Typically in a multi-panel control system setup there is a ground bar in
each panel. The incoming AC power ground is tied to this and the 24
volt power supply negative is tied to the local ground bar just as I
described before. If all of these panels are on a common machine frame,
the ground bars are oftentimes linked together with a heavy bonding
copper cable (like a 4 gauge or heavier bare cable).
You don't want to rely on the machine frame to be the ground
conductor. The systems I used to work on way back when were all AC,
120 volts for the controls and you could run control wire circuits for
1000 ft with only occasional problems. Way back when, a controls
electrician typically didn't carry a voltmeter with him. He used a wiggy.
https://en.wikipedia.org/wiki/Solenoid_voltmeter
Now you just pull Ethernet cables between control panels.
Not much 120 VAC control wiring is being installed any longer.
Dave
On 7/12/2018 6:33 PM, John Dammeyer wrote:
So what happens when the equipment with the 24V supply is 30m long in multiple
steel frames? There would be a bonding wire from frame to frame since you
wouldn't want to bond one end to one AC ground outlet and the other end to a
different AC ground outlet.
What if you have 30A, 24V supplies at each end of the machine or in each
module. If each 24V supply minus terminal is grounded to the frame and you
run a common ground bus through the machine you now have the frame and the
ground bus serving as the DC ground path. You wouldn't want to run DC ground
on the frame.
I worked on a machine once that had I think 42 modules. Each one was
independent and inserted two advertisement flyers into newspapers. A client
would buy the number of modules they thought they would need for the number of
insertions. I don't think they ran 24V from the front end all the way through
so probably AC to each module and a 24V supply for the motors and relays (and
air valves). The entire set of modules was also connected via CAN bus so we
have the CAN signal ground in that match too. It was a long time ago and I was
brought in to solve CAN bus problems so I never really looked closely at the
power.
What about if you have a vehicle instead. Might have equipment mounted on
frames that need to be bonded together. If they run an independent battery
pack and/or genset then the DC ground doesn't need to touch the frame. But
what if the vehicle 12V battery which does have negative connected to the frame
also provides some sort of vehicle connection. Say a radio that has a modem
that connects to a PC.
Logic would dictate you want the DC ground of everything connected to the frame
with some bond wires even if just for lightning protection. But now you run
risk of ground loops on the 12V circuits interfering with the system battery
pack.
Do the hybrid electric cars connect the negative DC of the high voltage battery
pack to the frame?
I'm amazed at the can of worms this question raises. We haven't even discussed
the electrical noise issues coupled through the lowest impedance path.
John
-----Original Message-----
From: Dave Cole [mailto:linuxcncro...@gmail.com]
Sent: July-12-18 2:57 PM
To: emc-users@lists.sourceforge.net
Subject: Re: [Emc-users] Grounding
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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