[ProAudio] Understanding the grounding electrode system (NEC article 250)

Bill Whitlock engineer_bill at verizon.net
Tue Nov 5 17:12:05 EST 2019

I recommend such a system but would also include twisting of all L and N conductors before pulling into conduit as well as IG outlets - to keep the ground system as clean as possible.  You might also need to bond the new clean ground to nearby water pipes, but I'm not clear on exactly when that's required.

Speaking of water pipes reminds me of another story. Short version:Complaint was that electric guitars hummed so badly on one side of rehearsal-hall stage that nobody could play there. Turned out there was 13 A flowing in a water pipe (just inbound from the meter) a couple of feet under that end of the stage (the field measured about 100 mG at the floor, about 70 mG at waist level).  Turned out that everything electrical was perfectly Code compliant.  A fraction of the current in the neutral between power poles was flowing between customers (on the same transformer) through the water main in the street - via the N-G to plumbing bonds at each customer site.  The solution was to put plastic coupler at my client's side of his water meter. Code says you must bond to plumbing in your premises - but there's no requirement to include the pipe run to the street. The fix worked, of course, but I suppose that, under some extreme circumstance, a dangerous voltage difference could exist across that insulated coupler (in this case only a couple of volts) that could "surprise" the next plumber! 

Bill WhitlockWhitlock ConsultingVentura, CA

-----Original Message-----
From: Scott Dorsey <kludge at panix.com>
To: proaudio <proaudio at bach.pgm.com>
Sent: Tue, Nov 5, 2019 4:32 pm
Subject: Re: [ProAudio] Understanding the grounding electrode system (NEC article 250)

Well, having a separately-derived ground system with its own ground rods
and a ground not bonded to the building ground IS a good idea for audio
systems.... it's just that you need an isolation transformer between the
main panel and the clean panel so that the grounds can be safely split.
And that transformer needs to be rated for sufficient fault voltage between
the two systems (as detailed in the NEC).  And hell, if you're doing that
you might as well include an electrostatic shield too.

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