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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