Stakeless Measurement

11-29-2018 | Grounding

The Fluke 1625 earth ground tester is able to measure earth ground loop resistances for multi-grounded systems using only current clamps. This test technique eliminates the dangerous, and time consuming activity of disconnecting parallel grounds, as well as the process of finding suitable locations for auxiliary ground stakes. You can also perform earth ground tests in places you have not considered before: inside buildings, on power pylons or anywhere you don't have access to soil.

With this test method, two clamps are placed around the earth ground rod or the connecting cable and each are connected to the tester. Earth ground stakes are not used at all. A known voltage is induced by one clamp, and the current is measured using the second clamp. The tester automatically determines the ground loop resistance at this ground rod. If there is only one path to ground, like at many residential situations, the Stakeless method will not provide an acceptable value and the Fall-of-Potential test method must be used.

The Fluke 1625 works on the principle that in parallel/multi-grounded systems, the net resistance of all ground paths will be extremely low as compared to any single path (the one under test). So, the net resistance of all the parallel return path resistances is effectively zero. Stakeless measurement only measures individual ground rod resistances in parallel to earth grounding systems. If the ground system is not parallel to earth then you will either have an open circuit, or be measuring ground loop resistance.

Ground Impedance Measurements

When attempting to calculate possible short circuit currents in power plants and other high voltage/current situations, determining the complex grounding impedance is important since the impedance will be made up of inductive and capacitive elements. Because inductivity and resistivity are known in most cases, actual impedance can be determined using a complex computation.

Since impedance is frequency dependent, the Fluke 1625 uses a 55 Hz signal for this calculation to be as close to voltage operating frequency as possible. This ensures that the measurement is close to the value at the true operating frequency. Using this feature of the Fluke 1625, accurate direct measurement of grounding impedance is possible.

Power utilities technicians, testing high voltage transmission lines, are interested in two things. The ground resistance in case of a lightning strike, and the impedance of the entire system in case of a short circuit on a specific point in the line. Short circuit in this case, means an active wire breaks loose and touches the metal grid of a tower.

Two-pole ground resistance

In situations where the driving of ground stakes is neither practical nor possible, the Fluke 1623 and 1625 testers give you the ability to do two-pole ground resistance/continuity measurements, as shown below.

To perform this test, the technician must have access to a good, known ground such as an all metal water pipe. The water pipe should be extensive enough and be metallic throughout without any insulating couplings or flanges. Unlike many testers, the Fluke 1623 and 1625 perform the test with a relatively high current (short circuit current > 250 mA) ensuring stable results.