I think you need to define what you mean by "grounding".
If you have a 24 volt DC powered control system, like an industrial
control panel, typically the 24 volt DC power supply/s will tie the 0V
terminal on the power supply to frame/panel ground. These are the big
10, 20, 40 amp 24 volt DC power supplies that power the control panel
components.
There are some good reasons to do this.
Many industrial control components have loose ties frame ground
internally and if you don't tie 0V to frame ground they malfunction!
Many of these components have specifications for the number of volts
that the M, 0V terminal can be away from chassis ground.
The grounding is normally done by a single green wire from the power
supply 0V terminal to the ground bus bar. That way if there are
grounding/common issues you can lift that wire to aid in debugging the
system.
When you get into 5V systems, breakout boards, etc, I tend to keep those
isolated from frame/panel ground. I think there are only downsides to
connecting the 0V terminal of a 5 volt power supply to frame ground.
If you look at industrial drives, they always separate the frame/safety
ground from the signal "ground" or "reference" terminal. They are
usually two different terminals. One is oftentimes a cable lug or
bolt-cable lug connection, and the other a small screw terminal.
FWIW, I am in the machine controls business. PCs are common components
in machine controls.
Dave
On 7/12/2018 4:31 PM, John Dammeyer wrote:
Thanks,
I'm not concerned about the AC ground side of things.
Internet searches on this subject generally seem to agree that DC ground
doesn't and shouldn't be connected to the metal frame earth ground at any
point. If it is either through a capacitor or a 100 ohm resistor.
I remember many years ago working on Trim & Form Equipment in The
Netherlands we ran into problems with the PCs used for the User interface
(Pentium 386) had the power supply internally connect the DC ground to the
frame. Caused all sorts of havoc. I don't remember what the solution was.
Jeff Birt also suggests not connecting DC ground to the frame on one group's
posting. Obviously there may be Break Out Boards that break this rule but
then they may also be made by hobbiests who have gone into the machine
controls business and don't really know.
John
-----Original Message-----
From: Gene Heskett [mailto:ghesk...@shentel.net]
Sent: July-12-18 1:16 PM
To: emc-users@lists.sourceforge.net
Subject: Re: [Emc-users] Grounding
On Thursday 12 July 2018 15:12:40 John Dammeyer wrote:
Is it standard practice to connect the DC Servo and DC Instrumentation
Bus to the machine frame ground which is connected to power line
earth? Or is it more normal practice to keep the DC isolated from the
'earth' ground.
John
Generally speaking its a good idea to have then all come together at a
common bolt, also called a star ground. The bolt is the star center
point and all other ground circuits radiate from it.. The machines
frame ground should connect to this common point, and it should be
ohm-meter verified that there is not another connection between that
bolt and the machine frame if the frame grounding wire is disconnected.
This means that its good practice to have shielded motor and sensor
cables, but the shield is cut short, not connected at the frame end of
the run. If there is another connection, then you have a ground loop
which can inject several tens of volts of noise back into the interface
card, potentially damaging it. Or worse, inject noise into a stepper
drive resulting in a gradual drift of the homed point which=wrecked, out
of spec parts.
IOW, the motor power supplies should be the only circuit that connects to
the 3rd pin of the power cord, and that 3rd pin should be connected only
to that common bolt. Do not connect this 3rd wire to the supplies, but
to this bolt, and take a separate wire from the bolt back to the ground
symbol on the PSU's. And if that ground has continuity to the shell of
the PSU, mount it insulated to open that ground loop. A piece of pcb
material, glued to the chassis, and the PSU's glued to the pcb should do
it nicely.
IHTH.
--
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
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