Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

6.4.2 Current transformers 
A number of power monitoring instruments have the capability for simultaneous voltage and current moni-
toring. Since it is difÞcult to perform in-line measurements of current without affecting the power system, a
CT is typically used. CTs clamp around a cable or busbar to facilitate non-intrusive measurement. When
selecting CTs for power monitoring applications, the following four points should be considered:
a)
The accuracy of the readings (a combination of CT and instrument accuracy)
b)
Phase shift if the instrument is capable of providing measurements of phase relationships
c)
Response and phase shift over the measurement bandwidth
d)
The crest factor capability
6.5 Monitoring instruments
6.5.1 Oscilloscopes 
Oscilloscopes can be used to provide a visual representation of voltages and, when combined with current
probes as discussed above, current. Digital oscilloscopes can store voltage and current waveforms. Further,
some digital scopes allow the direct calculation of peak, average, rms, and other values. A waveform from a
Hall effect current probe, a voltage probe, or other device may be fed into the oscilloscope for analysis. The
use of a de-coupler permits safe measurement of the 60 Hz voltage waveform as well as the high-frequency
normal mode and common mode noise into the RF spectrum. This measurement technique is useful to deter-
mine the ambient noise levels and can also be used to identify possible noise sources.
Measurement of current waveforms can also be conducted with a clamp-on CT. As mentioned earlier in this
document, caution should be used in the selection of the CT to assure the frequency response is high enough
to measure harmonic currents as well as the fundamental frequency of concern. Frequency response to the
50th harmonic (3000 Hz) is normally sufÞcient for most applications. Power waveforms can be displayed
and measured by storing a voltage and current waveform. A digital oscilloscope can increase the quality and
ease of data collection.
One area of major concern during the use of the oscilloscope to make measurements is the practice of Òßoat-
ing the scope.Ó This practice typically revolves around disconnecting the ground connection to the scope
chassis. It is essential that the equipment grounding conductor for the chassis of the scope not be bypassed or
defeated in any manner that results in operator exposure to electrical shock. If a standard scope, with one
side of the input connected to the chassis, is used, the voltage shall be isolated using an instrument-grade
potential transformer.