Low voltage distribution systems are common in commercial buildings, factories, and residential complexes. CTs in these systems are often used for sub-metering, load monitoring, and energy management. Selection is generally simpler than for medium-voltage switchgear, but accuracy, burden, ratio, and installation still require careful attention.
The first step is to determine the feeder current. The CT ratio should be chosen so that normal load falls within a suitable range, usually 60–80% of the primary rating. For example, a feeder with a normal load of 200 A may use a 250/5 A or 300/5 A CT. If the load is expected to grow, a slightly larger ratio may be appropriate, but oversizing should be avoided because it reduces accuracy at low currents.
The type of CT depends on the installation. Split-core CTs are popular for retrofits because they can be installed without disconnecting cables. Block-type CTs are used where space is limited. Cable-type CTs are suitable for individual cables, while busbar-type CTs are used for busbar feeders. The CT window must fit the cable or busbar.
Accuracy class depends on the application. For general monitoring, class 1.0 or 0.5 is often sufficient. For energy metering, class 0.5S or 0.2S may be required. The CT burden must match the meter or monitoring device. Long cable runs can increase burden, so larger conductors may be needed.
Installation safety is important. The CT must be installed with correct polarity and grounding. The secondary circuit must never be opened under load. In low-voltage systems, special attention should be paid to insulation and mechanical protection. The CT should be mounted securely and away from heat sources.
Testing and commissioning should verify ratio, polarity, and readings. The meter must be programmed with the correct CT ratio. If readings are inaccurate, check polarity, burden, and wiring.
In summary, choose a low voltage distribution CT by calculating load, selecting a suitable ratio, choosing the right type, matching accuracy and burden, and ensuring safe installation. A well-chosen CT will provide accurate monitoring and support efficient energy management.