Why CT's shall not be kept open?
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Answer Posted / azhar ali
Basically, current transform has a few turns one or may be two at primary sides and many turns on Secondary side.If the ammeter is removed, the secondary winding becomes open-circuited and the transformer acts as a step-up transformer due to the very large increase in magnetizing flux in the secondary core. This results in a high voltage being induced in the secondary winding equal to the ratio of: Vp(Ns/Np) being developed across the secondary winding.
Turn ratio=vp (NS/Np)=> VP=480 NS=160 V &Ns=1 Then,
Tr=480*160/1 = 76800 volts
So it will be very dangerous if the secondary is opened.Therefore short the secondary of CT if the current meter is removed from the CT having primary current.
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