Selective Measurement


Selective testing is very similar to the Fall-of-Potential testing, providing all the same measurements, but in a much safer and easier way. This is because with Selective testing, the earth electrode of interest does not need to be disconnected from its connection to the site! The technician does not have to endanger himself by disconnecting ground, nor endanger other personnel or electrical equipment inside a non-grounded structure.

Just as with the fall-of-potential test, two earth stakes are placed in the soil in a direct line, away from the earth electrode. Normally, spacing of 20 meters (65 feet) is sufficient. The tester is then connected to the earth electrode of interest, with the advantage that the connection to the site doesn't need to be disconnected. Instead, a special clamp is placed around the earth electrode, which eliminates the effects of parallel resistances in a grounded system, so only the earth electrode of interest is measured.

Just as before, a known current is generated by the Fluke 1625 between the outer stake (auxiliary earth stake) and the earth electrode, while the drop in voltage potential is measured between the inner earth stake and the earth electrode. Only the current flowing through the earth electrode of interest is measured using the clamp. The generated current will also flow through other parallel resistances, but only the current through the clamp (i.e. the current through the earth electrode of interest) is used to calculate resistance (V=IR).

If the total resistance of the ground system should be measured, then each earth electrode resistance must be measured by placing the clamp around each individual earth electrode. Then the total resistance of the ground system can be determined by calculation.

Testing individual ground electrode resistances of high voltage transmission towers with overhead ground or static wire, requires that these wires be disconnected. If a tower has more than one ground at its base, these must also be disconnected one by one and tested. However, the Fluke 1625 has an optional accessory, a 320 mm (12.7 in) diameter clamp-on current transformer, which can measure the individual resistances of each leg, without disconnecting any ground leads or overhead static/ground wires.