Technical Details
Selection criteria for accuracy class of CTs.
| Application |
Class of Accuracy |
| For precision testing or as a substandard for testing laboratory current transformers. |
0.1 |
| For laboratory and test work in conjunction with high accuracy indicating instruments, integrating meters and also substandard for testing industrial current transformers. |
0.2 |
| For precision industrial metering. |
0.5 |
| For commercial and industrial metering. |
0.5 or 1 |
| For use with indicating and graphic watt-meters and ammeters. |
1 or 3 |
| For purposes where the ratio is of less importance, for example, ammeters where values approximate are required. |
3 or 5 |
Selection criteria for burden of CTs.
| Description of the load |
Burden in VA |
| Ammeter |
1.0 |
| Current coil of watt/VAR meter |
1.5 |
| Current coil of Energy meter |
2.0 |
| Current coil of P.F. Indicator |
2.5 |
| Current coil of Trivector meter |
3.0 |
| Leads between C.T & meters |
2.0 |
Selection criteria for accuracy class of VTs.
| Application |
Class of accuracy |
| For protection testing or as substandard for testing laboratory voltage transformers. |
0.1 |
| For laboratory and test work in conjunction with high accuracy indicating instruments, integrating meters and also for substandard for testing for industrial transformers. |
0.2 |
| For precision industrial metering and for use with substandard and indicating wattmeters. |
0.5 or 1.0 |
| For commercial and industrial metering and for use with indicating and graphic wattmeter and voltmeters |
1.0 |
| For purpose where the phase angle is of less importance, for example, voltmeters. |
3.0 |
It is recommended that the accuracy class should be correctly selected along with correctly determined burden so that the PT’s can be produced at reasonable price.
Selection criteria for burden of VTs.
| Description of the load |
Burden in VA |
| Voltmeter |
1.0VA |
| Pressure coil of Watt/VAR meter |
1.5VA |
| Pressure coil of Energy meter |
2.5VA |
| Pressure coil of P.F. meter |
2.5VA |
| Pressure coil of Trivector meter |
5.0VA |
Selection criteria for Voltage Factor.
Method of connecting the
primary winding and system
earthing condition |
Rated Voltage Factor |
Rated time |
| Between line in any network between transformer star point and earth in any network. |
1.2 |
Continuous |
| Between line and earth in any effective earthed neutral system. |
1.2
1.5 |
Continuous
30second |
| Between line and earth in a non-effectively earthed neutral system with automatic earth fault tripping. |
1.2
1.9 |
Continuous
30second |
| Between line and earth in an isolated neutral system without automatic earth earth-fault tripping or in resonant earthed system without automatic earth fault-tripping. |
1.2
1.9 |
Continuous
8 Hours |
Note: - Reduced rated times are permissible by agreement between the manufacturer and the user.
Selection criteria for instrument security factor.
Instrument Security factor (I.F.S) for the metering core is often specified to be less than 5. while the IS specifications indicate that I.F.S shall be as low as practicable, a point is many time stretched by specifying I.F.S to be less than 5 for two or three ratio CTs., where the change of the ratio is oriented by taps on the secondary winding. It s apparent that in such cases I.F.S cannot be maintained to be less than 5 for all ratio as, if it is less than5 for the lowest ratio, it shall be around 10 for the next higher ratio and may be about 20 for the ratio (Ratio assumed to be in the proportion 1:2:4)
Besides the above, it is understood that the purpose of specifying I.F.S to be less than 5 is to prevent excessive current from flowing through the connected meters. From the specification of indicating instruments, it is seen that the current circuit of all indicating instruments can carry 10 times the rated current for the 5 seconds. If it is assumed that the fault on the system may be cleared within one second, the instruments can carry up to 22 times the rated current for one second which means that specifying I.F.S to be less than ‘5’ does not serve any useful purpose. The manufacturer in any case shall limit I.F.S to a minimum, consistent with specified burden and class of accuracy, to economies on the cost of inputs.
It may be mentioned that specification of a single rated primary current helps in keeping the value of I.F.S low.
It has been confirmed by instruments manufacturers that all current meters and coils of power meter have been tested to withstand 10 times the rated current for 5 seconds.
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