Control update: making grounds work for you. (News From Hench).

This chapter discusses the art of grounding electrical circuits. It seems like such a simple act, that we shouldn't even need to take the time to discuss it. On the other hand, when things are not making sense at your facility, one of the first aspects of your control installation you want to check is the adequacy of the grounds.

Purpose of Grounds --At its most basic level, a ground is a way to eliminate a wire to complete a circuit. As all serious students of electrical theory know, current can only flow if a complete circuit is present for the electrons to return to their source at a lower voltage. If you have a car frame, you can eliminate the wire returning to the low voltage terminal by grounding the return wire to the frame, and allowing the current to flow through the steel metal of the frame. "Ah", you say, "but isn't steel a poor conductor of electricity?" Well, it isn't as good as copper, aluminum or silver, but since the car frame has a huge cross sectional area for the current to flow through, the return resistance will be very low. For the same reason, the dirt the earth is made of can be used as a ground because, while the conductivity of dirt is poor, the cross-sectional area of the earth is very large.

This brings us to one of the most common uses of grounds: providing a "true" zero voltage for current coming from extraneous sources, to allow the current to flow to ground to complete the circuit. The magnetic fields causing the stray current come from AC circuits, radios, lightning, cell phones, motor starters, welding, and many other sources in an industrial environment.

Grounding Induced Voltages - Electro Motive Force (EMF) is created whenever a conductor is moved through a magnetic field or when a magnetic field changes in the presence of a conductor. The best example of a moving conductor and a fixed magnetic field is a generator, where a moving conductor on a rotor passes through a magnetic field. A common example of an EMF being generated by a stationary wire in the presence of a changing magnetic field is a transformer. An EMF being generated by a stationary wire in the presence of a changing magnetic field is a common cause of problems in low voltage sensor circuits. To counter these problems, we protect these circuits with a conductive shield, which absorbs most of the magnetic field and creates a voltage in the shield, which then has to be drained to ground.

Proper Grounding--For the induced shield voltage to be drained properly, you must have a good electrical pathway to the earth. This means both a large metal conductor into the ground and a significant penetration into the ground to provide the large cross-sectional area discussed above. A steel cold water pipe is a commonly used ground, and usually is more than adequate. This means your ground wire to the cold water pipe must be large enough to carry the current without much voltage drop. It is also important to ground your devices at the same ground, since voltages can build up in the earth and cause a significant voltage difference from one place to the other. This is why you should never ground the shield at both ends: if there is a voltage difference between the ground at one end of the wire and the other, you have set up a path for current to flow through your shield, or a "ground loop", which can cause as much trouble as not grounding your shield properly.