Current Affairs: Have An Earthly Idea
Most electrical designers are comfortable with grounding of 60-Hz power systems for safety and fault protection, but they may find themselves challenged when confronted with grounding for instrumentation, communication, surge protection, or data processing facilities; at the higher frequencies involved, these ground systems do not behave according to simple circuit theory like the familiar 60-Hz systems, leaving many designers without the tools to fully understand their performance. As a result, trial-and-error solutions prevail, and grounding receives a reputation as a "black magic" art that cannot be understood by the uninitiated.
While it's not possible in a series of one-page columns to explain grounding adequately to get you completely past the "black magic" stage, I will try, over the next several months, to cover the basics of grounding for electrical safety, for surge protection, and for instrumentation and control. This should provide a basic understanding of the principles involved, and enough knowledge to know what you don't know, which is always useful.
What Is Ground?From a power system standpoint, when we talk about ground, we are literally talking about the dirt under our feet; in fact, in European convention, electrical circuits are not grounded, they are "earthed." While you may not think soil has much in common with copper wire, the earth is actually a very effective conductor. The formula for the resistance that any object offers to the flow of current is R = rL/A, where R is resistance; r is resistivity, a fundamental property of the material; and L and A are the length and cross sectional area of the path. Thus, in spite of relatively high resistivity, the surface of the earth can have very low resistance over long distances due to its large cross section.
We're interested in the electrical characteristics of the earth for two reasons which are actually related to one another: First, because we walk around on it and our projects are built in contact with it, and second, because most electrical systems are designed with an intentional connection to the earth.
Grounded vs. Ungrounded SystemsWhen we intentionally connect part of an electrical system to the earth, we refer to it as a "grounded system." The most common method of doing so is to connect the neutral point of a wye-connected three-phase transformer winding, or the center-tap of a single- phase transformer winding directly to ground, as shown in Figure 1. Voltage(s) produced by that winding are then clearly referenced to ground, and should any phase conductor supplied by it inadvertently contact the earth or any grounded conductive object, a short circuit will occur, high current should flow, and the fuse or circuit breaker will open to de-energize the conductor. Similarly, should a person make contact a phase conductor and ground, the voltage of the system will be applied across them, resulting in current flow through the body and possible electrocution.
An "ungrounded" system has no intentional connection from the transformer winding to earth. The voltage produced by the transformer is isolated from ground, and a phase conductor contacting earth or a grounded conductive object does not provide a completed current path, so the fuse or circuit breaker does not open.
On the face of it, this system would seem to be preferable; one would think that a person contacting a phase conductor and ground would also not complete a current path, eliminating the danger of electrocution.
In fact, for alternating current systems, there is an always an unintentional connection of the system to ground through an effect called capacitance. This creates a weak connection that won't pass enough current to open a fuse or circuit breaker but will pass enough current to electrocute a person. For this reason, it is generally accepted that intentionally grounding the system is safer because it increases the chance that a fallen conductor or other inadvertent contact with earth will be cleared by a fuse or circuit breaker before a person can make contact with it.
In the next column, we'll start the discussion of grounding for electrical safety with the assumption that the power system is intentionally grounded. ES