The Mighty 477 Blog

In the world of Relay Technicians Impedance remains the same and voltage drives the current in the circuit. Lets go back to the CT and we will say that the CT has a ratio of 2000 to 5. That means if 2000 amps are flowing through the primary wire, there will be 5 amps flowing from X1 to X2 in the CT. CTs exist because it is impractical to make a 2000 amp ammeter. The manufacturer designs the ammeter to read full scale at 5 amps, so a 2000 amp ammeter must have a 2000:5 CT in order to read correctly. When 2000 amps flow through the primary wire, a magnetic field is produced that is proportional to the current. This magnetic field induces a voltage in the CT, but because the CT is shorted the voltage will drop significantly. If you were to try and measure the voltage at the X1 and X2 terminals you would find that the meter would read almost 0, giving the illusion that a CT has only current flow. This voltage drop is largely due to the fact that there is only one turn in the primary circuit. If you were to short the secondary of a power transformer, the magnetic field in the core would drop, lowering the impedance in the primary circuit and high current would flow in the primary. In a CT, shorting out the secondary also caused the magnetic field to lower in the core, but the single turn of wire cannot replace the magnetic field in the core, therefore the short has no effect on the primary wire. If, however, the CT were not shorted the 2000 to 5 Current Transformer would become a 5 to 2000 Power Transformer and on an 18kV line there would be 7,200,000 volts at the X1 and X2 terminals. Let me say that again, SEVEN MILLION, TWO HUNDRED THOUSAND VOLTS. This is why we use ring terminals on all CT circuits so even if the screw were to become loose it might not fall off altogether.

Posted by Hello