DC
Shunts
DC Shunts are
separately mounted units used for extending measurement
of currents beyond the available limits of self-contained
DC ammeters. Due to the heat build up, and subsequent meter
"melt-down", self-contained meters (internal shunts) seldom
exceed 0-50 DC amps. In actual usage, few meters are sold
for direct DC amperage input with almost universal usage
of DC shunts. Shunts are offered as a ratio of DC current
input to 0-50 or 100 mv dc output.
Any
shunt by definition is a resistor and hence will
generate heat with passage of current. Because of this,
the resistance blades of the shunt should be mounted in
a vertical position with the longitudinal axis of the shunt
in a horizontal position in order to promote the free convectional
flow of air. Where this mounting position is impractical
and installations where the shunt is in a confined area,
forced air cooling should be provided. Under no conditions
should the manganin shunt strip be allowed to surpass 145
deg C, as this will cause permanent change in resistance.
Busbar size and
position will influence the temperature rise of the shunt,
so the installation must be planned for use with adequate
bus bar sizing and ventilation.
Utilizing extended
lead lengths will, by Ohm's Law, reduce the mv DC output
to the instrument.
For negative
to zero to positive dc amperages, the shunt does not care
which direction the amperage is flowing. Use a shunt with
maximum DC amperage input for either direction, i.e. 500-0-500
DC amps utilize a 500:50 mv shunt and 50-0-50 mv meter scaled
500-0-500 DC amps.
Shunts are manufactured
with copper blades and manganin shunt strips, and are available
mounted on a plastic insulating base or for direct mounting
in a bus bar.
Where possible,
all shunts should be mounted on the grounded side of the
circuit.
All shunts must
be wired in a series connection with any load; the mv outputs
should be wired in parallel with multiple readouts. Ohm's
Law must be obeyed.
Usually the larger
post connections are for the input line connection, whereas
the smaller connections are for the 0-50 or 0-100 mv output.
