Some days ago read Things Every Electrical Engineer Should Know about EMC slide set by Todd H. Hubing Michelin Professor of Vehicular Electronics Clemson University. I must say that it is an interesting slide set to check. It is a short 25 slide set that introduces EMC fundamentals and ground loops. Here is one fact that is not often presented clearly elsewhere:
Current takes the path of least impedance!
> 100 kHz this is generally the path of least inductance
< 10 kHz this is generally the path(s) of least resistance