Corner-Grounded Delta Circuits
Occasionally, (especially in older facilities or in oil well and irrigation applications), you may encounter a three-phase delta service with one phase (leg) connected to ground, called a “corner grounded delta” or “grounded leg service”. You can determine if you have a corner grounded delta service by using a multimeter (DMM) to measure the voltage between each phase and ground. If you see a reading between 0 and 5 Vac for a phase, that phase (leg) is probably grounded. Note that a reading of zero volts could also be caused by a blown fuse on that phase or a ground fault on that phase.
A 3D model WattNode meter, with the appropriate voltage range, will correctly measure grounded delta services, but the measured voltage and power for the grounded phase will be zero and the status LED will not light, because the voltage is near zero. Also, one or both of the ungrounded phases may indicate low power factor because this type of service results in unusual power factors.
For the best accuracy with a grounded delta service, you should connect the N (neutral) terminal on the meter’s green screw terminal block to the meter’s ground terminal and then connect the ground terminal to the electrical service safety ground. This will not cause any ground current to flow because the neutral terminal is not used to power a delta (3D) WattNode model.
If you have a grounded leg configuration, you can save money by removing the CT for the grounded phase, since all the power will be measured on the non-grounded phases. We recommend connecting the grounded phase to either the ØB or ØC (Phase B or C) voltage input and attaching a note to the meter for future reference indicating that this is a grounded delta circuit.
Keywords: corner-grounded, grounded delta, grounded leg service, three-phase corner-grounded delta circuits