Well Grounded

| 9/1/2015 1:11:38 PM

Brian Foster -- Well GroundedThe terms “2-Wire” and “4-Wire” are often used when discussing RS-422/485 installations.  These terms can be misleading, as they suggest that there is no need for an additional ground wire.  That isn’t the case.  Without proper grounding you’ll be vulnerable to common mode voltage transients that can compromise your data or damage your equipment.

So although we talk about “2-Wire” and “4-Wire” installations, there will always be a need for an extra wire to connect the signal ground, and this signal ground conductor is often overlooked when ordering cable.  Cable with a twisted pair and a third conductor does exist, as does cable with two twisted pairs and a fifth conductor.  But it’s easier to just use a cable with an extra twisted pair.  You can then use one or both conductors for the signal ground.  A “2-Wire” system, therefore, would actually require two twisted pairs.  A “4-Wire” system would require three.

RS-422/485 systems can sometimes communicate successfully without the signal ground. This can happen when the nodes are located in very close proximity and the local ground is at the same potential, as in a controlled lab environment. But it’s not recommended. If nodes are separated by any significant distance, and there is no signal ground, lightning strikes and other electrical noise can cause the common mode voltage to rise to levels that can prohibit communications and do serious damage.

Connecting signal grounds on both ends while keeping them separated from the Earth Ground is not  sufficient to prevent issues in long RS-422/485 runs, even if you have a good physical earth connection and external surge protection. I’ve heard people refer to this kind of installation as “Partially Isolated”. I prefer to call it what it really is: “Non-isolated”.  Providing true isolation is a much better strategy.

When using isolation, you should connect the signal ground between both ends. You want to keep the “isolated side” on the long distance run. External surge protection is only required if large lightning-related surges to ground are expected. But exercise caution.  Surge suppressors on each end of the circuit might just provide a path for current to flow. And don’t overlook the power. If you are powering equipment on both ends of the circuit with the same power supply, this could very well provide a path around the isolation. If this is the case, you’ll need to use a “triple isolated” product like our 485OPDRI.

For more information about RS-422 and RS-485 grounding, download our RS-422/485 application guide here. If you need to select a serial isolator, click here. 

Figure 44 - as referenced in the comments below: 
Figure 44: Signal Ground connection between two nodes with 100 ohm resistor


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