Grounding Considerations

 

If there is anywhere a user of

Accutrak™ can get into trouble, it is with grounds. The historical reason for using grounds has been safety of persons and equipment. The usual result of improper safety grounds is a fire. Thus one should not be amazed to find that the owner of the National Electrical Code is The National Fire Prevention Association. Grounding is one thing about which the code is very specific. All electricians are generally well versed in the requirements for safety grounds.

 

However, safety is not the only reasons that grounds are required. In modern solid state circuits which use very small currents to transfer information, grounds are required to keep out unwanted electrical interference usually referred to as "noise." Noise arises because all current carrying conductors are surrounded by a magnetic field. This magnetic field in turn through the actions of the physical laws governing electro-magnetic circuits can induce another current in an adjacent conductor where it is not wanted. This unwanted, induced current is the noise.

Noise is eliminated in electrical circuits by a combination of remedies with proper grounding being one of the tools. The theory is to shunt all unwanted current to the ground where it will not cause noise problems. In order to do this there must be a single path from the potential source of unwanted noise to ground. This means that if one starts at the ground rod, the wiring looks like branches on a tree. There is one and only one path that exists from the leaf on a tree to the root. Branches on a tree are not connected together between the leaves and the trunk of the tree. In the same fashion, when one starts from a potential noise source and traces the wiring to the ground, there must not be multiple paths between the source and the ground.

The most common way that multiple paths to the ground arise is by intentionally or unintentionally grounding both ends of a shielded cable. The shield on a cable should anly be connected at one end to the ground. The other end should have the shield cut off and taped with insulating electrician's tape to prevent accidental grounding.

Another problem that arises with noise grounds is with the relative magnitude of the voltages. Voltages which cause noise can be low. But it is voltage which is the driving force that pushes electrical current through conductors and junctions. With low voltages, the connections between conductors in the ground path have to be low impedance which is the technical way of saying that these connections must be solid. That is, the conductors must be soldered or connected with mechanical connectors such that it is easy for electrical current to pass. In the realm of safety grounds, small currents and voltages do not cause fires or harm people, so this is not so critical in the safety arena. However, in the noise business, even a little noise is bad. So the average electrician who is not familiar with noise requirements must be re trained.

Connections are not the only way that one ensures that unwanted noise currents have an easy time reaching the ground. Another way is using relatively large wire. A large wire is to electrical current what a wide river is to water current. It provides a low impedance (an easy path,) to the ground.

The final problem to be discussed here is the last connection to the ground itself. If your location uses city water, then the water pipes themselves, assuming they are constructed of metal and not plastic, are an excellent connection to the ground. When the water pipes are used, it is important to verify that from the ground connection all the way to the street all of the pipes are metal --- preferably copper. Also care should be taken to make sure the connection to the pipe is a low impedance connection. This is done by: >

    1. Buffing the pipe with emory cloth to remove all oxide and dirt in the area of the connection.
    2. Using a ground clamp that is tightened to the clamp manufacturer's specifications.

 

If water main is not available, then the next best alternative is a driven ground rod. The primary requirement of a driven ground rod is that it must extend into the local water table. Connection to the driven rod is made the same as the connection to the water pipe discussed above.