A current transformer (CT) is an electrical device used to measure electrical current in high-voltage circuits by transforming it into a proportionally reduced current that can be easily measured with conventional measuring instruments. The main purpose of a current transformer is therefore to provide accurate and safe current measurement in high-voltage electrical circuits. It is widely used in electrical power distribution networks, transformer stations, industrial plants, and other electrical installations.
Current transformers can also serve protection functions, where they detect fault currents and trigger protective devices to isolate equipment and people from dangers associated with these high currents.
Current measurements can be made using meters. These meters employ two technologies: either with integrated CTs or remote CTs. This guide applies when using remote CTs, which are generally required for installations above 125 amps or when meters with integrated CTs cannot be incorporated into the installation.
Current transformers are sensors that measure the current flowing through an electrical conductor.
The sensor is a CT that measures a magnetic field proportional to the inducing current (the current passing through the cable). This variable magnetic field generates an induced current in the CT. The current created at the CT output will also be proportional to the magnetic field (depending on the number of turns in the CT).
In metering applications, the current leaving the CT must be between 0 and 5 amps. There are two criteria to consider when choosing the appropriate CT:
- The maximum current that will pass through the conductor depends on the rating of the directly upstream circuit breaker or its setting. If the setting is 95 amps, use the next highest rating of the circuit breaker, in this case 100 amps.
- The component where the CT will be installed varies depending on whether it's connected to a cable or a busbar.
Therefore, it's necessary to account for the dimensions of the cable (the diameter, not the cross-sectional area) or the busbar (height and width).
