Listening through the noise to get the right read
It is funny to think of the earth’s soil as being noisy as well as part of an electrical circuit. But in fact it is. When an electrical connection is made to the earth’s soil in the form of ‘ground’, the earth itself becomes a part of the extended electrical system.
Utilities in the United States often ground secondary distribution transformers by tying the system neutral to a ground bus located on a panel board, then grounding it to a rod or grid on the utilities’ premises. When thinking in terms of fault to ground, the soil acts as a return path parallel for current with a neutral system. Although it seems unconventional compared to what is generally pictured as a circuit, the soil acts as such by means of its charged ions of salts dissolved in moisture creating a large area for the current to map its course.
In a perfect world where electrical systems function properly, the soil would serve as an emergency back-up and a zero voltage reference. System imbalances, leakage current, fault currents, inductive influences, harmonics and static discharges all may travel in the ground path. Mix all of that with the biggest static discharge there is – lightning – and it adds up to making the earth’s soil a very active part of the power grid.
By employing the use of Megger’s DET2/2 ground tester the theory can be proven true. Typically, the first measurement taken will display a steady reading with little to no digital displacement. Adjust the test frequency toward 120 Hz and take a second measurement.
Depending on what is happening in the soil at the time the measurement is taken, the one or two digits will destabilize noticeably. This is a visible indicator of ‘noise’ in the soil. This is a primary reason why an ordinary multimeter should not be used when performing a ground test. Such noise may influence the reading without the operator being aware.
Using a Megger ground tester gives the operator maximum protection against these influences. Megger’s ground tester not only establishes the tester’s own current, but also protects the voltage detection circuit against transient-voltage saturation up to an industry best 40 V peak-to-peak. In addition to the much-publicized and dreaded harmonics from the 60 Hz power grid, sources of DC interference also abound.
Utility sub-stations, computer rooms, hospitals as well as many manufacturing processes and other critical functions that cannot afford to go off-line are supported by battery back-up systems. Deteriorating insulation and high resistance faults that are not yet sever enough to trip protective devices will contribute a DC signal to the grounding system. Solar panels generate DC which is then converted to 60 Hz, and if not perfectly balanced and insulated can contribute to ground ‘noise’. These and numerous other sources can contribute an error to meters reading with DC.
Don’t let the noise interfere with your ground testing requirements. Armed with the right tools and knowledge on the market, effective noise suppression is as easy as 1, 2, 3.