15.6 Accessories
149
effects of source variations cancel out. Furthermore, since the quantity being measured is a small
error, equal to at most about 1% of the secondary current, then the accuracy required of the
measuring instrument could be relaxed as well.
The ideal instrument to make this comparison is the Arbiter Systems Model 931A Power System
Analyzer. This instrument samples both of its selected input channels simultaneously, meeting
the requirement stated above. Best of all, measuring the error current with 0.05% accuracy, the
Model 931A is capable of making this comparison at a level of 5 ppm of the secondary current
(0.05% of 1%). This standards-lab performance can readily be achieved in the field using rugged,
portable equipment designed for field use, operated by service technicians with little or no training
in metrology techniques.
Figure 15.2: Comparison Technique B
15.6
Accessories
This setup will be provided with the necessary accessories to perform the required calibrations.
The cable carrying the high primary current will be AWG 4/0 welding cable, capable of up to 500
amps current. For applications where the cable can be run through the center of the test CT, a
single long length (about 15 m or 50 ft) of cable will be provided, allowing for multi-turn setups.
To perform a calibration of, for example, a 1000:5 CT at currents up to 2000 amperes, four turns
are required through each of the excitation CT, the test CT, and the reference CT.
For applications where the current must be provided to a bar running through the test CT, four
shorter (3 m or 10 ft) cables will also be provided. The lugged ends of all of the cables are connected
to the primary of the test CT. This would form a single-turn loop, with each cable carrying 500 of
the 2000 amperes of current.
Software will be provided to automate the data gathering and reduction process, using a serial
connection to the Model 931A and prompting the operator to perform the proper hookup and
settings of the other equipment. The data could later be printed out, and it can be stored in a
data file suitable for use when configuring the 1133A, eliminating a manual step where errors could
enter into the process.
Hardware required for connecting the lugged ends of the cables together, completing the high-
current loop, and tools to tighten the hardware to specification, will also be a part of the kit.
