Chapter 15
Accurate CT Calibration for the
Model 1133A
15.1
Overview
The Model 1133A Power Sentinel offers accuracy in power measurement of 0.025%. Existing CTs
provide accuracy of a few tenths of one percent. How then can the 1133A actually provide accuracy
of 0.025% in the field?
The answer is by calibrating the existing CT’s. By comparing each CT to an accurate reference
CT, traceable to national standards (for example, NIST in the USA), the errors of the test CT can
be determined at several currents and the correction factors entered into a table in the 1133A. The
1133A uses these correction factors, interpolating between them based on the measured current, to
correct measured power (and current) to the actual values, in real time, within the instrument’s
stated accuracy.
15.2
Calibration Process
To calibrate the CT’s requires three things: (1) a calibrated reference CT, (2) a source of high
current, and (3) some means of comparison. Each of these will be addressed below.
15.3
Calibrated Reference CT
Arbiter Systems has developed the Model 936A, an accurate, multi-ratio reference CT suitable for
these calibrations, and suitable for calibration by national standards laboratories at an accuracy
which exceeds 0.01% (the present limit of traceability at NIST). This CT will have ratios of 1200 : 5,
1000 : 5, 800 : 5, and 600 : 5. It will operate accurately at currents from 0 to 200% of rating. Using
a two-stage, amplifier-aided design, this CT should be fundamentally capable of accuracy better
than 10 ppm (0.001%).
By using a multi-turn primary through the center of the reference CT, current ratios of (1200
÷
N
) : 5, (1000
÷
N
) : 5, (800
÷
N
) : 5, and (600
÷
N
) : 5 can be provided as well (with N equal to the
number of turns). By using a multi-turn primary on the CT under test, ratios of (1000
×
M
) : 5,
(800
×
M
) : 5, and so on can also be provided. And finally, by using both N turns through the
