cRio Grounding

[Corelab]

We have a cRio 9047 here. I see that the "Getting Started" guide on page 5 has these instructions: "You must connect the cRIO-904x grounding terminal to the grounding electrode system of the facility."

Our chassis has been installed without any physical connection from the chassis grounding terminal to any explicit ground. The installers are inferring that the chassis is equivalently grounded by virtue of the fact that the chassis is connected to a power supply, the power supply is connected to a power strip, the power strip is connected to facility power, and facility power is appropriately grounded.

Is this approach sound? This question is in regards to a larger issue of improving sampling quality. Thanks in advance...

[Corelab]

Correction to my initial post...the chassis grounding terminal is connected to the ground terminal of the NI power supply (the power supply is connected to the power strip).

[CFER_STS · Accepted solution]

If you have a DC power supply in the same enclosure as your CompactRIO, and have, for example, single phase AC (comprising line, neutral, and ground wires) running into the enclosure, the ground should be tied down to the enclosure / panel immedately upon entering the enclosure. The line and neutral wires should supply the DC power supply inputs, and the DC power supply should also have a ground terminal or chassis earth connection which has to be tied to ground.  This ground may be run back to the same point at which the incoming AC power supply cable is tied to the enclosure, but this isn't necessary if the enclosure is metal and the mounting panel is electrically connected to that ground point.  The supply ground in that case may be tied down locally to the panel, since it is electrically at the same potential and connected to the safety ground from the AC supply.  The DC power supply outputs (e.g. +24VDC and COM) then run to the CompactRIO inputs, but the COM / DC - connection does not constitute a safety ground, as this may be floating with respect to the DC power supply ground (depends on the model of power supply), and is not connected to the CompactRIO chassis in any case.  Hence, the CompactRIO chassis itself must be tied to safety ground via the provided ground screw.  Again, this may be run back to the same tie down point as the ground from the incoming AC supply cable, or may be tied down locally to anything at the same electrical potential, or to any other point at the same potential.  Running it back to the chassis ground terminal on the DC power supply (which in turn is connected to the enclosure / panel / AC supply ground wire) satisfies this requirement.

[CFER_STS]

I'll just add that I typically employ DIN rail mounting of my controllers and power supplies, so I usually ensure that my mounting panel and enclosure are properly grounded and that the DIN rails themselves have good electrical contact with the mounting panel, and then I employ ground-specific DIN rail mount terminal blocks (where the terminals are connected through the block to the mounting rail) adjacent to installed equipment.  This keeps my wiring nice and clean, because all of my component chassis grounds can simply terminate in an adjacent terminal block.

[CFER_STS]

One more thing - Don't forget to internally connect your enclosure door to the main enclosure body with a suitable ground wire. Such doors can be (inadvertently) electrically isolated from the enclosure by the gasket / paint and poorly-conducting hinges, in which case you don't achieve an effective Faraday cage against noise.

[Corelab]

Thanks so much for the replies to this post. The engineering guys are actually just starting the process of assembling the enclosure.