The “PASS” or “FAIL” message is generated using the CT accuracy class (see
Section 13.9.1
)
and the selected error limit from the User Preferences menu. If no CT nameplates are selected in
the Site Editor, the PowerMaster
®
estimates a nameplate according to the readings and a default
accuracy class of 0.3. Once the measured ratio is displayed, the PowerMaster
®
calculates the
error to see if it is within limits. For example, if the measured ratio is 201.86:5, the nameplate is
200:5, the accuracy class is 0.1, and the error limit is set to “2.0x Specification Accuracy,” then
the PowerMaster
®
will display a “FAIL” message. Here is how it is calculated:
[(201.86-200) / 200]*100 = 0.93% error
(Accuracy class = 0.1) * (Error limit = 2) = ±0.2%
0.93% > 0.2% = FAIL
Errors that could cause a CT to fail could be improper labeling of the CT, overburdened, loose
wires, or a damaged CT. In addition to physical problems such as those listed above, there are
other reasons a CT could fail a test. A CT”s accuracy class is only valid if the CT is operating
within its temperature, rating factor, burden, and current limits.
If a CT is overburdened, it can cause the secondary current to drop, and reduce the accuracy of
the CT in the test results. For the above listed transformer, the 0.1% accuracy is only valid if the
CT is fully loaded on the primary. The CT would then also only be 0.2% accurate from 10% of its
rated load, to maximum load. Below 10% of the rated load, there is no guarantee of accuracy.
Because of these problems, using a CT with a higher rating factor will help with increased
accuracy. The rating factor on a CT is a multiplier of the ratio which allows more operating range
at its rated accuracy. If a CT has a rating factor of 2, a ratio of 20:5 and is rated at 0.3%
accuracy, it will maintain that 0.3% accuracy from 20A to 40A, or Primary rated maximum current
* rating factor.
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