28
Operation
For active power measurements with the 918B, both the voltage and
current inputs are used.
CAUTION
Do not exceed ±25 volts peak, with respect to earth
ground, on the (-) voltage input. The 918B is designed to operate
with the (-) input connected to earth ground or circuit neutral.
Procedure
1.
To perform watt measurements with the 918B, switch the
instrument to the watts (power) mode by pressing "W".
2.
Connect the output of the clamp-on current transformer (CT) to
the current input of the 918B, using the supplied cable. Be sure
that the (+) and (-) terminals on the CT correspond directly to
the (+) and (-) terminals of the 918B current input.
3.
Clamp the CT around the wire carrying the current to be
measured, making certain that the jaws close completely. Note
the (+) and (-) symbols and the arrow, indicating the phase
relationship between the CT input and the CT output.
4.
If a current flowing in the same direction as the arrow is exactly
in phase with a voltage present on the (+) voltage lead (with
respect to the (-) voltage lead), the watts value displayed on the
918B will be maximum. Figure 3 illustrates the relationship
between voltage lead polarity and current transformer
orientation.
5.
Plug the supplied test probes into the voltage input terminals,
with the red wire connected to the (+) terminal and the black
wire connected to the (-) terminal.
6.
Connect the black lead to the circuit neutral or ground, or the
point to be used as the reference for the measurement.
7.
Connect the red test lead to the voltage to be measured. The
value displayed is the active power in the circuit, in watts.