Current Transformers

If you have worked at Mountainview and have been involved with wire pulling or termination you have probably run into a few CTs or Current Transformers. So how does a Current Transformer differ from a Power Transformer (also known as a Voltage Transformer)? Not by as much as you would think. Normally we don't think of transformers in terms of current. Voltage comes into a transformer at 480 volts and is stepped down to 208/120 volts for instance, but nothing changes for free. For the voltage to step down, the current had to step up. That's why the 480 volt leads are smaller than the 120/208 volt leads on a power transformer, larger current means larger wire. In the case of a CT, wire is wound around a laminated iron core in the shape of a doughnut. The wire in which the current is to be measured passes though the hole in the center of the CT which has a polarity mark H1. This could be considered as the primary of the transformer. The secondary side of the CT is marked X1 and X2. This is where the similarity between VTs and CTs end. CTs must remain shorted between X1 and X2, either by a jumper or by an ammeter or some other low resistance device or disaster will result. As electricians it is our experience that voltage is a constant while the current varies with the resistance or impedance of the circuit. When a light is turned on, current flows and is determined by the resistance of the filament in the light. The voltage drop across the lamp is insignificant and we would still measure 120 volts before and after the light was turned on.
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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