Stepping away from the standard words for a second, it may help to realise that from a comms/emc perspective, we usually are considering much higher frequencies than the dc or 50hz needed to blow fuses, and the same piece of metal can be both solidly earthed at low frequencies an yet decidedly live at higher ones. An extreme case could be a vertical antenna mast, earthed solidly enough to be its own lightning conductor, but at the same time 'live' to the transmitter's RF and creating an E- field around it of hundreds or even thousands of volts per metre. So an earth that is clean for one application such as a VSD switching at a few kHz may be entirely inadequate or even act as a resonant antenna for sharp edges for superficially similar switching waveforms but with unfiltered fast edges giving frequency components in the 100s of MHz.

Given that the fields around a wire are created with a wave speed cira 'c' or about 300m per microsecond, a full signal at say 10MHz takes about 30m, and much as you can slam a skipping rope in a door and then set up a standing wave by flapping the free end at a frequency that is a quarter wave, a length of wire 7.5 m long forms a resonator and or antenna  for 10MHz - giving a voltage maximum at that frequency when the other end is earthed. Higher frequencies are even more obvious - in a length of say a few 10s of metres of cable carrying 100Mbit Ethernet, any given bit only starts  coming out of the far end after quite a few of the  subsequent bits have already been clocking in  - in a sense the bits of the signal are queued up inside the cable,

Such delays and phasing effects make for a situation that is very hard to generalize how best to organize high speed systems especially those where the earth also carries signal 'return' currents , beyond, ' keep it short and low impedance or keep it separate' Both approaches do work, but which to employ in any situation is not an easy call.

Good modern systems tend to sidestep this altogether (and wired Ethernet wired to standard with the transformers at both ends is a good example) by providing a balanced transmission line pair, with out and return currents in a controlled geometry ensuring that earth connections are not really part of the signal loop at all and that if the earth 'bounces' the link is unaffected.

Mike.

Stepping away from the standard words for a second, it may help to realise that from a comms/emc perspective, we usually are considering much higher frequencies than the dc or 50hz needed to blow fuses, and the same piece of metal can be both solidly earthed at low frequencies an yet decidedly live at higher ones. An extreme case could be a vertical antenna mast, earthed solidly enough to be its own lightning conductor, but at the same time 'live' to the transmitter's RF and creating an E- field around it of hundreds or even thousands of volts per metre. So an earth that is clean for one application such as a VSD switching at a few kHz may be entirely inadequate or even act as a resonant antenna for sharp edges for superficially similar switching waveforms but with unfiltered fast edges giving frequency components in the 100s of MHz.

Given that the fields around a wire are created with a wave speed cira 'c' or about 300m per microsecond, a full signal at say 10MHz takes about 30m, and much as you can slam a skipping rope in a door and then set up a standing wave by flapping the free end at a frequency that is a quarter wave, a length of wire 7.5 m long forms a resonator and or antenna  for 10MHz - giving a voltage maximum at that frequency when the other end is earthed. Higher frequencies are even more obvious - in a length of say a few 10s of metres of cable carrying 100Mbit Ethernet, any given bit only starts  coming out of the far end after quite a few of the  subsequent bits have already been clocking in  - in a sense the bits of the signal are queued up inside the cable,

Such delays and phasing effects make for a situation that is very hard to generalize how best to organize high speed systems especially those where the earth also carries signal 'return' currents , beyond, ' keep it short and low impedance or keep it separate' Both approaches do work, but which to employ in any situation is not an easy call.

Good modern systems tend to sidestep this altogether (and wired Ethernet wired to standard with the transformers at both ends is a good example) by providing a balanced transmission line pair, with out and return currents in a controlled geometry ensuring that earth connections are not really part of the signal loop at all and that if the earth 'bounces' the link is unaffected.

Mike.