Fluke 1625 FE 1625 Users Manual - Page 47
Measurements on High Voltage Pylons
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Earth/Ground Tester Procedure of Measurements reliable values, the instrument offers the possibility of averaging over a longer period. 1. Select a fixed frequency (see "Control loop", Operation) 2. Keep the "START TEST" button pressed until the warning "measured value unstable" disappears. Max. averaging time is approx. 1 min. Measurements on High Voltage Pylons Measuring the Earthing Resistance without Disengaging the Overhead Earth Wire Using the Selective Clamp Method The measurement of the earth resistance of a single high voltage pylon usually requires the overhead earth wire to be disengaged (lifted off) or the seperation of the earthing system from the pylon construction. Otherwise false reading of the resistance of the pylon earth electrode are liable to occur because of the parallel circuit of the other pylons connected to each other by an overhead earth wire. The new measuring method employed in this instrument - with its external current transformer to measure the true current flowing through the earth electrode - allows measurements of earth electrode resistances without disconnection of the earthing system or disengaging the overhead earth wire. As all four pylon stubs are connected to the foundation earth of the pylon, the measuring current Imeas is divided into five components according to the present resistances involved. One part flows via pylon construction to the overhead earth wire and further to the parallelly circuited pylon earthing resistances. The other four current components (I1... I4) flow via the individual pylon foots. The addition of all currents result in a current IE going through the earthing resistance, i.e. the resistance of the "composite"earth electrode to the soil. If the current transformer is fixed to each pylon stub, one after the other, four resistances have to be measured which show a behaviour inversely proportional to the corresponding current components I1 ... I4. The feeding point of the measuring current is to be left unchanged to avoid a change in the current distribution. 39