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Number of Secondary Windings
When P.T. is required for metering application as also for protection application, it is usual to specify two independent secondary windings (Not cores) for two different applications. This has been detailed in I.S. 3156/1992. The accuracy classes for metering as also for protection application have been standardized and specified in the standard. The accuracy classes for metering application are 0.1, 0.2, 0.5, 1 & 3. The limits of permissible errors of these classes of accuracy are indicated in the said standard. The errors of the P.T. are required to be within the specified limit for any voltage between 80% and 120% of the rated voltage and with burden of between 25% and 100% of rated burden, at a power factor of 0.8 lagging. For protection application the specified accuracy classes are 3P and 6P and the limits at 5% of the rated voltage and at rated voltage multiplied by the rated voltage factor with burdens of between 25% and 100% of rated burden at a power factor of 0.8 lagging.

It is possible that for certain metering applications, from amongst the instrument/meters proposed to be connected across the P.T. secondary, one or two of the meters may require P.T. of accuracy P.T. of accuracy class 0.5 or better, whereas for other meters/instruments an inferior accuracy class P.T. may be acceptable. In such cases it is suggested that two separate metering windings be specified, one with lesser burden and required class of accuracy (say class 0.5 or better) and other with the balance burden and with inferior class of accuracy, (say class 1 & 3). This practice, if followed, may considerably reduce the cost of the P.T. as compared to P.T. specified with total burden and superior class of accuracy. e.g. Consider a P.T. with total burden of 100VA, out of which 10 VA is required for a trivector meter requiring accuracy class 0.5. The balance 90 VA is required for driving meters and instruments for which accuracy class 1 is acceptable. It may then be more economical to specify two separate windings for the P.Ts. one with 10 VA burden and accuracy class 0.5 and another with 90 VA burden and class 1 accuracy.