“Ground” is a term that has taken on many meanings with regard to power distribution and audio systems. This article looks at the safety ramifications of grounding with regard to permanent and portable power sources. Part 2 will examine the role of grounding in audio systems.
The Hums, Buzzes and Things That Go ZAP! workshop is drawing near. This primer on safety grounding will help lay a foundation for many of the principles that will be discussed.
Grounds serve a number of purposes in electrical systems. The first is reducing the possibility of lightning entering a building. This is the primary responsibility of the ground rod. Lightning is a high voltage that is referenced to the earth. An equalizing current flow wants to find the path of least resistance, which may include man-made structures or objects. We prefer that the path bypass the building and its occupants. Therefore we connect a terminal to the dirt, in the form of a ground rod, and connect one side of our electrical system to it. If our electrical system were allowed to float or follow some distant ground reference, our refrigerator door could rise to several thousand volts relative to our fingers when lightning strikes in the area. This makes the munchies an unpleasant experience.
The purpose of the ground wire on a branch circuit is to provide a low impedance, high current path to the supply neutral, sufficient to blow the fuse or breaker on the electrical supply in the event of a simple insulation failure between the high voltage side of an electrical circuit and the metal case of an appliance (Figure 1). The vast majority of ground wires live their entire lives and die of old age without ever having conducted more than a few milliamps. But those few who heroically serve their life’s calling by conducting several hundred amperes for a few milliseconds until the breaker blows, have saved countless human lives.
The principle is rather simple. An insulation failure should not leave two conductors that can be touched simultaneously at significantly different voltages. It should instead trip a breaker. If an incidental conductor, meaning something that conducts electricity even though it is not intended to be part of a circuit, can be touched, and can reasonably be expected to become energized in the event of an insulation failure, then that conductor should be grounded.
Let’s look at a few examples. I’m quite certain a pair of scissors conducts electricity, but it doesn’t normally need a ground wire, because it’s not expected to become energized by an insulation failure. Only if some inquisitive 2 year old sticks the points into an outlet will it become energized. Been there, done that. It may have sparked a lifelong obsession.
The generator running a parade float doesn’t need a stake in the dirt because it’s not really possible for an insulation fault to energize the earth. However it is quite possible for an insulation fault to energize the trailer frame.Therefore the generator needs to be “grounded”, bonded actually, to the vehicle frame.
When the power from a generator is used within a building, it’s quite possible for an insulation failure to energize the building steel, therefore the generator’s ground needs to be bonded to the building ground with a conductor of sufficient capacity to trip any circuit breakers used.
It should be obvious that defeating this protection by cutting off the ground pin of an AC cord, makes exactly as much sense as disconnecting the airbags and seat belts in your car. Your car still runs just as good, and makes the same gas mileage, but it is no longer as safe as it could be. But you’ll never know it until it’s too late. And you are endangering the lives of others, whether the passengers in your car, or the pastor of your church, by foolishly disabling a safety device.
Additional Safety Measures
The ground wire is the second line of defense, backing up the primary wire insulation. A Ground Fault Current Interrupter (GFCI), also known in some places as a Residual Current Detector (RCD), will also provide protection in the event of insulation failure. They make sure all current flowing to the load through the hot conductor returns via the neutral conductor and not some other route. They will protect people when there is no metal case to ground or when other substances such as water help to liberate the excess electrons inside a device. The NEC requires all outlets within 6 feet of water to have GFCIs. It seems the old hair dryer in the bathtub trick was reducing the world’s supply of blondes.
The ground rod, the ground-to-neutral bond, and the bond to building steel, if available, insures that the building, it’s occupants, all electrical equipment, and even the dirt outside the door will all be within a volt or two of each other. This becomes very helpful when we start running signal cables around to various places in the building. It’s extremely convenient when the two locations our signal cable is connecting are at the same or nearly the same ground voltage.
In Part 2 I will show how poor grounding practice can produce hum and buzz problems in audio systems, and how an isolated ground system can improve things. ds