I want to know what must the CT ratio be on a load of 300A
is a 3PH nutral to earh installation?

Answer Posted / a.p.nanjappa

The neutral is a point in power transformer or in motor
connection where all phases are connected.In this point
voltage is zero. when you connect thisoint to earth, the
fault current or capacitance circulating current flows to
earth.
when a fault occurs , current is high.Therefore,CT ratio
should be 1000-1200/5 or 1A.

It is usually upto designer or protection device supplier.

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