A consultant called in to investigate an EMI problem at a sewage plant worries about his shoes
By Daryl Gerke, Contributing Writer
Many people have heard me describe grounds as the “sewer system” of electronics. When the electrons are finished doing their job, we flush them away to “ground.” But since current must return to the source, it still continues to flow in the return path. This is as opposed to the “cesspool theory” of grounding, where misbehaving electronics are simply dumped somewhere.
Obviously, I’ve had some fun with this analogy. But several years ago, I got to work on a real sewer problem. Radiated emissions from a sewage lift station were jamming nearby radio receivers in a neighborhood. As part of the review, I got to look into a real sewage pit, and it was not pretty. My initial fear was that I might have to troubleshoot in the pit itself. It turns out my client (a civil engineer) was just having some fun with me. He quickly assured me that no electronics were in the pit, only a submerged motor.
As first I suspected electromagnetic radiation from the motor drive wiring. A three phase motor was connected to a variable speed drive by a 30 foot cable (antenna) that descended into the pit. I tried adding a three phase EMI filter to the motor drive electronics, but to no avail. Now I was worried that I might have to go into the pit after all.
It turned out, however, that the real problem was crosstalk in the cable, coupling from the noisy motor drive lines to a pair of unfiltered control lines. The noise then coupled from the control lines to the input AC power, and finally radiated from the overhead AC lines. Adding a small filter to the control lines solved the problem. But whenever I talk about grounding as an “electronic sewer,” it brings back vivid memories of this case.
Contributing Writer Daryl Gerke is an expert on EMI issues and is the co-owner of the consulting firm Kimmel Gerke Associates Ltd.