[ProAudio] Proper Grounding Replies

Bill Whitlock engineer_bill at verizon.net
Tue Nov 5 10:09:25 EST 2019

I absolutely agree with you Scott!  For electricians that really want to understand what they're doing (I've run across one or two), Code is truly miserable about explaining, even briefly, the reasoning behind "the rules."
Your last-paragraph scenario is one I experienced personally a few years back.  It often gets quite windy in California (witness our fire-storms).  This time the wind apparently work-hardened the neutral connection at the pole-mounted transformer that fed my home and 3 others.  I noticed a sudden brightening of some lights in the house and, at the same instant, my refrigerator stopped running. Within seconds, I realized that my neutral was disconnected, so I ran to the breaker panel and killed the main breaker.  All was saved except for the charger for my shaver. Over the next couple of days, I watched a series of appliance service trucks come and go from those neighboring houses. After the utility company fixed the break, I took a look at my ground rod connection ... it was OK but I noticed that the ground clamp on my CATV feeder had completely burned the cable braid. Apparently, it sacrificed itself as it attempted to become a substitute neutral, likely carrying over 20 A of unbalanced current.  Based on the brightness of the lamps during the incident, I'd say voltage on one leg rose to about 180 V while the other dropped to about 60 V.
It reminds me of the "expert" audio system grounding advice I read on the internet:  "As long as one piece of your equipment is grounded, the rest is protected via the signal interconnects."  Can you imagine fault currents of 150 A to 1,000 A (typical, from a UL field study) flowing in RCA cables for a few seconds?  I see nothing but flames!
Thanks again for your post,Bill Whitlock

-----Original Message-----
From: Scott Dorsey <kludge at panix.com>
To: proaudio <proaudio at bach.pgm.com>
Sent: Tue, Nov 5, 2019 4:39 am
Subject: Re: [ProAudio] Proper Grounding Replies

> 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.

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