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 On three-phase three-wire circuits, two transformers are sufficient for metering, although a third
transformer is sometimes used for checking the ratio of the others. On a three-phase four-wire
grounded system, three transformers are required. Usually the turns ratio of a current transformer
is such that dangerously high potentials result when the secondary circuit is opened. A test
switch or current jack, therefore, is normally provided to short-circuit the transformer secondary
while testing the instrument, or for using plug-in portable meters. The secondary circuits of
current transformers are always grounded.
5.1.3.2 Potential Transformers. Potential transformers reduce voltages to values within
the rating of instrument potential coils. Single-phase transformers are usually employed with two
transformers connected in open delta for three-phase three-wire circuits. For three-phase
four-wire systems, three transformers are required. Switches are provided in the potential
transformer secondary circuit to disconnect the instrument for testing. Potential transformer
secondaries are also grounded.
5.1.4 Direct Current Measurements. Direct current measurements employ either shunts or
DC transformers to carry the main current to be measured.
5.1.4.1 Shunts. A shunt is made of metal with a low-temperature coefficient of resistance
and low thermoelectric effect with respect to copper. Strips of resistance metal are brazed into
heavy copper blocks which become both the terminals for the line and the leads to the
instrument. Ordinarily, the shunt must be calibrated with the leads attached. The direct current
ammeter actually measures the millivolt drop across its shunt and is calibrated in terms of the
current rating of its associated shunt. Meters reading up to 50 A or less may have the shunt
within the meter case. External shunts are available in ratings up to many thousands of amperes.
5.1.4.2 Direct Current Transformers. A direct current transformer is a form of magnetic
amplifier or saturable core reactor. Two double-circuit transformers are used, with one winding
of each connected in the direct current circuit and the secondary windings excited by an
alternating voltage. As the direct current varies, it induces varying degrees of core saturation. As
the transformer core saturation changes, so does the magnetic flux linkage of the AC secondary
circuit. An alternating current instrument senses the changes in the flux linkages of the
secondary circuit and is calibrated to read direct current amperes. This method has two definite
advantages over the shunt method. The secondary circuit is isolated from the measured source
and the user may add one or more instruments, relays, or other current-operated devices to the
secondary of the transformer. The lower cost and greater reliability of the direct current
transformer provide a definite advantage over the shunt, for very high current values. Direct
current transformers are especially useful where remote metering of large direct currents is
involved, because calibrated leads are not required.
5.1.5 Transducers. Electrical quantities of alternating current systems can be measured by
using a transducer, that is referred to as a Hall generator and based on the Hall effect. The Hall
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