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Is the Soil Too Noisy to Test?

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Soil isn’t typically thought of as being “noisy” … but it is.  It’s not often considered as part of an electrical circuit either … but it is.  When an electrical connection is made to the earth in the form of a ground electrode (commonly referred to simply as a “ground”), the earth becomes an extended part of the electrical system.  Utility companies commonly ground the secondary of distribution transformers, and tie the system neutral to the ground bus at the panel board. This in turn is grounded locally to a rod or grid on the premises.  Thus, in the case of a fault to ground, the soil acts as a return path in parallel with the system neutral.  Although unconventional in terms of what is commonly pictured as a  “circuit”, soil functions well by means of the charged ions of salts dissolved in moisture and the vast area a current has in which to map its course.

If electrical systems functioned perfectly, the soil would serve as an emergency backup and a zero voltage reference. In fact, system imbalances, leakage currents, fault currents, inductive influences, harmonics, and static discharges all may travel in the ground path.  Add to that, the biggest static discharge of them all…lightning, and it adds up to making the soil a very active part of the power grid!

If using a high performance Digital Earth Tester (DET), the operator has maximum protection from these “noise” influences.  Therefore, all that stringing of leads and spacing of probes isn’t just extra work to make the job difficult.  What it really does is free the operator to have maximum control of the test environment.  A quality earth tester will establish a unique test current through the soil as a basis for measurement, despite the presence of noise.  In the case of many Megger brand Digital Earth Testers (DETs) a default square wave frequency of 128 Hz is used, distinct from the second harmonic of the utility.  The DET bases the measurement on this signal, and rejects all else.

But what if “noise” is influencing your measurement?  Counter measures may be limited, but effective.  Voltage transients in the soil are just that … transient.  One counter measure is to wait awhile.  Go to lunch, or work on another aspect of the job. Try again. Chances are the transient will be gone.  If time doesn’t permit waiting, pull up the probes and move the test setup in another direction. There is a reasonable chance of getting away from the interference field.  Inductive influences can be reduced or eliminated by avoiding any paralleling of the test leads with the offending source (e.g., overhead lines or buried cable) or with each other.  Leads can be widely spaced and snaked, and the direction of test made on the order of a 90 degree to 45 degree angle from the line of the inductive source.

If you rented a high end test set like the Megger DET4 from Protec Equipment Resources, additional capabilities are available making the DET4 virtually unstoppable in the performance of a reliable test. One key feature is the ability to adjust test frequency, which allows the operator to quickly dial away from offending noise.  The ability to select a higher test current improves the signal-to-noise ratio. With these added features, the operator can deal with a noise problem quickly and effectively, without moving beyond the DET4’s keypad.

Don’t let noise interfere with your ground-testing requirements! Call Protec and get armed with the proper knowledge and most effective instrumentation on the market, effective noise suppression is no more than a phone call away.