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CT Installation and Wiring

SAFETY WARNING! Current transformers (CTs) are typically install in electrical equipment with lethal high voltage levels. Before attempting to install CTs read the CT Installation Safety page.

CAUTION! WattNode meters are designed to work only with CTs that have a 333 mV output. This type of CT has a built-in burden resistor that produces a safe low voltage output signal. The use of any other type of CT will result in incorrect power measurements, and may permanently damage the WattNode meter.

Key Points

Opening and Closing 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.

Split-core and bus-bar model CTs can be opened for installation around a conductor. These CTs have two parts, a ‘C’ shaped body and an ‘I’ section which is removed for installation. To open a split-core CT pull the “I” section straight-out of the “C” shaped body. To open a bus bar CT, first remove the thumb-screws that secure the ‘I’ section. A strong pull is required, especially when the CT is brand new.

The removable section only fits one way, so as it is removed, notice how the steel core pieces fit together. When closing the CT, be sure to match up the ends the same way. If the CT seems to jam and does not close, the steel core pieces are probably not aligned correctly. Do Not Use Excessive Force! Instead, reposition or rock the removable portion until the CT closes without excessive force.

After a CTS model split-core CT has been reassembled, a nylon cable tie may be secured around the perimeter of the CT to prevent accidental opening. On CTB bus bar models, reinstall the nylon screws and tighten them with your fingers. Do Not Use A Screwdriver!

Note that the ‘C’ shaped body and the removable “I” section of opening style CT are calibrated as a single unit. For best accuracy, these parts should not be interchanged with other CTs.

Phase 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. For example, the ØA CT must be installed on the same phase conductor that is connected to ØA Voltage input. Likewise, the ØB CT is installed on the same phase as the ØB Voltage input, and the ØC CT input is installed on the ØC Voltage input. 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.

For best accuracy, the CT opening shouldn’t be more than 50% larger than the phase conductor. If the CT opening is much bigger than the conductor, position the conductor in the center in the CT opening. If that is not possible, try to position the conductor away from the opening end of the CT. Refer to the CT Selection page for more information choosing CTs.

When using split core CTs, it’s best to position the conductor in the bottom of the ‘U’ shaped half of the CT, away from the opening end where there is magnetic flux leakage. 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 CTS series CTs to prevent them for accidentally coming apart.

Orientation And Polarity

CTs are marked with a symbol (arrow) or label which indicates the correct mechanical orientation of the CT on the conductor being measured. Find the arrow or label “THIS SIDE TOWARD SOURCE” on the CT and install the CT with the label or arrow facing toward the current source: generally the utility meter or the circuit breaker.

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 to the black six position screw terminal block. The terminals are labeled ØA CT, ØB CT, and ØC CT. 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.

CT Lead Wires

If the CT lead wires are longer then necessary, they can be shortened. Short CT lead wires help to minimize interference from electrical noise. If the CT lead wires need to be longer then 8 feet, they can be extended. It is generally better to install the WattNode near the conductors being measured instead of extending the CT wires. However, it is possible to extend the CT wires by 300 feet (100 m) or more by using shielded twisted-pair cable and by not running the CT wires close to high current or high voltage conductors. Refer to the Current Transformer Wire Extension page for more information.

Making Connections

Because the CT inputs of the WattNode transducer are susceptible to damage from electrostatic discharge (ESD), always ground yourself momentarily by touching the electrical enclosure or other grounded metal object before touching the transducer. This is a good practice to follow with all electronic equipment sensitive to ESD.

To connect the CT lead wires to the CT input terminals, first strip about 1/4" (6 mm) of insulation off the end of one of the wires, twist the bare strands together, insert the end into the terminal block, and tighten the screw securely. It will be easier to connect the wires to the terminal block if the block is first plugged into the meter.

Unused CT inputs could possibly pickup electrical noise, so it is a good practice to short out unused CT input buy connecting a wire jumper about 1 inch long between the white and black CT terminals.

See Also


Keywords: CT, current transformer, installation, wiring, connecting

 

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