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When a CT is used in a metering circuit it will have a
secondary current loop connecting its output through a test
block to a meter . This current loop will consist of a
secondary wire from its X1 terminal to a test block and go
through a shorting switch. From the shorting switch the
current loop will have a wire connection over to the meter,
go through the meter, and have a wire connection back to
the test block. From there the current loop will go through
the secondary current test switch and have a wire
connection return to the X2 terminal of the CT with a
connection to neutral either at the output of the current
test switch or at the X2 terminal of the CT . This
secondary current loop has an electrical impedance called
its burden. This burden will have a resistance component
and may have an inductance component. Usually the
inductance component is small and is treated as negligible.
The burden of the secondary circuit, as installed, is
designated the natural burden of the circuit. If installed
properly the natural burden should be less than a few
milliohms. The CT is designed to operate accurately under
certain burden conditions as specified. The accuracy class
certification of a CT is presented for a stated upper value
of burden, called the CT’s rated burden. For example it may
have an accuracy class of 0.3 for burden of 1, meaning that
the CT’s rated burden is 1 ohm and the CT will maintain
accurate rated ratio performance within 0.3 percent for
secondary circuit burdens up to 1 ohm. This rated burden
classification is also stamped on the CT nameplate. If the
natural burden of the secondary circuit is larger than the
rated burden, then the CT is not certified to have accurate
performance. For this condition, called over-burdened, the
secondary current will not be accurate tending to be
somewhat less to significantly less than the accurate value
causing the metering to be in error in favor of the
customer. Furthermore, secondary circuits may have burden
build up as they age due to loose connections, corrosion,
broken stranded wires, etc. CT’s, as installed, need to
have enough excess burden capacity over the natural burden
to allow accurate performance over time even if there is
additional burden build up in the secondary circuit.
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