11kV - 3 Phase Star System, with Neutral Point Earthed.
Theoretical Voltage Calculations and
Explanation of using the Live Line Tester to measure Resultant Phase to Earth
Voltage, and to determine the In Phase and Out of Phase Voltage relationships
3 Phase Star Diagram, with Neutral Point Earthed. Resultant Phase to Earth Voltage
Red
Phase to Phase (R-Y) = 11kV
Phase to Earth (Y-E) = 11kV / √3
(√3 = 1.7320508)
Ph to E (Y-E) = 11kV / 1.7320508
Ph to E (Y-E) = 6.35kV
Phase to Phase (R-Y) = 11kV
Cos 30 = 5.5kV / (Y-E) Ph to E
(Cos 30 = 0.866)
Ph to E (Y-E) = 5.5kV / 0.866
Ph to E (Y-E) = 6.35kV
11kV
120°
E
90°
5.5kV
30°
Yellow
Blue
3 Phase Star Diagram
For 11kV System
Phase to Earth Voltage = 6.35kV
For 6.6kV System
Phase to Earth Voltage = 3.81kV
11kV Tester
Showing
Scale 0-15kV and
Front Cover with
Phasing
Information
The table on the front of the 11kV Tester details the In-Phase and Out of
Phase Voltages expected on an 11kV System and on a 6.6kV System.
From the Scale the In-Phase and Out of Phase Voltages can be easily
measured
Phase to Earth
Fig.13- Phasing Connections
The previous diagrams show how connections are made to the systems under test.
Phase to Earth
Current passes through the resistor rod and meter to Earth, this current is
displayed on the meter as a Voltage
Fig 13 – Phasing Connections
The current passing through the Phasing Rod is led into the Tester on the right by
means of the Phasing Lead which screws into the handle of the phasing Rod and
into the back of the Tester housing (remove blanking plug).
The Tester is Earthed as usual, and connected to the phases as shown.
If the two lines are in phase the currents through each resistor rod are in phase,
consequently twice normal current will pass through the Tester on the right.
If the phasing is incorrect, the currents are 120° out of phase, the vector sum being
equivalent to normal deflection between one phase and earth.
Correct Phasing is thus shown on the Tester by an indication of twice the normal
line/earth voltage and the
Incorrect Phasing is thus shown on the Tester by an indication of normal line/earth
voltage.
Phase to Earth and Phase to Phase Voltages - when using a Live Line Tester
Theory diagram for the LLT