(i) Insulation Resistance Test
The Insulation-resistance shall be taken with all the winding earthed, expect, the one being tested. The entire terminal shall be cleared thoroughly, with clean cotton cloth. A 5000/2500/1000V Megger should be used in the measurement. Care should be taken that the lead wires of the megger do not have joints or come in contact with each other or with current transformer. It is known that the value of Insulation Resistance is continuous to increases initially, with the duration of measurement and hence for the purpose of comparison, the reading is normally taken at the end of 1 minute
Minimum Insulation Resistance after 1 minute is Rated Voltage + 1 Mega Ohms. The 5000/2500V Megger is used for Primary connection and 1000V megger is used for secondary connection.
(ii) Polarity Test
The main objective of the test is to check the polarity of the P.T;the polarity may be changed due to the connection made in the circuit. Change in polarity may lead to malfunction of the equipment which is connected to the potential transformer. To conduct this test we connect a voltage source to the primary of current transformer (P1-P2), in secondary we connect a Galvanometer and look for the deflection. If it‟s in positive then the polarity is satisfactory for connecting the equipment, if negative deflection is observed necessary action should be taken.
(iii) Ratio Test
The objective of test is to check the P.T ratio transformation. The potential transformer primary was excited by means of 3 phase voltage source (415V/433V). The voltage supply will be connected to the primary and check the voltage in the secondary, if the ratio of change is as per the rated ratio, C.T operation is satisfactory. The ratio was calculated using the formula,
Actual Ratio = (Rated primary Voltage/Rated secondary voltage)
Measured Ratio = (Applied primary voltage/measured secondary voltage)
Then these two ratios are compared to identify the error.