Any sound person who has been in the biz for a while usually has a GFCI tale of woe to tell. Ground Fault Circuit Interrupters (GFCIs) are those pesky receptacles found in bathrooms, kitchens and outdoor locations to prevent a shock hazard when water and electricity could mix. Unfortunately, the National Electric Code (NEC) requires GFCI circuits if the public (read: musicians) has access to outdoor electricity. There are exceptions on GFCI usage for carnival vendors, sound and lighting gear and other utilizations where the public normally is not likely to engage electrical appliances directly. These GFCI tales of woe usually come from accidental tripping during performances at the most inopportune times. Most sound persons will either cheat by substituting a non-GFCI circuit, or try another GFCI circuit on some of the artist's backline gear in the hope that another GFCI has a less "trippy" condition. The focus of this Theory and Practice column is to introduce you to the inner workings of a GFCI device, and show you tips to help you prevent false circuit interrupts (trips) and still stay compliant with the NEC.
What is a GFCI?
A Ground Fault Circuit Interrupter is a form of circuit breaker that seeks out a current path that is not the hot and neutral wires. Most typical circuit breakers are set up for high amperage circuit overloads or short circuits by monitoring the current in only the hot wire. A GFCI looks at both hot and neutral wires, and if the difference in current (a "leak") is more than 6 milli-amperes, the GFCI interrupts the circuit by mechanically opening both the hot and neutral wires together with its integral circuit breaker. See Figure 1 for a schematic view of a GFCI and a typical piece of gear as an electrical load.
The 6 milli-ampere trip threshold is set by government edict, as a reasonable threshold of when a human can endure an electrical shock safely or become a lifeending event based on gruesome statistics. Every GFCI manufactured is tested at 7 milliamperes for a trip, and includes a "test" button to apply such a leak current between hot and ground wires for recommended periodic testing. Because a circuit board full of electronics is used to sense these 7 milliampere or higher current leaks, the components are prone to fail after many tens of thousands of hours of being on duty. That is why you must replace all GFCIs that do not trip the breaker when tested.
Most GFCIs employ a small transformer that has two wires passing through the transformer center from the hot and neutral screw terminals on to the receptacle contacts. If the currents in the hot and neutral wires are identical, no magnetic flux is induced in the transformer core and picked up by secondary wraps of wire leading into the trip threshold electronics. If a greater than 6 milli-ampere current difference is present between hot and neutral, significant magnetic flux flow in the transformer tickles the electronics and releases the circuit breaker placed before the transformer. This trip time is almost instantaneous, and prevents severe shocks from reaching innocent humans.
Under today's National Electric Code rules, most gear is completely tested, and the consumer is legally informed if the electrical products are safe to be used indoors only or outdoors as well. For musicians, modern instrument amplifiers and sound system gear all have three-prong plugs and direct chassis- to-ground prong connections. But this was not always so.
Old Work
To electricians, "old work" means commercial or residential electrical wiring that was likely back in a time when two-prong receptacles were the norm and safety grounding may not have been in place. This also applies to musical instrument gear and sound equipment that was meant to coexist with old work wiring. Most old work venue wiring was done before the early 1960s, but most sound system gear provided accommodation for old work wiring all the way into the 1980s.
A good example of old work electronics is my 1959 Supro Guitar Amplifier. This amplifier has an unpolarized two-prong Edison plug that can be inserted into receptacles both ways. Even the user's manual that came with the amplifier states that "If you have excessive hum, just unplug the amplifier and flip the plug over." This was the precursor of "polarity" switches that were placed on music equipment amplifiers from the 1960s through the 1980s.
To make guitar and PA amplifiers safe for the public in the old work days, one wire from the AC plug had a "safety capacitor" connected between it and the chassis. When the safety capacitor wire was plugged to the venue wiring neutral, the chassis hum was minimal and no tingle on the mic or guitar strings was noticed. If the safety cap wire was connected to the venue wiring hot wire, typically the hum in the speakers was a touch louder, and musicians got a slight tingle from the mic or guitar if they were partly grounded. By definition this was a ground-fault or out-of-polarity condition.
Polarity Switches
To avoid the plug-flipping trial and error on polarity, many music equipment manufacturers started incorporating a polarity switch or an on-off-on switch that reversed hot and neutral connections to the chassis safety capacitor for quick minimum hum determination. Safety capacitors also varied a bit in value as well, with values between 0.01uf and 0.1uf the most common. The size of the capacitor also determined the amount of tingle possible, since at 60Hz power line frequency you received a couple milli-amperes leak current if grounded better than your amplifier and with damp skin.
Referring to Figure 1 again, the example amplifier is shown with a polarity switch in the "out-of-polarity" position leaking the maximum amount of current due to the largest size safety capacitor commonly used. Without going through the capacitive reactance formula math, at 120 volts the capacitor current is 4.5 milli-amperes from the hot wire to ground. Since the 4.5 milli-amperes is smaller than the 7 milli-ampere GFCI trip level, all is well. But put two or more of these amplifiers on the same GFCI circuit, and a breaker trip is almost guaranteed.
Solutions
If you have a case of the GFCI false trips, the best solution is to give each piece of gear its own GFCI. Many extension cords (service cords) have in-line GFCIs and can be employed from on-GFCI circuits with success. Many of these cords are yellow colored and designed for outdoor construction equipment use, but industrious sound companies can make their own versions with black rubber cabling and GFCI-equipped receptacle boxes on one end.
Do not connect GFCI extension cords from an existing GFCI venue wall receptacle, as the common GFCI will still trip out from the sum of all the difference currents. Most sound company power distro panels will have plenty of non-GFCI'd circuits to connect to. And making a single backline AC circuit GFCI compliant can be done by taking quad receptacle service cord (quad box) and placing in-line GFCI extensions from that quad box location. The legal challenge with this approach is that you must prove that the quad box is watched by soundco staff so that musicians are not tampering with the non-protected box, much like a circuit breaker sub-panel (e.g. Motion Labs Rack Pack).
Contact Mark at marka@fohonline.com.
