Do Not Use A Screwdriver! For best accuracy, these parts should not be interchanged with other CTs. Solid-core CTs require that the phase conductor being measured be disconnected at one end so that it can be passed it through the opening in the CT. This is not difficult when the wire gauge is small but becomes impractical with larger wire gauges and multi parallel conductors.
For correct measurements, CTs must be installed on the phase conductor that corresponds to the voltage input connection. The voltage input connections are on the green, five position, screw terminal block. It may help to use colored tape or labels to identify the wires. To reduce magnetic interference between CTs on adjacent phases, it is a good practice to separate them by about 1 inch 25 mm.
This also helps to prevent dust and debris from forming a bridge across the phase conductor terminals or bus bars and possibly causing a flashover arc. If the CT opening is much bigger than the conductor, position the conductor in the center in the CT opening.
Plastic cable ties can be used to secure the position of the CT on the phase conductor. A cable tie can also be secured around the perimeter of some models of CTs to prevent them from accidentally opening. CTs are marked with a symbol arrow or label which indicates the correct mechanical orientation of the CT on the conductor being measured. In addition to installing CTs with the correct mechanical orientation, electrical polarity, as indicated by their white and black wires must also be correct.
Each pair of CT wires connects to the appropriate terminal on the black six position screw terminal block. The polarity of each pair of terminals is indicated by a white and black dot on the label. Be sure to connect the white wire to the phase terminal aligned with the white dot, and the black wire to the terminal with the black dot. Remember that both the physical orientation and the electrical polarity of each phase must be correct for proper operation.
If a phase is reversed either electrically or mechanically, and current flows in the reverse direction, the WattNode meter will measure, depending on the model, zero or negative energy for that phase. With this connection the directions of primary and secondary currents are:. When testing a CT using an Omicron test equipment it is possible to verify if the CT is correctly connected:. The doughnut fits over the primary conductor, which constitutes one primary turn.
Figure 6 — On line CT. Ring type CT are commonly used in cables, busbars and transformers bushings. CT may have one or more cores; typical applications of these cores are:. The use of more than one core for protection is justified when the installation have two protections sets — main and backup.
The ratio of a CT is the relation between the values of the primary and the secondary currents ; the usual secondary values are 1 A and 5 A. Figure 8 — Schematic connection of primary CT windings with double ratio. The accuracy class of a CT is the percent allowable error , and is associated to burden power , the apparent power , expressed in VA , that is taken up from the secondary core secondary load , and for which the accuracy is assured.
Class PX is the most accurate and is used usually for main protections. This accuracy class was retained by IEC in in the Amendment nr. Common accuracies and burden powers, as well as error limits, according to IEC Standard are indicated in Table 1. Table 1 — Common accuracies and burden powers of CT and error limits. Common value for RF is 1.
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