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By Mike Sokol
Chief Instructor, Live Sound Advice
I’ve been a pro audio engineer for 40-plus years and a musician for 50-plus years, and during that time, I’ve witnessed hundreds of shock events on performance stages, recording studios, and even factory floors. A survey we ran a couple of years ago on ProSoundWeb revealed that 70 percent of the 3,000 musicians who responded had been shocked at least once on stage, some so severely that they were knocked unconscious. I’ve also witnessed dozens of ground-fault current events where signal cables interconnecting sound gear plugged into different electrical outlets mysteriously arced, sometimes turning red hot and melting before my eyes.
The cause behind most of these guitar-to-microphone shocks appears to be incorrectly wired electrical outlet grounds or damaged extension cords. But while a broken-off ground pin on a power cord is the obvious culprit in most stage (and home) shock situations, many power outlets that appear to be wired correctly when checked with a 3-light outlet tester or even a voltmeter reading between H-N, H-G, and N-G still present a shock hazard.
 Standard outlet testing methods fail to reveal one of the most dangerous miswiring situations possible, which I refer to as the “reverse polarity bootleg ground” (RPBG), as seen in Figure 1.
As the illustration shows, a bootleg ground (or false ground) occurs when an ungrounded electrical outlet in an older building or stage has been improperly upgraded to a modern NEMA 5-15 or 5-20 grounded outlet.
Because sound stage, office building, and home wiring installed before 1965 didn’t require a safety ground, there’s no easy way to install a grounded NEMA 5-15 outlet. Per Sec. 250.130(C) of the 2011 NEC, in that situation, a GFCI outlet should be installed with the ground wire unattached.
Under what condition can a 2-wire receptacle be replaced with a 3-wire receptacle when no ground is available in the box? Where no equipment bonding means exists in the outlet box, nongrounding-type receptacles can be replaced with [406.3(D)(3)]:
— Another nongrounding-type receptacle.
— A GFCI grounding-type receptacle marked “No Equipment Ground.”
— A grounding-type receptacle, if GFCI protected and marked “GFCI Protected” and “No Equipment Ground.”
Note: GFCI protection functions properly on a 2-wire circuit without an equipment grounding (bonding) conductor because the equipment grounding (bonding) conductor serves no role in the operation of the GFCI-protection device.
A word of caution: permission to replace nongrounding-type receptacles with GFCI-protected grounding-type receptacles doesn’t apply to new receptacle outlets that extend from an existing ungrounded outlet box. Once you add a receptacle outlet (branch circuit extension), the receptacle must be of the grounding (bonding) type and must have its grounding terminal grounded (bonded) to an effective ground-fault current path in accordance with 250.130(C).
However, the Code states this outlet must be clearly marked on front as being ungrounded. Because GFCI breakers don’t need a ground wire to function properly, this type of outlet isn’t a shock hazard.
Even if the chassis of an appliance becomes electrically energized due to a high-pot failure, for example, anyone touching the appliance and ground simultaneously would exceed the 4 to 6 milliamps (mA) GFCI trip threshold and be protected from electrocution.
This article continues to include information on:
- How RPBG situations occur in the field, and why they're so dangerous
- A simple test using a NCVT to find RPBG mis-wiring in the field
- a Real-world example of a licensed electrician creating an RPBG just to save time - but it cost him $6,000 in replacing damaged gear in the studio he was wiring!
Download and read the full article here.
Mike Sokol is the lead instructor for Live Sound Co, an AV integration and installation company in western Maryland, and lead writer of the Live Sound Advice blog. He’s a veteran audio educator as well as an adjunct professor at Shenandoah Conservatory in Winchester, VA. Mike's also the founder and moderator of the AC Power & Grounding forum on PSW, where they discuss electrical safety issues for musicians and pro-sound engineers.
Click here for more articles by Mike Sokol
or email mike@livesoundco.com
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