I get a call from time to time from a customer that explains that they are being shocked while playing on the worship platform. Upon inspection, I have 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 of most of these (usually guitar-to-microphone) shocks appears to be from incorrectly wired electrical outlet grounds or damaged extension cords.
Yet while a broken-off ground pin on a power cord is the obvious culprit in most home or stage shock situations, many power outlets show they’re wired correctly when checked with a 3-light outlet tester or even a voltmeter reading between H-N, H-G, and N-G, yet 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 Section 250.130(C) of the 2011 NEC (National Electrical Code), in that situation, a GFCI (Ground Fault Circuit Interrupter) 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 that 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.
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 a receptacle outlet (branch circuit extension) has been added, it 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).
Improper grounding also sets the stage for lightning to destroy a good sound system or other electrical equipment.
I always recommend adding a 3 pin grounded circuit to any non grounded equipment such as an older Hammond organ which did not use a ground yet has exposed metal surfaces that can come into contact with the player.
some ifno take from Live Sound Magazine