Q: CH.9 Section 7 : Transformers

Discussion in 'General Electronics Chat' started by foolios, Dec 29, 2009.

  1. foolios

    Thread Starter Senior Member

    Feb 4, 2009


    1) When the voltage is stepped down with the PT doesn't the current then get stepped up to a dangerous level for the voltmeter to measure the circuit?

    2) When the current gets stepped down via the CT, the voltage is now so high, why is this acceptable here vs. in the PT scenario?

    3) Why isn't this ground(grounded for safety) causing a direct short?
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    1. No - that's the very purpose of having the PT and CT - to galvanically isolate the measurement side from the high voltage side and to transform the current [CT case] or voltage [PT case] to a safe magnitude.

    2. The voltage is not high on the secondary measurement side for the reason stated above in point 1. The CT primary connection is usually made in the HV active side - i.e. before the feed to the load rather than in the return line from the load. There is a condition in which a CT secondary can be hazardous - if the secondary is left open rather than being terminated in the ammeter or intentionally shorted if not in use.

    3. Where would this "short" occur? The ground simply ensures that if a fault develops from the high voltage to the low voltage side of either the CT or the PT, the secondary side hardware will not be "live" relative to ground. This (hopefully) ensures an operator (at ground potential) touching these devices on the low voltage side would not be harmed in the event of such a failure or breakover. One would expect the CT and PT to have some form of protection which would isolate the HV side in the event of a HV to LV breakover occurring.

    Even if a fault doesn't occur there may also be sufficient capacitance from primary to secondary in either the CT or the PT which could permit a hazardous voltage rise on an ungrounded secondary side - a condition which can be circumvented through the intentional ground link.