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

7.3.7 Current monitoring
If simultaneous voltage and current monitoring can be performed, the information available for problem
solving is increased tremendously. Clamp-on CTs can be used to measure the current associated with the
voltage deviation. If rms current increases substantially at the time when rms voltage drops, the voltage drop

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
43
is a result of a fault or load downstream of the monitoring point being energized. Fast changes in current
(< 1 ms) may not be accurately measured by some CTs. When CTs are used, they should be arranged such
that panel covers or some means of covering the service power source is possible. CTs should not be
clamped-on to the conductor to be measured until they are connected to the monitoring instrument.
As with the voltage connections, the quality of the current connections can affect the recorded data. The fol-
lowing are three common problems when using clamp-on CTs:
a)
The conductor or bus bar is not properly positioned within the clamped area.
b)
The two split core ends do not make a solid contact.
c)
The wrong type or number of conductors are enclosed within the CT.
Included in this last item is the improper polarity problem where a CT is positioned backwards or the CT
polarity is incorrect.
CT speciÞcations typically assume the conductor or bus bar is positioned dead center within the clamped
area. Any other location will incur some accuracy error. If the clamp ends do not mate solidly, then the
resulting gap between the ends will also introduce error into the measurement. Unlike the error from posi-
tioning, however, this gap error may also affect recorded waveshapes. Whenever multiple conductors are
being measured, care should be taken to ensure that no return conductors are also being measured. This
would cancel some or all of the magnetic Þeld of the conductor being measured, thus changing the reading.