For a bus with 24kA available fault current, which class of current transformer should be used to avoid saturation?

Choosing the correct class of current transformer (CT) is crucial for ensuring accurate measurements and preventing saturation during fault conditions. In this instance, the available fault current is 24kA.

A C200 class CT can handle fault currents of up to 200% of its rated current without saturating. Given a maximum fault level of 24kA, it is essential that the CT selected can adequately withstand this without entering saturation, which would lead to inaccuracies in fault detection and protection system operation. C200 is designed for situations where there is a need for a higher degree of linearity and less chance of saturation under heavy fault conditions.

Conversely, classes with lower ratings, such as C100, may not provide the necessary capability to handle this level of fault current without saturation, leading to potential operational issues during faults. Higher classes like C300 and C400 may be more than needed for this specific fault current level, potentially resulting in unnecessary costs or over-specification.

In summary, the C200 current transformer is appropriate because it offers a suitable balance of performance and capability to manage the specified fault current of 24kA, thus effectively avoiding saturation and maintaining accurate measurements during fault conditions.