A Current Transformer (CT) is used to obtain reduced current signals for purpose of measurement , control and protection.They reduce the heavy current to lower values which are suitable for operation of Relays and other instruments connected to their secondary winding.
While a potential transformer(PT) used to reduce the voltage to lower values and to provide isolation between high voltage power network and the relays and the other instruments that are connected to their secondaries.
CT is used for current measurement and PT is used for voltage measurement.
CT is connected with primaries connected in series with the power system. CT has a large number of turns on its secondary winding whereas fewer number of turns on primary winding.( since primary is connected in series, so it carries Full Load current and hence fewer turns)
While a PT has large number of turns on primary and fewer turns in secondary winding. the reason is obvious(i. e to get reduced voltage in secondary and V is proportional to Number of turns).And here primary winding is connected across the supply.
Secondary of CT should not be opened.If happens, there won't be any secondary mmf to oppose the mmf produced due to current flowing in primary and hence all the primary mmf acts on the core as magnetizing quantity. This produces a very high flux density in the core which produces a very high induced emf in secondary. this very high voltage is severe to the operator as well as to the other instruments connected to secondary.
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