Tying 24VDC- to Ground

[NetNathan]

You should start a new thread. It will get faster response.

 

[BobB]

My answer before was no and still is - I do not ground 0VDC from a switchmode power supply and I do not ground 0VAC from an isolation transformer either. You los your isolation straight away.

 

[Firejo]

For those who don’t run a ground (for isolation) my only comment is you better make sure there isn’t a path to ground somewhere in the system. I run into this from time to time with our repairs (wireless modems). Most common (pardon the pun) is our I/O radios. We’ll get a unit back that has a blown discrete or analog output and when quizzing the customer about how the unit is installed it’s almost always in a panel that does not tie the DC ground to Earth ground. What gets missed is that the modem is connected to some other piece of equipment that is internally grounded to its case and that case is attached to the panel (which is almost always a metal mounting plate in the panel). With the modems power connected to a floating ground and the outputs connected to a device that is tied to Earth ground anytime the floating ground “floats” up or down compared to the Earth ground the differential current goes through the modems outputs and if it’s a high enough differential “pop goes the modem” (and usually the device it’s attached to).
If you have a piece of equipment that is truly isolated then isolation will work but if there is a connection somewhere that has an “unintended” path to ground then you run the risk of damaging equipment.

 

[Mill_Control]

Firejo,

Just because a system doesn't have any paths from 0VDC, or +VDC, to ground when new doesn't mean it'll stay that way. IMO tying to ground is the best solution unless it's unavoidable. It's the difference between troubleshooting blown fuses and random, usually intermittent, wierdness.

 

[Gene Bond]

Well, here we normally gound the low supply of our DC supplies.

But, at the last place I worked, we didn't.

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I was just reading A/B publication #2711-TD001, "Wiring and Grounding Guidelines for PanelView Plus and VersaView CE devices."

On page 19, it states, "Most PanelView Plus terminals have dc- connected to earth ground internally in the product. Circuits powering these terminals must not have another dc- point connected to earth ground. Otherwise, the dc- current will flow through the earth-ground wiring and earth-ground structure. Current flowing through earth ground can damage the terminal."

Sure enough, I popped the wires off the PanelView+ 600 at my desk. My Fluke shows very little resistence between the dc- and earth ground terminal.

Hmm, so lets assume one is in the habit of grounding the low side of the DC power supply at the supply. Other than adding a separate, isolated, power supply is there any thing else a person can do?

Everybody always talks about how power flowing through a ground loop is bad, but in this case how likely is it to damage a PV+?

Aaron






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Just to beat a long dead horse a little more:

Only *OUR* device can be grounded. BS! Poor design!

This is preposterous and ridiculous of Rockwell! What if a system has 2 Panelviews? It's just stupid and irresponsible of them!

If there is noise on the ground plane, fix it. End of problem.