Overview
The WattNode® meters (WNB and WNC series) have two connections for each current transformer (CT): white and black circles on the label indicate where the white and a black CT wires connect to. Internally, the meter CT terminals marked with the black dots are all tied together (they are the common or reference terminals), while the signals are measured on the terminals with the white dots.
CT Common Bias
For the WattNode Pulse (WNB series), the CT common terminals will be approximately 2.5 Vdc above earth ground (connected to the green terminal block). This occurs to bias the CT signals to the midpoint between circuit ground (0.0 Vdc) and 5.0 Vdc.
For the WattNode (WNC series) BACnet®, Modbus®, and LonWorks® meters, the CT common terminals are biased approximately to earth ground. In this case, the internal circuitry uses ±2.5 Vdc supply voltages.
For all WNB and WNC series WattNode meters, there is a 20 kΩ to 60 kΩ resistor (the resistance depends on the model) between the earth ground connection on the green terminal block and the internal circuit ground. This limits fault currents to safe levels, but it also means the CT common terminals may float somewhat with respect to earth ground. In particular, it is common to see between a few hundred millivolts of AC and a few volts of AC voltage difference between the earth ground connection and the CT common terminals. This does not cause any harm or affect the accuracy, but you should not treat the CT common terminals as an earth ground reference. Also, do not connect the CT common terminals to earth ground!
Notes
On occasion, there are cases where it would be helpful to feed the output from one CT into two or more input phases on the meter, or even into two or more WattNode meters. This can be done, but with some precautions:
- You cannot feed the signal from one CT into two input phases and swap the white / black wires on second input phase, because you will be shorting out the CT signal.
- Very low amperage CTs (less than 30 amps rated current) will suffer reduced accuracy if the CT signal is fed into two or more CT inputs. The added error may be in the 0.1% to 1% range, depending on the CT model.
- Do not feed the signal from one CT into a WattNode Pulse meter and a WattNode BACnet, Modbus or LonWorks meter, because the 2.5 Vdc difference between the CT common bias will cause problems.
- If you feed the signal from one CT into multiple WattNode meters, there is a greater risk of interference and accuracy degradation. We cannot predict if any given installation would suffer these problems.