Amprobe Telaris-Earth-Test Earth-Resistance-Tester - Manual (Page 10)

General Information concerning Earth Measurement
The earth resistance is the resistance between reference earth and the connecting point of
the earthing system. The earthing system is required to bring each system parts and circuits
to a reference potential as close as possible to the reference earth. It is recommended to
perform earth measurements in systems as lightning protection, telecommunication and
tank parks.
The built-in earth measurement of the Amprobe EARTH-TEST is an earth resistance
measurement in accordance with the voltage/current measurement method. The power is
supplied by the built-in batteries. The earth resistance is determined in accordance with the
voltage/current measurement method. Earthing represents an essential part of a power
supply system. Earthing is required to bring the individual system parts and current circuits
to a common reference potential close to the reference earth. Earthing is also used to protect
systems from overvoltage or short-circuit currents.
The earth resistance must be of sufficiently low Ohm. The limit values are defined within
national or international standards. Earthing or earth resistance is comprised of the earth
conductor (equipotential bonding conductor or PE), the earth (foundation earth, rod earth,
band earth...), and the earth spreading resistance, i.e. the resistance between earth and
reference earth. A voltage funnel is formed around any earth depending on the earth shape
and the surrounding soil. Assuming that the soil is uniform throughout with a uniform
temperature as well as uniform humidity, the voltage funnels around the earth are of
concentric shape.
The lower the earth resistance, the smaller the voltage funnels. When determining the earth
resistance, the voltage drop is measured which has been generated by a known and constant
current applied to the earth resistance to be measured.
Influence on Measurement Accuracy
The instrument Amprobe EARTH-TEST offers the option to set a maximum measurement
voltage using the UL 25/50V key. The contact voltage limit for medical or agricultural
applications should be set to 25V. For all other applications, it is advised to set the limit to 50V.
The measurement current supplied by the instrument is above 10mA, thus ensuring a precise
measurement result. If, due to the total earthing resistance caused by the voltage which is
generated by the measurement current, the pre-set contact voltage limit of 25V is reached,
the measurement is only carried out at the current present at the contact voltage limit. If the
50V contact voltage limit has been selected the measurement current up to 50V is not limited.
For any measurement voltages exceeding 50V, the measurement current is limited to 3mA.
Thus, a precise measurement result at a sufficiently high measurement current is obtained,
even when encountering difficult measurement conditions, e.g. stony soils.
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