what is the CT of protection and CT of metering ? what is
class 5P10 & 5P20 and class PS ?
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Answer Posted / sindhu sl
There are three types of core basically used in Current Transformers
they are
1)Metering
2)Protection
3)PS core or Protection Special
Basically CT are the one which is used to measure the current in the lower value as Ammeters and other measuring meters are rated at a lower value.Thus CTs step down the current and this is used for metering or protection purpose.
1)METERING:
Metering cores are used only for measurement.for a 500/1 A CT the core should essentially saturate after 120% of the rated current then only it will give accurate measurements.So saturation must be very much higher for metering core.
it is identified as 0.1S,0.2S,0.5S.These are the accuracy class explained by Indian Standars. 0.1S is more accurate than 0.2 and 0.5.
2)Protection Core:
Protection cores are used when a high surge current enters or in the case of any abnormal current.So the core should not saturate when the primary experiences high current.
it is designated as 5P10,5P20 etc
here 5 denotes the percentage of error
P-Protection
10-Accuracy Limit Factor(ratio of accuracy limit Primary current/Rated Primary Current)
If ALF is high then the Class of protection is also high.
3)Protection Special or PS Core:
This is also used for special type of protection in case of heavy faults particularly differential faults. It is determined by Knee Point voltage or Short time current
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