If the CT secondary circuit ground connection is removed without the
current transformer primary being de-energized, dangerous voltages may
result in the secondary CT circuits.
4.8 Checking VT circuits
Check that the wiring is in strict accordance with the supplied connection diagram.
Correct possible errors before continuing to test the circuitry.
Test the circuitry.
• Polarity check when applicable; this test is often omitted for CVTs
• VT circuit voltage measurement (primary injection test)
• Grounding check
• Phase relationship
• Insulation resistance check
The primary injection test verifies the VT ratio and the wiring all the way from the primary
system to the IED. Injection must be performed for each phase-to-neutral circuit.
While testing VT secondary circuit and associated secondary equipment,
care shall be exercised to isolate the VT from the circuit being tested to
avoid backcharging the VT from the secondary side.
4.9 Using the RTXP test switch
The RTXP test switch is designed to provide the means of safe testing of the IED. This is
achieved by the electromechanical design of the test switch and test plug handle. When the
test plug handle is inserted, it first blocks the trip and alarm circuits then it short circuits the
CT secondary circuit and opens the VT secondary circuits making the IED available for
secondary injection.
When pulled out, the test handle is mechanically stopped in half withdrawn position. In
this position, the current and voltage enter the protection, but the alarm and trip circuits are
Section 4 1MRK 504 165-UUS -
Starting up
54 Transformer protection RET670 2.2 ANSI
Commissioning manual
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