[TowerTalk] Does prevailing grounding scheme promote large ground loop?

[Jim Brown]

On Tue,7/26/2016 10:30 PM, Christopher Brown wrote:
> The NEC has strict standards for the Grounding Electrode System, and
> exactly where and when neutral (ground_ed_ conductor) can be bonded to
> ground (ground_ing_ conductor).
>
> It does not limit the number of rods/other that can be used in addition
> to the minimums.
>
>
> Also, additional ground rods/other can be added basically_anywhere_
> bonded to a ground lead or equipment chassis.  These are simply NOT part
> of the grounding electrode system and cannot substitute for any other
> grounding requirement.

Correct on all counts.

> If you dig a bit there is language about supplemental and supplementary
> grounding electrodes.  I forget which is which, but one is bonded to the
> GES via #6 or larger bonding run, the other is not.

NEC places very little importance on the quality of the connection to 
earth. It calls for some maximum value of resistance to earth (25 
ohms?), and if a single rod does not provide it, a second rod must be 
driven. That's ALL it says!  What matters, and what is says a LOT about, 
is BONDING -- that is, how grounded equipment and earth electrodes and 
parts of the system are connected together.

> They are both allowed, but_do not do not do not_  replace/substitute for
> any required grounding/bonding...You can drive a rod and bond it to the
> chassis of a AC powered machine, but this does not in any way remove the
> requirement of bonding the chassis to the AC grounding conductor in the
> AC feed.

Exactly right. The equipment ground (the green wire) MUST follow the 
path of the phase and neutral conductors and be bonded to exposed metal 
of equipment. The fundamental principle here is that the primary purpose 
of the equipment ground (the green wire) is to blow the fuse or breaker 
if something in the equipment fails and causes the equipment chassis to 
be hot. In other words, it's electrical safety. Using a separate ground 
conductor for this function, even if it is bonded to other grounds, is 
prohibited because the return path is highly inductive, making the 
fuse/breaker blow more slowly.

And using only a driven rod at the equipment with NO ground conductor 
from the panel to the load is CRAZY unsafe, because the earth is 
basically a big resistor, and doesn't conduct enough current to blow the 
breaker. In my life in pro audio, we ran into dumbos who insisted on a 
separate "clean audio ground," not connected to the building ground. 
Mondo unsafe, for the reasons stated, and it does nothing to make audio 
systems cleaner.

73, Jim K9YC