Fluke 1625 FE 1625 Users Manual - Page 50
Correcting Clip-on Transformer Errors
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1625 Users Manual Time average measurement: If there is a warning "measured value unstable" (see "Description of displays", Procedure of Measurement) after a test sequence, most likely it is caused by strong interference signals (e.g. unsteady noise voltage). Nevertheless, to get reliable values, the instrument offers the possibility of averanging 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. Measuring Earthing Impedance with 55 Hz (R*) For the calculation of short circuit currents in power supply plants, the complex earthing impedance is important. Direct measurement is possible under the following conditions: Phase angle at 50 Hz: 30 °... 60 ° inductive auxiliary earth electrode (ohmic): >100 • ZE Measuring process: The measurement of the earthing impedance (R*) is only possible if it is activated by putting in a personalized user's code (see "Change of Setup Data with Personalized Code"). If this measuring function is activated, in every measurement of the four RE positions, the earthing impedance R* is displayed before all other measured values. Correcting Clip-on Transformer Errors If the measurement of an earthing resistance by means of a clip-on transformer results in a significantly different value as if measured without the clip-on, the deviation may be due to the tolerances of the clip-on current transformer. This error can be corrected by fine tuning the clip-on transformation ratio (basic settings 1000:1). This correction applies to the transformer current range it was performed with. For other ranges a different correction may be necessary. 1. Connect a low Ohm resistor (approx. 1 Ohm - in the range you want to correct) as described in the picture below. 42