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The first soil resistance measuring instrument was invented in the 1950s by Evershed & Vignoles Meggers who made the first insulation and earth resistance testers. [2] One of the most used analog grounding testers in USSR were М416. [3] From the 21st century several companies produced digital earth resistance meters and testers.
An insulation resistance test (IR test) measures the electrical resistance of insulation by applying a voltage between two locations, and measuring the resultant current flow. Proper safety precautions must be taken when doing this test, such as exclusion zones, making sure no wires are exposed, and personal protective equipment is worn.
An insulation test set; in this pattern, a hand-cranked generator provides the high voltage and the scale is directly calibrated in megohms. Voltage withstand testing is done with a high-voltage source and voltage and current meters. A single instrument called a "pressure test set" or "hipot tester" is often used to perform this test.
A digital clamp meter A multimeter with built-in clamp. Pushing the large button at the bottom opens the lower jaw of the clamp, allowing the clamp to be placed around a conductor. An electrical meter with integral AC current clamp is known as a clamp meter, clamp-on ammeter, tong tester, or colloquially as an amp clamp.
For choosing a grounding rod there are several selection criteria such as: corrosion resistance, diameter depending on the fault current, conductivity and others. [30] There are several types derived from copper and steel: copper-bonded, stainless-steel, solid copper, galvanized steel ground.
For resistance measurements, usually a small constant current is passed through the device under test and the digital multimeter reads the resultant voltage drop; this eliminates the scale compression found in analog meters, but requires a source of precise current.
A good earth connection is normally a 6 m stake of copper-clad steel driven vertically into the ground, and bonded to the transformer earth and tank. A good ground resistance is 5–10 ohms which can be measured using specialist earth test equipment.
It does this by having a controlled low resistance: a metal mat would keep parts grounded but would short out exposed parts; an insulating mat would provide no ground reference and so would not provide grounding. Typical resistance is on the order of 10 5 to 10 8 ohms between points on the mat and to ground.