IMHO, if the card reader, etc are part of the display which is part of the dispenser and the dispenser power feed contains an EGC, and the reader is bonded to the rest of the dispenser, then for safety purposes the reader is adequately grounded already.
The regulatory aspect is thornier. Since the data cable does not supply any power, I do not see any applicable guidelines for the EGC size to run in that raceway.

 

[amatsu42]

You are correct--the power conduit has an adequately sized EGC, and everything is bonded together inside the dispenser itself.

I am trying to find something in the communication section etc. that would give us an exception for not running an EGC with RS-485 and a 24VDC digital signal. The requirement of 514.8 does not seem applicable for CNG as the finished grade acts as the barrier of the hazardous location (CNG is lighter than air and doesn't seep through the ground as gasoline does). What adds to the confusion is that the manufacturer doesn't reference an EGC in their wiring diagram (again, besides the power conduit EGC). I can't blindly accept their wiring, but they reference conformance with NEC and it would be surprising if they missed something like an EGC in hazardous locations.