In North America, getting an occasional shock from a 120VAC household mains supply is almost a way of life for some of us. In Europe, getting shocked by their 230VAC or 240VAC is a good way to lose your life.
Ohm’s law is ever vigilant. A higher voltage across a given impedance produces higher current, and if you happen to be the impedance in question then you’d better hope you have lots of it. It doesn’t take much current to change your status from active to “he was such a nice guy.” As little as 60 milli-amps (that’s six one-hundredths of an amp) can cause your heart to go into fibrillation.
In Germany, the utility companies ground the electrical service at the utility pole and at the point of consumption. It’s a multi-grounded system as opposed to a uni-grounded system such as we use in North America. It saves the utility money because they don’t have to run a grounding wire back to the utility pole. Instead, the earth becomes the return path for the current in the event of a ground fault or an accidental short from a live conductor to ground. What could possibly go wrong with that?
If the soil conditions are such that the impedance is too high then there might not be enough ground fault current to trip the circuit breaker. And circuit breakers are inverse-time devices; the higher the current, the faster they trip. So you want a low impedance path back to the source to make sure the grounding system does its job of protecting people and equipment by tripping the circuit breaker in the event of a ground fault. In the United States, the National Electrical Code calls for no more than 25 ohms to ground.
But the Germans are an ingenious lot. Their response was to figure out how to sense a ground fault and shut down the circuit before anything can go wrong. They came up with the idea of running both the phase and neutral conductors through a sensor that picks up the magnetic fields produced by the flow of current. Since the phase current is flowing towards the load while the neutral current is flowing in the opposite direction, the magnetic fields oppose each other and cancel — as long as they are balanced. But if there was a ground fault, then some of the current would “leak” through the grounding wire and the magnetic fields of the phase and neutral currents wouldn’t cancel. This so-called residual current would cause the sensor to pick up the difference in the currents and trip the circuit breaker.
The sensor they used was a donut-shaped current transformer that would generate a voltage in the presence of a varying magnetic field like that produced by alternating current. The output is tied to a solenoid which would in turn open two switches to interrupt the phase and neutral conductors and stop the ground fault before it could do much harm.
They called this apparatus a residual circuit device (RCD). The first RCDs were sensitive enough to trip if there was a 500 milliamp difference between the outgoing and incoming currents. They eventually improved them to trip at 100 milliamps and then again to 30 milliamps.
RCDs gained popularity in the mines of South Africa in the 1950s because they had problems with electrical safety. When University of California Berkeley Professor Charles Dalziel began his work in the area of electrical safety, he learned about RCDs during a meeting in Geneva, Switzerland in 1962. He was impressed with the idea but he thought they could be improved. When he returned from Europe, he met with a manufacturer and together they developed a version of an RCD that tripped at 15 milliamps. They called it a ground fault circuit interrupter (GFCI). Eventually, the company started manufacturing them in various forms. In 1968, the National Electrical Code started requiring their use in swimming pool lighting circuits in the United States.
The popularity of GFCIs has grown tremendously. Today, there is a harmonized tri-nation standard among Canada, Mexico, and the United States for Class A GFCIs, which is intended to protect personnel. The standard says that it must trip at a minimum of 6mA and it must not trip below 4mA. Class B GFCIs were very early devices with a trip value of 15mA. They are now obsolete, but there are still many of them around from the early days.
To borrow from Dominique Bouhours, electricity is a good servant but a cruel master. There are many ways to protect equipment and personnel from the hazards of electricity, including insulation (insulate the wires), isolation (keep unqualified personnel away), grounding (to avoid energized metal parts) and overcurrent protection (to avoid overloading and burning up a system). But in his book Undercurrents and Overcurrents: All About GFCIs, AFCIs, and Similar Devices, (available at www.plsnbookshelf.com), Earl Roberts explains why GFCIs are superior to grounding for the protection of personnel.
To paraphrase him, there are two at least two things that can go wrong when you get tangled up in a live circuit; you can come in series contact with ground or in parallel contact with ground. Grounding can only protect you from the hazards of parallel contact with a live circuit and ground. In fact, in the series scenario — where current flows from a live wire through a person and then to ground — the grounding wire only makes the situation worse. It helps complete the circuit and you pay the price. In the parallel scenario — where current flows from a live wire in parallel with a person and a grounding wire — the current will divide in inverse proportion to the impedance. So what if you happen to come in contact with an energized metallic enclosure? If the equipment is grounded and bonded properly, what could possibly go wrong? Not much, unless you are standing barefoot in a puddle of water.
Unlike the grounding system, GFCIs will protect you in either of these situations regardless of the condition of the grounding wire or the condition of your judgment. Up until now, the live event production industry has been left to its own devices to use or not use GFCI protection in their power distribution systems. But ESTA’s Technical Standards Program has a new proposal regarding the use of GFCIs. BSR E1.19, Recommended Practice for the use of Class A Ground-Fault Circuit Interrupters (GFCIs) intended for personnel protection in the Entertainment Industry is in public review and is expected to be approved soon. This standard spells out the whats, wheres, whens and hows of using GFCIs in our industry.
In brief, it recommends their use in outdoor shows and/or in any situation that might be wet or damp. It also describes various types of GFCIs that are available including those in duplex receptacles, portable adapters, portable PDs, quad strings and circuit breakers, all with GFCI protection.
And if you are wondering about putting a GFCI on a dimming circuit, you can’t — at least not your typical GFCI and dimming circuit. The control circuitry in a GFCI is solid-state and needs non-dim power for the electronics to operate properly. Dimming the control circuitry won’t do. But there are specially-made GFCIs with a separate power input for the electronics.
Had RCDs and GFCIs been more widely used in the 1960s and 1970s, we might not have lost Leslie Harvey of Stone the Crows, John Rostill of the Shadows and Keith Relf of the Yardbirds. Back then, little was known about the potential dangers of improper grounding and less was known about RCDs. Today, we know better. Let’s not let one more person be injured in an accident that could have been prevented.
Thanks to Roger Lattin for recommending Overcurrents and Undercurrents: All About GFCIs, AFCIs, and Similar Devices.
