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CHAPTER 2: ELECTRICAL BACKGROUND
EPM 6000 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE 2–5
Another common delta connection is the four-wire, grounded delta used for lighting loads.
In this connection the center point of one winding is grounded. On a 120/240 volt, four-
wire, grounded delta service the phase-to-ground voltage would be 120 volts on two
phases and 208 volts on the third phase. The phasor diagram for the voltages in a three-
phase, four-wire delta system is shown below.
FIGURE 2–5: Three-Phase, Four-Wire Delta Phasors
2.2.4 Blondell's Theorem and Three-Phase Measurement
In 1893 an engineer and mathematician named Andre E. Blondell set forth the first
scientific basis for poly phase metering. His theorem states:
If energy is supplied to any system of conductors through N wires, the total power in the
system is given by the algebraic sum of the readings of N watt-meters so arranged that
each of the N wires contains one current coil, the corresponding potential coil being
connected between that wire and some common point. If this common point is on one
of the N wires, the measurement may be made by the use of N-1 wattmeters.
The theorem may be stated more simply, in modern language:
In a system of N conductors, N – 1 meter elements will measure the power or energy
taken provided that all the potential coils have a common tie to the conductor in which
there is no current coil.
Three-phase power measurement is accomplished by measuring the three individual
phases and adding them together to obtain the total three phase value. In older analog
meters, this measurement was made using up to three separate elements. Each element
combined the single-phase voltage and current to produce a torque on the meter disk. All
three elements were arranged around the disk so that the disk was subjected to the
combined torque of the three elements. As a result the disk would turn at a higher speed
and register power supplied by each of the three wires.
According to Blondell's Theorem, it was possible to reduce the number of elements under
certain conditions. For example, a three-phase, three-wire delta system could be correctly
measured with two elements (two potential coils and two current coils) if the potential coils
were connected between the three phases with one phase in common.
Vca
Vab
Vbc
Vnc
Vbn
120 V
120 V