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Electrical Thumb Rules You MUST Follow (Part 8)PostedMAR 10 2014byJIGUPARMARinENERGY AND POWER,METERINGwith4 COMMENTS
Electrical Thumb Rules You MUST Follow (Part 8)
Continued from part 7:Electrical Thumb Rules You MUST Follow (Part 7)
Three EE thumb rules to follow:1. Accuracy Class of Metering CT2. Accuracy Class Letter of CT3. Accuracy Class of Protection CT
Accuracy Class of Metering CTMetering Class CT
ClassApplications
0.1 To 0.2Precision measurements
0.5High gradekilowatt hour metersfor commercial grade kilowatt hour meters
3General industrial measurements
3 OR 5Approximate measurements
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Accuracy Class Letter of CTMetering Class CT
Accuracy ClassApplications
BMetering Purpose
Protection Class CT
CCT has low leakage flux.
TCT can have significant leakage flux.
HCT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)
LCT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.)
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Accuracy Class of Protection CTClassApplications
10P5Instantaneous over current relays & trip coils: 2.5VA
10P10Thermal inverse time relays: 7.5VA
10P10Low consumption Relay: 2.5VA
10P10/5Inverse definite min. time relays(IDMT) overcurrent
10P10IDMT Earth fault relays with approximate time grading:15VA
5P10IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA
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