Why ct secondary shorted

The question arises why? As the secondary mmf is slightly less than the primary mmf, the net mmf is small. Now, in case secondary winding is kept open then secondary current will be zero while the primary current of CT will remain same. Therefore the opposing mmf of secondary will no longer exist. Hence the net mmf is due to primary current only i. N1I1 which is very large. This resultant MMF is responsible for the production of flux in the core and as this MMF is small under normal operating conditions, a small voltage is induced in the secondary winding of the CT.

If the secondary winding is open-circuited with energized primary, the primary MMF remains the same while the opposing secondary winding MMF reduces to zero. In this condition, the resultant MMF becomes very large.

This large MMF produces a large flux in the core till it saturates. This large flux links with secondary winding and induces a high voltage in the secondary winding. Think of the CT as a transformer, with a1 turn primary and many turns on the secondary. When current is flowing through the primary, the resulting voltage induced in the secondary can be quite high, on the order of kilovolts.

When a CT fails under open circuit conditions, the cause of failure is insulation breakdown, either at the shorting terminal strip, or at the feedthrough in the case of oil filled apparatus , because the distances between terminals are not sufficient for the voltages presen. It is basically a safety precaution that's why we short secondary winding because a heavy blast can be if applied voltage on primary side during testing of a transformer e.

We want the primary to look as much like a short circuit as possible so it does not change the current being measured. For this reason the load across the secondary wants to be low impedance. The secondary voltage reflected back to the primary is then very small and the magnetising inductance appears to be shorted out meaning primary current is not affected by adding the current transformer.

In practice the secondary usually has a small value resistor across it so you can sense the primary current as a voltage on the secondary.

If this resistance is too big the primary voltage increases and the current transformer affects the primary current. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why is the secondary winding of a current transformer shorted?

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