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Home / Technical Articles / Electrical Thumb Rules You MUST Follow (8)

Three EE thumb rules to follow:

  1. Accuracy Class of Metering CT
  2. Accuracy Class Letter of CT
  3. Accuracy Class of Protection CT
Electrical Thumb Rules You MUST Follow (Part 8)
Electrical Thumb Rules You MUST Follow (Part 8)

Continued from part 7: Electrical Thumb Rules You MUST Follow (Part 7)

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

Go back to Index ↑


Accuracy Class Letter of CT

Metering Class CT
Accuracy Class Applications
B Metering Purpose
Protection Class CT
C CT has low leakage flux.
T CT can have significant leakage flux.
H CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)
L 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.)

Go back to Index ↑


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) overcurrent
10P10 IDMT Earth fault relays with approximate time grading:15VA
5P10 IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA

Go back to Index ↑

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Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

12 Comments


  1. AJAY DAMOR
    Dec 27, 2018

    THANKS SIR,
    VERY NICE MATERIAL FOR PRACTICAL KNOWLEDGE IMPROVEMENT


  2. Loveleen
    May 08, 2018

    How to prepare ELECTRICAL BOQ ? Can you pls help ? Do you have some study material or book on that. URGENT !


  3. nagaraja
    Oct 25, 2015

    thanks sir, your articles helping me to boost my kowledge.


  4. milind
    May 26, 2015

    Thank you for providing such important information which will help to update my knowledge.


  5. Ahmed Eltayeb
    Jul 24, 2014

    Thanks Jungesh ..Keep it up.
    Good stuff ..


  6. niraj patil
    Jul 21, 2014

    thanks Jignesh sir, for giving us the information about electric thumb rules.

    i want help from you,

    can you give me a thumb rules for the use of estimation and costing of electrical project work ?


  7. sushil sasane
    Jul 18, 2014

    Great electric engineering.


  8. ANKIT JANGID
    Apr 21, 2014

    Please send newsletter


  9. sasikumar
    Mar 27, 2014

    Dear Jigu Sir,
    Good and Thank you so much. It is use full for us.


  10. Prakash
    Mar 11, 2014

    I wanted to query the last part of your article regarding the accuracy class of protection CT’s. What was the intention of this part of the article?

    Is it to give one an idea as to how to select CT’s?
    To my knowledge, the CT ratings are dependent on the fault level of the application, this is one of the determining factors for the CT sizing.
    If you have some time, please look into writing an article on this topic as well.
    Thanks once again.


  11. Gife
    Mar 10, 2014

    Your materials are very useful. How do I download them? Keep doing the great job.


    • Dhaval
      Mar 11, 2014

      You can find “Get PDF” Tag on middle left og the webpage,by clicking on that you can have pdf copy of what u want…have a great day

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