Forums

Grounding

Started by Dave Moore April 20, 2004
I'm curious about a couple of things regarding grounding.

- Chassis ground vs. "system" ground.  Your "system" ground may be digital,
analog and power supply grounds.  They are usually tied together on the pcb
at a single point.  I've then seen schematics that tie that "system" ground
point to the chassis ground through a resistor and capacitor.  I've also
seen that system ground tied directly to the chassis both on the PCB and
through an external wire to chassis ground.  What's the scoop?  Separate,
together?

- Telco -48VDC grounding.  So, -48VDC is the potential.  You can use an
isolated DC/DC converter to generate the voltage needed for your circuit.
This would be done by applying the -48VDC to the negative input of the DC/DC
and the -48V return to the positive input of the DC/DC.  Do you end up
connecting the generated ground of your voltage (the output side of the
DC/DC) to the -48V return?  Do this through a resistor/cap network?  Bring
the signals out and have someone do it externally with wires?


On Tue, 20 Apr 2004 03:45:36 GMT, "Dave Moore"
<daveoNOSPAMman@NOSPAMaol.com> wrote:

>- Chassis ground vs. "system" ground. Your "system" ground may be digital, >analog and power supply grounds. They are usually tied together on the pcb >at a single point. I've then seen schematics that tie that "system" ground >point to the chassis ground through a resistor and capacitor. I've also >seen that system ground tied directly to the chassis both on the PCB and >through an external wire to chassis ground. What's the scoop? Separate, >together?
In grounded mains powered systems the chassis is sooner or later (somewhere in the building/area electric distribution system) tied to the mains neutral wire. Any currents flowing in the neutral wire or grounding electrode will cause voltage drops and if the equipment (PE) grounds are connected to the neutral at different places, the equipment chassis potential will be different in these two equipment. If these equipment are connected together, with say RS-232 serial line, a very large (several Amperes) 50/60 Hz current can flow in the serial cable signal ground or the cable shield, if the RS-232 electronics is tied directly to the chassis ground. This is somewhat inaccurately referred as "ground loop" currents. If at least the electronics in one equipment is floating relative to the chassis, such ground loop currents can not flow in the signal cable. If only capacitors are used between the electronics and chassis, this will effectively break the 50/60 Hz current loop, but still ground effectively the circuit at RF. Resistors in the order of 100-1000 ohms are also used to discharge any electrostatic charge, but still limit the ground loop current to a few milliamperes. An other method is to have a completely separate technical earth network (TE), which is connected the mains grounds at exactly one point. All electronic grounds are connected to the TE ground, but since no current is flowing in this network, the electronics will be in the same potential all over the plant. It is imperative that the PE and TE grounds are kept separate everywhere except in a single point in the electric distribution system main bar. A single equipment with electronics tied directly to the chassis will ruin this system. Paul