Is 3 wire cable needed for 2 wire RS485 systems?

[sesa71277_bridg]

When wiring for a 2 wire system using an EGX100, usually only a two pair and shielded cable is used.

As per the PM800 manual (Belden 9841) is recommended.

What should be done with the 0V terminal of the iEM3150 in this situation. 

Should 3 wire communication cable be used or should the shield terminal be connected to the 0V terminal?

[VictorMarincus]

The GND or COMM or COMMON or '0V' (different notations on different devices) is used to obtain a balanced data transmission line. The function of the GND connection is to tie the signal grounds of all nodes on a network to one common ground potential. This is the only wiring rule for this connector - to connect the reference pin (of each RS485 driver) to the same reference. This ensures that the common mode voltage cannot exceed the specified value (see spec. for a RS-485 transceiver – voltage levels). If the devices have the ground connector (0V) this should be connected to each common connector of the other devices. This (loop) shall be earthed at one point only assuring that all notes have reference to the same potential (0V).

The shield (cable’s shield – notation SH, SHLD or symbol for Shielded Multiconductor Cable – see this) is used - tied at one point to earth ground - to shunt coupled noise and avoid creating ground loops. The SH or SHLD is present in all our devices. The documentation states whether the SH is connected to chassis or not (take the example of the ION7650). The chassis, further, shall be connected to a good earth ground – one end only.

So, getting back to your question:

1. when you have devices like PM800 you'll use a 2wire+shield cable

2. when you have devices like iEM3150/PM5000, etc you'll use a 3wire+shield cable

And here you can find a good FAQ on this topic: 3 Wire RS485 Vs. 2 Wire RS485.  | Chipkin Automation Systems

Regards,

Victor

[JamesAZZO]

Great explanation Victor.

[sesa71277_bridg]

Thanks for the great answer. Therefore from your response, I am assuming if I have a two wire system, it is Ok to connect the 0V terminal to the Shield terminal in the meters and tie to the earth ground at one point?

[VictorMarincus]

Yes that is correct. Here is the schema presented on the PM5000 installation guide on which I highlighted the ground connections. Hope this is clear now.

[michael_franz]

Victor:

Great write up.  Some RS485 systems get by with two wires but with metering I like the shield.

Michael