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.
Posted by Hello