Electrical Thumb Rules-(Part-8)

Accuracy Class of Metering CT:

Metering Class CT

Class Applications
0.1 To 0.2 Precision measurements
0.5 High grade kilowatt hour meters for commercial grade kilowatt hour meters
3 General industrial measurements
3 OR 5 Approximate measurements

 Accuracy Class Letter of CT:

Metering Class CT

Accuracy Class Applications


Metering Purpose

Protection Class CT


CT has low leakage flux.


 CT can have significant leakage flux.


 CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)


 CT 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.)

 Accuracy Class of Protection CT:

Class Applications
10P5 Instantaneous over current relays & trip coils: 2.5VA
10P10 Thermal inverse time relays: 7.5VA
10P10 Low consumption Relay: 2.5VA
10P10/5 Inverse definite min. time relays (IDMT) over current
10P10 IDMT Earth fault relays with approximate time grading:15VA
5P10 IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 13 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

11 Responses to Electrical Thumb Rules-(Part-8)

  1. murali sir says:

    good help to eee technical people and
    updating eee dedcator experience

  2. rama says:

    some times i see 0.2s 0.5s as accuracy class…what exactly it mean…specifically that ‘s’
    thankyou in advance sir….

  3. asimali856 says:

    thanks sir

    Thanks & regards

    Asam Ali

    Al Mariah United Group Delma Hospital Abu Dhai – U.A.E Cell No.: +971 567077962


  4. Ruid says:

    Sir, please tell me the tolerance of Copper Strip of size 32 x 6mm used for earthing, or ligtning protection ground conductor with IS.

  5. Muthukumar says:

    Thanks. all the data’s are very use full for site engineers

  6. Yasir Bashir says:


  7. Raut says:

    please tell me the horizontal Clarence require between 220Kv & 132 Kv over head transmission line both are parallel running.

  8. Iraki jishan says:

    Sir i just want to know about complete information of Installation,Testing & Commissioning of Electrical Equipment like motor,generator, transformer, substation etc…step by step complete procedure.. pls help sir

  9. pravincares says:

    very useful informations.thanks

  10. Mangesh says:

    Thanks for improving my electrical knowledge.

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