Correcting Clip-On Transformer Errors - Fluke 1625 User Manual

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 com-
plex earthing impedance is important. Direct measurement is possible under
the following conditions:
Phase angle at 50 Hz:
auxiliary earth electrode (ohmic):
Measuring process:
The measurement of the earthing impedance (R
vated by putting in a personalized user's code (see "Change of Setup Data with
Personalized Code"). If this measuring function is activated, in every meas-
urement of the four RE positions, the earthing impedance R* is displayed be-
fore 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
30 ... 60 inductive
>100 • Z
E
*
) is only possible if it is acti-