In facility design, power engineers can recommend many different layout approaches where grounding is just one of the considerations. It is possible to meet the requirements of the National Electrical Code and minimize user-generated interference. Subsequent articles will discuss such topics as ground planes, separately derived power, and isolation transformers. First, a discussion of the lore surrounding grounding will help to explain some of the misinformation that persists in this area.

Grounding

The definition of grounding used in this series of articles is that it is an electrical connection to earth or its equivalent

It is useful to start with the history of power distribution. When electrical power was first introduced to cities, there were very few rules. The power industry grew very fast and so did the problems. Fires resulting from power wiring were commonplace. The National Fire Protection Association, under pressure from insurance companies, backed a National Electrical Code (NEC) to control the way power is distributed and installed. This code with some variation has become the law and controls how power is handled in all facilities. These rules are intended to provide fire protection as well as safety and lightning protection. These rules are based on years of practical experience and in some cases they are quite arbitrary. Fortunately the rules are flexible enough that there is always a way to solve every problem. The rules are basically the same as they were 50 years ago when there were no computers or cell phones.

Why is a Facility Grounded?

The basic reason for grounding is to keep lightning from entering a facility via the power conductors. This is accomplished by "earthing" one of the power conductors that enters a facility at the service entrance. This conductor is the neutral in three-phase power or the grounded conductor in single-phase power distribution. The idea is to provide an earth path for lightning that is essentially outside of the facility. If the lightning path is inside a facility, there can be a real threat to life and property. The rules are the same whether the power conductors enter overhead or enter in underground conduit.

Grounding (earthing) one of the power conductors creates several safety issues that must be addressed. There are many conductors in a facility that are earthed including plumbing, gas lines, and building steel. Suppose there is an electrical fault to any one of these earthed items. If a breaker or fuse does not disconnect the power, there is a real chance of electrical shock.

Unfortunately, earth connections are rarely below 10 ohms and a fault to an earthed conductor might only draw 5 amperes. This current is not sufficient to trip a breaker. If some piece of plumbing is electrically "hot," there is danger of electrical shock. The NEC thus requires that all facility conductors that could come in contact with power wiring must be electrically connected together. The fault path resistance must be milliohms not ohms. This set of conductors is known as the grounding electrode system in a facility. When there is a fault to this grounding electrode system, a high current will flow thus tripping a breaker or blowing a fuse.

The NEC requires that there can only be one grounding electrode system in a facility. Splitting the grounding electrode system into two sections is against the law. A building that rests on earth must have all of its exposed conductors connected together and connected to earth at the service entrance. The neutral power conductor must also be earthed once and only once at the service entrance. It is illegal to earth the neutral a second time. The grounding electrode system may be earthed at many points. These rules must be followed or the fault protection system may be compromised. A diagram of this arrangement for single phase power is shown in Figure 1.

Power Distribution and the Interference Problem

Consider electrical hardware in a facility that is powered from a common source of power. The power conductors have resistance. If one piece of hardware draws a spike of current the resulting voltage spike on the power line is brought to every other piece of hardware. Depending on how the hardware is designed, this spike of voltage can cause current to flow through the hardware onto signal conductors. There are many mechanisms that allow this spike to add to the signals being processed by the system. There are hardware design techniques that avoid this coupling but this is not always provided.

An Early Solution to the Power Line Coupling Problem

If the spike of current returns to earth ground at the hardware, the flow into an input cable is reduced and this can help to avoid interference. For individual pieces of hardware this grounding may solve a noise problem. Early experimenters found out that some ground connections were better than others. As an example, a water pipe might be better than a connection to building steel. This grounding technique worked on a limited basis for small systems where the electronics was not very sophisticated. This experimentation led to the observation that some grounds were better than others. It also reinforced the idea that grounding was the way to limit interference.

Grounding in Larger Systems

Many new problems can appear when a single-point earth ground is provided for a larger system. A "good" ground for one portion of a facility may be a "bad" ground for another. This leads engineers into looking for ways to have separate grounds or ways to "isolate" grounds. These techniques violate the National Electrical Code yet in many cases engineers have had their way and safety issues have been skirted. These techniques are unsafe, illegal, and unnecessary.

There are many facility specifications that call for a special "ground" for the electronics. These special "grounds" may be no better than the water pipe ground that is already in place but they are sure to be far more expensive. These grounds are made by burying copper conductors in a deep well treated with chemicals to reduce the contact resistance to the earth. The belief is that connecting the hardware to this "good" ground will reduce interference.

It is very difficult to prove that a special grounding conductor has a beneficial effect. It is also hard to install a special grounding well after a building is completed. Rather than take chances, a special grounding point for the facility is often specified in building plans. This approach has been taken so many times that it is hardly ever challenged. In fact the presence of a grounding well is assumed to be an asset. In today's electronics this grounding is a solution looking for a problem. On the negative side it provides a false sense of security.

Many organizations have a grounding procedure or plan written into their building operations manual. It is very difficult to challenge these requirements as the writers of the plan are not available. Often it is politically wise not to argue. These articles are not intended to solve the political problem but to shed light on good electrical grounding practices.

Some Myths about Grounding

• A grounding conductor provides a "sink" for noise currents.

Fact: There is no such thing as a current sink. The noise current that flows in a grounding conductor must flow in a loop. After the current enters the earth it returns via other conductors to the circuits generating the interference. These paths may couple interference into other hardware.

• Grounding is necessary to limit interference.

Fact: In general this statement is false. The sophisticated electronics aboard an aircraft can function even though the aircraft is not grounded (connected to earth). Grounding is necessary in facilities to provide a path for lightning current and provide shock protection. Here is a problem to ponder: consider an astronomical observatory built on a mountain top where the surface is lava. How is it to be connected to earth? Remember, lava is an insulator.

• Noise can be controlled by providing special grounding conductors that only carry noise current.

Fact: The only reason current flows in a conductor is because there is an associated electric and magnetic field. This same field causes current to flow in nearby conductors. This means it is impossible to restrict the flow of noise current to specific open conductors. At frequencies where interference can be a real problem, the electric and magnetic fields must be controlled by different techniques.

Next in This Series

Most of the electrical interference in a facility is generated by users. The way the power is distributed can help to limit this interference. This subject is discussed in the next article.

By Ralph Morrison