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

6.3 AC current measurements
The measurements of ac currents may be accomplished with the use of an ac probe, a Hall effect probe (also
used for dc current measurement), or a shunt resistor. It is important to note that the techniques and limita-
tions discussed above concerning analog and digital voltmeters also apply to the measurement of ac currents
in the power system. The ac current probe makes use of transformer action to detect current. This type of
probe, also referred to as a current transformer (CT), has a limited bandwidth. Limits at the lower frequen-
cies occur due to saturation of the probe and at higher frequencies due to parasitic inductances and capaci-
tances. Further, the excessive amplitude of the signal may cause saturation. Wide bandwidth transformers
are available, which are more than adequate for power quality measurements.
The bandwidth problems encountered with CTs can be eliminated through the use of a Hall effect probe. The
Hall effect probe does not use a transformer but senses the magnetic Þeld produced by the electrical current
ßow using a semiconductor device. The output of the probe is proportional to the current ßowing in the wire,
which is then read with a meter. The advantage of using this type of probe is that it will accurately measure
distorted wave forms without the concerns of limited bandwidth as experienced by the current transformer
technique.
The oldest method of measuring current is the shunt resistor. The shunt resistor is a low value precision
resistor that is inserted into the circuit to be measured. Current ßow produces a voltage drop across the resis-
tor in direct proportion to the shunt resistance. The resulting voltage is then converted into a current value.
Using a shunt requires breaking into the circuit, and thus can be difÞcult to install and use. One major beneÞt
of the shunt is that it does not suffer from the bandwidth limitations that are experienced with the current
transformer technique.