Video: AMRU-120

Video transcript

What the AMRU-120 measures, how it is connected for each earth test, from 3 poles to two clamps, and how soil resistivity is measured. An earth electrode only protects if its resistance stays low. Soil dries out, connections corrode, and the value drifts. So earthing is measured at commissioning and at every inspection, and the soil is measured before a new earthing system is designed. Three and 4 pole earth resistance up to 19.9 kiloohms. One clamp for multiple earths, two clamps where no probe can be driven.

And soil resistivity by the Wenner method. The sockets are along the top edge: the clamp socket, then E, ES, S and H, each with its own colour. The rotary switch selects the method, and START runs the measurement. The F keys set the test voltage and the clamp. The kit holds everything for a 3 or 4 pole test: Four earth probes, leads on reels of 25 and 50 metres, short leads and test probes. The clamps, and longer reels, are options.

A test current flows from E through the soil to the current probe H. The voltage probe S picks up the voltage the electrode takes on. Voltage over current is the earth resistance. The test signal runs at 125 hertz, away from mains frequency. Each measurement follows the same routine. Turn the switch to the method. Set the test voltage with F one. Connect. Press START. Read the earth resistance, the probe resistances

and the uncertainty, and store the result. For the 3 pole method, open the test link so that the electrode is measured on its own. E goes to the electrode, S to the voltage probe, and H to the current probe, the furthest away. Electrode and probes stand in one line. Then check the result. Move the voltage probe a few metres towards the electrode and away from it.

If the readings differ by more than 3%, move the current probe much further out. High probe resistance adds uncertainty: water the ground, or drive the probe deeper. For very low earth resistance, use 4 wires. Connect as for 3 poles, and add the ES lead on the electrode, below the E lead. The meter then measures the voltage right at the electrode, and the resistance of the E lead drops out. With a multiple earth, the test link can stay closed.

Connect E, S and H as for 3 poles, then put the measuring clamp on the electrode, below the E connection. The clamp sees only the current in this electrode, so only its resistance is measured. Where no probe can be driven, two clamps do the job. The transmitting clamp connects to H and E, and the measuring clamp to the clamp socket. Both go round the electrode, at least 30 centimetres apart.

This method works only on multiple earths. For soil resistivity, drive 4 probes in a straight line, all at the same spacing L. From the first to the last, they go to E, ES, S and H. The outer pair carries the current; the inner pair measures the voltage. The meter asks for the spacing L and calculates the resistivity as two pi times L times R. A wider spacing looks deeper into the ground,

so a series of spacings shows how the soil changes with depth. That is the input for designing a new earthing system. Bonding and protective conductors are tested with two leads and at least 200 milliamps. Zero the leads first with F one. Then E goes to the main earthing terminal, and S to the bonded metalwork. Results go into 990 memory cells, in 10 banks, each with date and time. They transfer to a computer over USB, or wirelessly, for the measurement report.

Measuring category 4 at 300 volts. IP 54. Minus 10 to 50 degrees. And about two kilos, with a rechargeable pack good for more than 800 earth measurements. Earthing of buildings, industrial plants and substations, towers and lightning protection, at commissioning and in maintenance, and soil surveys before a new earth is built. Earth resistance testing, from Amperis.