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> > >> > >> > > ---------------------------------------------------------------------------- > > -- > >> Check out the vibrant tech community on one of the world's most > >> engaging tech sites, Slashdot.org! http://sdm.link/slashdot > >> _______________________________________________ > >> Emc-users mailing list > >> Emc-users@lists.sourceforge.net > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > ------------------------------------------------------------------------------ > > Check out the vibrant tech community on one of the world's most > > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > ------------------------------------------------------------------------------ > Check out the vibrant tech community on one of the world's most > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
