[ProAudio] Proper Grounding Replies

[Scott Dorsey]

> THX! I didn't think of GFI's. It will take about 50 of them but it's 
> doable, a few at a time. As to the iso-circuits: Not in this house. This 
> house was originally built in 1932 and has suffered several bad 
> remodels. It was listed as a fixer-upper when we bought it (31 years 
> ago) but, I had no idea until I really got into it, the initial buyer 
> walk-through simply didn't show all of the gremlins.  The bedroom that 
> was converted into a studio has no AC grounds so, I drove a stake which 
> solved the problem. So, now the household neutral is, most likely 
> grounded to that stake.

Right, and this is terrifying.  Because now you have some outlets whose
ground pin is connected to your ground rod, while the neutral pin is bonded
to ground at the main panel and connected to the building's ground rod.
And there's what, maybe fifty or a hundred feet between them?

Let's say lightning strikes the earth a couple thousand feet from your
house.  Current flows through the earth and for a fraction of a second
there's a voltage gradient across the earth because the earth does not
have zero impedance.

And that means you're likely to have a few hundred volts between your two
ground rods, meaning you have a few hundred volts between neutral and PE
pins on the outlet, and that is not beneficial for your equipment.

This is why "unbonded double grounds" are very specifically called out 
in the NEC as being forbidden.

Now, the problem with the NEC is that it very explicitly tells you what
to do, but it doesn't explain why to do it that way.  Section 250 on 
grounding lays out very good practice, but sometimes it is difficult to
understand why these things are good practice.

The unbonded double ground problem is mostly seen when cable TV companies
hire homeless crackheads to do installations and they do not correctly
install the cable system ground.... next thing you know there's a storm
and people wonder why their TV set exploded.  You can run a lot of current
from the building ground to the cable ground but only so much before something
has to give.
--scott