Panel Building

[debco99]

A few times a year I build panels for machine tool retrofits usally 3 phase 480. I always ground one side of the 115 vac control transformer as well as the 24vdc and -+15 vdc supplies.

These are grounded to coppper plate (in the panel) and usally connected to a ground rod and the primary service ground as well.

Is the the industry standard?

Thanks in Advance

 

[buzzp]

Seems reasonable but it all depends on what service it will be in (isolation requirements of the power sources you mention - if these feed some field device or interface with other equipment then this should be considered). Also, if these are grounded elsewhere in the plant, make sure have no ground loops.

 

[jraef]

Stricly from a "panel builder" standpoint, what you are doing is accepted common practice (albeit not absolutely necessary). I assume you are in the US (115V) and are not a UL listed panel builder, because it WOULD be a requirement for UL listing for you to ground one side of the CPT. Be that as it may, the issues of how your ground point is connected on-site is actually the responsibility of the installer, because as buzzp mentions, there are other considerations relating to what already exists.

"Venditori de oleum-vipera non vigere excordis populi"

 

[analogkid2digitalman]

I am curious, is it standard practice to ground the common lines of the DC supplies?

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[debco99]

I should have added that in the panels are a combination of plc's, vfd's and pc,s along with both hydraulic valve amps and digital servo drives.

This application includes both scale and rotary encoder fedback

I have never had any problems ever with this method with noise etc.

Just thought I would throw that out as they are not simple applications.

Thanks to all that responded

 

[buzzp]

I have seen DC commons grounded and ungrounded. Again, I think it depends on what it has to work with. Certainly, ungrounded DC systems require 2 pole breakers.
As an example, the DC supply in our power plants is isolated from the ground grid. However, over the course of many upgrades, etc, somewhere in the system at one of the plants, the DC common was tied to earth ground, this, of course, defeated the isolation. I believe this could be an external wire an electrician added or could be from some piece of equipment, which uses AC and DC power, tying the two commons together in their equipment (I think this happens alot).
So to answer your question, I would say many DC systems have the return tied to ground. It would all depend on the final installation and internal design of the device.
As far as DC supplies in a panel being tied to ground, I have seen some that do and some that dont but mostly they do.

 

[DRWeig]

Sometimes when powering multiple transducers and controllers from a single power supply, it's necessary to have a floating (ungrounded) DC supply to avoid creating a fuse-popper... Most often, this happens when 24 VAC-powered devices are hooked to the same transformer that also powers a full-wave rectified DC power supply. If the AC transformer and DC supply are both grounded, it creates an every-half-cycle short circuit in the diode bridge.

In most other cases it's best to ground if you can. Prevents large potential differences between the DC circuit and earth ground in the event of system faults or lightning.

Old Dave

 

[dbaird]

debco99,
Please read thread748-106713.
I posed a simular question to EU Standards forum and got no affirmative response from them.








David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[afterhrs]

Your issue should be addressed in NFPA 79, if you do not have a copy, I strongly suggest you order one.

Regards,
Afterhrs