Minimum Acceptable Specification of C.T & P.T for Metering

Min. Acceptable Specification of Current Transformer for Metering:

Sr. No Particulars 11 kV 33 kV 132 kV 220 kV 
1 Highest System Voltage (kV rms) 12 36 145 245
2 CT ratio. 2000-1000/1-1 800-400/1-1 400/1-1 800/1-1
1600-800/1-1 600-300/1-1    
1200-600/1-1 400-200/1-1    
800-400/1-1 300-150/1-1    
600-300/1-1 100-50/1-1    
3 Number of metering cores Two Nos Two Nos Two Nos Two Nos
4 Rated continuous thermal current. 120% of rated primary  current 120% of rated primary  current 120% of rated primary  current 120% of rated primary  current
5 Rated short time thermal current of primary for 1 sec. (kA) 25 25 31.5 40
6 CT characteristics :a) Rated primary current (Amps.)  2000-1000  800-400  400  800
1600-800 600-300    
1200-600 400-200    
800-400 300-150    
600-300 100-50    
(b) Rated Secondary current (Amps.) 1 1 1 1
(c) Class of accuracy. 0.2 0.2 0.2 0.2
(d) Max. instrument security factor 5 5 5 5
(e) Rated burden (VA). 30 30 30 40
7 IS to which CT conforms.        
8 IS to which insulating oil conforms.        

 Min. Acceptable Specification of Voltage Transformer for Metering:

Sr. No Particulars     245 kV CVTs      145 kV CVTs 
1 Highest SystemVoltage (kV)          245 kV              145 kV
2 Rated Capacitance (pF)  4400 pf with tolerance + 10% and – 5%
3 For low voltage terminal over entire carrier frequency range.    
  (a) Stray capacitance  Shall not exceed 200 pf
  (b) Stray conductance Shall not exceed 20 us
4 (a) High frequency            capacitance for entirecarrier frequency range within 80% to 150% of rated capacitance
  (b) Equivalent series resistance over the entire frequency range. less than 40 Ohms
5 No. of secondary windings for potential device.             Two              Two
6 Transformation ratio:  
  (i) Winding –I   20 kV- \/3/110 -\/3V
  (ii) Winding –II   20 kV- \/3/110 -\/3V
7 Rated secondary burden    
  (i)Winding –I   (VA) 50 VA 50 VA
  (ii) Winding –II  (VA) 50 VA 50 VA
8 Accuracy Class :    
  (i)Winding –I   (VA)                        0.2 for metering
  (ii) Winding –II  (VA)                        0.2 for metering
9 Voltage factor for winding – IVoltage factor for winding – II 1.2 Cont. & 1.5 for 30 secs.1.2 Cont. & 1.5 for 30 secs.
10 IS to which CVTs conform. IS 3156 with latest amendment
11 IS to which Insulating Oil conform. IS 335 with latest amendment

Minimum Acceptable Specification of Single Phase PT for Metering:

Sr.No Particulars        33 kV         11 kV 
1 Highest System Voltage (kV rms)           36             12
2 Transformation ratio.   33kV/ V3/     110/ V3   11 kV/110 V
3 Number of windings.          Two           Two
4 Rated output/ burden (VA) per winding /phase.           50             50
5 Accuracy class. (At 10 to 100% of VA burden)          0.2             0.2
6 Rated voltage factor and duration. 1.2 continuous & 1.5 for 30 secs.
7 IS to which PT conforms.   3156 with latest amendment


About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. 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.

9 Responses to Minimum Acceptable Specification of C.T & P.T for Metering

  1. waqas ahmed says:

    dear sir!!

    your post are very informative..i request you to please guide us on realy testing for different schemes of cable and OHTL protection..please share us your practical knowledge..

    Waqas Ahmed

  2. rajiv pandey says:

    It is really informative. Please guide me about the following:
    We have a new steel plant coming up in the state of west bengal. The plant is still in installation & commissioning stage. We have erected one 220 KV line from supplying utility to the 220/33 KV Main receiving station of the plant. the CT ratio in the line is 400/1, 0.2 Class. We need about 1.5 MVA at present to keep the MRS charged and for commissioning of few of the equipment. However, supplying utility has told us that they will not be able to supply less than 5 MVA in the 220 KV line, as supply of less than 5 MVA may cause error in metering. Kindly guide us whether the stand of supplying utility is correct or not with reasoning.



  3. r c says:

    Very good…. keep it up

  4. parijat says:

    in maharastra it is required to put check meter on the consumer side for high MD cosumer. please could u tell me its possible to connect with same core of ct or PT or we have to insatall separate metering temple

  5. Adeel Arif says:

    Dear sir

    Your site is very useful. Please tell me that are thumb rules same at both 50 or 60Hz?

  6. vijay says:

    please sir provide ct,pt sizing and selection procedure

  7. sir i am arvind sharma
    i am electrician . i am information for basic electrical control connection
    so please help me all

    Arvind Sharma

  8. Nasser says:

    We have plain feeders with CT ratio 800-400/1-1 specified at 800/1 with circuit breaker of 400A. Now would specifying the CT at 400/1 or 800/1 have any impact on reducing or increasing the reading of the total consumption read by the meter?
    Please explain.

  9. K.L Patra says:

    Dear Sir,
    we have a 1MVA Transformer having the Voltage ratio is of 11KV/433V and a NGR on LV side to limit the fault current. what should be the CT ratio is to be installed in NGR , that should protect against the earth fault.

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