Help with Understanding Ground Loops

[fog37]

TL;DR Summary

Understanding the issue with ground loops

Hello,

When connecting different circuits together, the reference electric potential $V_{ref}$ (the $0 Volt$) for each circuit should be the same electric potential so the potential at all other points is the same.

If circuit 1 has reference $V_{ref1}$ and circuit 2 has $V_{ref2}$ with $V_{ref1}\neq V_{ref2}$, when the two reference potentials are connected, a current will flow along that connection...What is the problem with that? Does a return current always flow anyway along the ground/reference conductor?

thanks for any clarifications

 

[fog37]

Maybe this figure helps:

The two Earth connection points are there solely for protection. However, if their electric potential is different, then it is like adding an extra circuit with an extraneous current in parallel to the main circuit (lower portion of the figure)...

 

[zoki85]

DC source current is simply divided between wire and ground in proportion R_ground/R_wire. In a well designed DC network this isn't an issue since allowed voltage drop is small and R_ground>>R_wire. If local ground currents are sufficiently strong that may cause unwanted DC corrosion effects.
Situation is more complicated in AC large networks due to induced EMF in the loop and self-inductance of the "return wire". Good example is an AC monophase railway network where rails must be solidly grounded as often as possible and special attention is paid to such issues.

 

[Rive]

Let's say you have a ground plane in a high speed, low voltage digital design. For example, just a CPU and a RAM.
Let's say you have a ground loop across that plane (by any reason).
What will that current do with the signal to/from the RAM? What will the devices 'see'?