Sizing Metering CT's 1

[tmaly1]

Hi folks,

is sizing secondary metering CT's 25% above normal load is sufficient to accomodate for any short-term overload? in this case what role does the Thermal Rating Factor play, should it be conisdered when sizing the CT's and how?

Thanks for your input.

 

[scottf]

The correct way to size a metering CT is to pick the ratio as low as possible and use the rating factor to cover up to the maximum current level.

In the IEEE standard,the normal metering class is 0.3%. The way this class is defined is that the CT will be guaranteed to meet a 0.3% accuracy from the rated current up to the rating factor current (RF x Irated). A 0.6% accuracy will be guaranteed from rated current down to 10% rated current.

Therefore, to the greatest extent possible, try to keep the primary current level above the rated current of the CT.

Depending on the voltage level of the CT, rating factors from 2.0 up to 4.0 are available. Normally, for a given type of CT, the higher the rating factor given, the lower the maximum burden rating of the CT.

If you have questions about how to size the CT and the effect on cost, I recommend you contact your CT supplier.