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Failures in AC Outlet Testing and What it Means for Pro Audio

It might come as a shock to your system, but all testers are not created equal, and the damage can be expensive..

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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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Comments

  • Excellent article.

    A friend of mine came over to my house to play guitar and sing. He kept complaining of feeling a shock when his lips touched the mic while he was playing guitar. He said this happened to him often but not always.

    I looked over his equipment... he had an old tube amp (Fender I think), and I discovered that the the power switch on the guitar amp was a three position switch (center off). The left and right positions of the switch reversed the polarity of the power cord feeding the amplifier i.e. one position had a hot chassis ground.

    Apparently, this is how the amplifier was made back in the day, but I was shocked (pardon the pun) to discover this.

    Jon Rice  August 25 2017, 4:53 pm EDT
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  • I am a professional audio technician working in theater and corporate audio/visual(A/V). Fortunately, working with pro A/V companies, I rarely see bootleg gear come my way any more. Through experience, I guess that we have all learned that prevention, meaning properly wired gear, is cheaper than spending time troubleshooting ground problems in the field. The most important tools in my bag of tricks on every show are a three light outlet tester and a multimeter. Thank you for showing the false readings displayed when testing bootleg plugs using a three light tester. On shows, before anyone plugs anything in to the power distribution center(PD), which is usually a two or three phase unit, I check voltage to determine proper wiring from the disconnect panel. You would not believe the stupid and unsafe wiring that we encounter. Not only is hot to hot, hot to neutral, and hot to ground measured, I also check hot to chassis, neutral to chassis, and ground to chassis as well. I also measure the voltage between neutral and ground. When necessary I also identify the different phases of the plugs on the PD. These simple tests are very important in knowing what we are dealing with before using the venue electrical system as ground hums and buzzes in any part of the A/V system are highly undesirable. Quite often I find that the venue provides a three phase system using only 4 wires. The ground and neutral are improperly bonded inside the local PD. Usually, as you mentioned, a reading of absolute zero volts ground to neutral is the first identifier of this condition. I always wonder if the bonded connection is derived from the load panel ground buss or the neutral buss. One of the most frustrating things that we encounter is using power from separately derived sources/transformers within a building. This frequently occurs when the front of house(FOH) mix position is plugged into a different power source than backstage, hence, different ground path. If there is a buzz or hum in the sound system we have several tricks and tools to use. One solution is to run a power cable to the FOH position from the same PD that all of backstage is plugged into. Another might be to use digital snakes instead of a traditional copper snakes. These may include fiber, ethernet, or coaxial. These methods can sometimes help eliminate undesirable ground loops/noise in the system. As for individual "local" problems, as was the case recently, it turned out that a computer power supply was creating noise on one of the computer audio playback channels. I magically found a power strip with a missing ground pin to use on the computer PSU. That helped, even as illegal as its use may have been. Improvisation is a very big part of creating a "clean" temporary A/V system. In all my years of doing this I have hardly ever encountered a GFCI outlet in any venue unless there is a generator company supplying show power or we are plugging things in to the kitchen/catering outlets.

    Zippy  August 19 2017, 7:12 am EDT
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