Clamp on Method
This measurement method is innovative and quite unique. It offers the ability to measure the resistance without disconnection the ground. This type of measurement also offers the advantage of including the bonding to ground and the overall grounding connection resistance.
The Induced Frequency test or commonly called the “Clamp-On” test is one of the newest test methods for measuring the ground resistance of a grounding system or electrode. This test uses a special transformer to induce an oscillating voltage (often 1.7 kHz) into the grounding system. Unlike the 3-point Test which requires the grounding system to be completely disconnected and isolated before testing, Induced Frequency method requires that the grounding system under test be connected to the electric utilities (or other large grounding system such as from the telephone company) grounding system (typically via the neutral return wire) to provide the return path for the signal. Induced Frequency test is the only test that can be used on live or ‘hot” systems. However, there are some limitations, primarily being:
- The amount of amperage running through the tested system must be below the equipment manufacturer’s limits.
- The test signal must be injected at the proper location, so that the signal is forced through the grounding system and into the earth.
- This instrument actually measures the sum of the resistance of the grounding system under test and the impedance of the utility neutral grounding, including the neutral wiring. Due to the high frequency used, the impedance of the neutral wiring is non-negligible and can be greater than the ground resistance of a very low resistance grounding system, which can therefore not be measured accurately.
- The ground resistance of a large grounding system at 60 Hz can be significantly lower than at 1.7 kHz.
Many erroneous ground resistance tests have been conducted where the technician only measured metallic loops and not the true resistance-to-ground of the grounding system. The veracity of the Induced Frequency Test has been questioned due to testing errors, however when properly applied to a small to medium sized, self-standing grounding system, this ground resistance test is rapid and reasonably accurate