On Friday 27 December 2019 00:04:32 Chris Albertson wrote: > I think you are confusing ground and neutral. Ground should never > move off zero. But the neutral can be up to about 5 volts above > ground. > Which is why, as a CET, I like to specify static ground, which on this side of the pond, is the round pin of the duplex socket IF the electrician has done it right. Neutral is the wider flat pin, and AC hot is the narrower flat pin. All control signals should use the static ground as a ground reference connected to that single bolt. On power supplies, the ground symbol should also share this bolt, while the 2 line connections should carry the supplies draw current, and may or may not be interchangeable. Switch mode supplies that have a universal 110 to 240 input will generally be interchangeable.
The input lowside current for 110 volt environments is the neutral from the service, and is NOT connected to this static ground at anyplace but the common point of the earthen grounds in the service entrance. The supply outputs can be used as required but should have one terminal connected to this static ground bolt for safety reasons. Relay/solenoid coils being controlled by semiconductor switches in the interface card should have a flywheel diode across the coils which is reverse biased and therefore non-conductive when the semiconductor is "off". This diode should be faster than the semi can switch off for best protection of the semi from inductive spikes. The point of this single bolt common static "ground" connection is that it CAN bounce but everything being connected there BOUNCES in unison, and when it bounces, carry's all the signal references with it, and nothing properly connected to this single bolt sees the bounce even if its a direct lightning strike on the substation pole 30 feet away from the building. The operating circuit itself is unaware its just been struck by lightning. When I moved into this house 30 years ago, I lost a modem everytime I heard thunder because this house was wired by an idiot. I put 2 and 2 together and opened up every duplex between this room and the service fixing the f-ups as I went, even soldering poorly done static connections when they weren't even made up. I have not had a lightning damaged piece of gear in this room since, even though I know this whole circuit has probably been 100 kv or more from ground hundreds of times. Then in 2007 in prep for building the garage, I made this house a legal subcircuit of a properly grounded 200 amp service I installed myself. Ditto for the shed in the back yard. Zero damage from lightning strikes since including one hit that blew the fuse link at the transformer pole across the street. If using expander strips to distribute input power, its recommended that all the static grounds be broken off but one, and that one is then the line feed into the box including the static connection to the house. All others s/b terminated at this single bolt by separate wiring so that there is just one connection between this bolt and the "house" static ground. Break that ground loop before its ever plugged in. > On Thu, Dec 26, 2019 at 2:26 PM N <nicklas.karlsso...@gmail.com> wrote: > > Current used to power device will cause some voltage drop in ground > > so grond potential at device will be higher. Ground potential will > > also vary with power used by device. This higher potential might > > cause a problem if there for example is digital communicatin between > > devices. > > > > > A ground loop is then a single device is connected to ground more > > > than once. A good example is a motor driver. It might in a > > > "power" input called "+" and "-" with the minus side grounded to > > > the AC mains ground or a chassis frame ground. The in > > > addition there is a logic level control signal that is "signal" > > > and "ground" wires. This is a classic gound loop. > > > > > > How to break it? Use optical isolation on the signal. This > > > places an air-gap in the control signal. > > > > > > Most of the time the system is not so simple as the above but the > > > concept is the same, multiple ground connections are not good. > > > Why? Because in theory current can flow if you have a loop but can > > > never flow if there is not a closed loop. Then Ohm's law applies > > > -- if there is current flow there is voltage drop. If the > > > voltage drops across a gound then you have tow "grounds" that are > > > not the same voltage. This can be really serious if the motors > > > are large. > > > > > > There are a number of conventions that work. but they all do the > > > same thing, they reduce the number of ground connects to one per > > > "part" of the system. > > > > > > All the rules try to do the same thing, connect nuetral to ground > > > ONLY at the building service entrance, use opto's on all signal > > > lines. It is all the same idea > > > > > > > > > _______________________________________________ > > > Emc-users mailing list > > > Emc-users@lists.sourceforge.net > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users 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) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
