š Key Concepts
ā¢ Three-phase ungrounded system with one accidental ground on phase C
ā¢ How a three-lamp ground detector is connected between each phase and a common junction
ā¢ What happens to the voltage from each phase to ground when (1) one leg is already grounded accidentally and (2) you press the test pushbutton to intentionally ground the detector junction
š Think About
ā¢ First, picture phase C already tied to ground by the fault. When you now press the test button, which point in the lamp circuit is also forced to ground potential?
ā¢ Compare the voltage from each phase (A, B, C) to the grounded point at the bottom of the lamps, before and after pressing the button. On which lamps does the full line-to-line voltage appear, and on which does it drop toward zero?
ā¢ Remember that lamp brightness depends on the voltage across that particular lamp. For each lamp A, B, and C, is the voltage across it now higher, lower, or essentially zero?
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Before You Answer
ā¢ Trace the exact connection of the test pushbutton to ground and how it relates to the bottom of the three lamps
ā¢ Identify which conductor already has a solid connection to ground due to the accidental fault and whether that is on the line side or lamp side
ā¢ For each lamp, determine the potential difference (voltage) between its top and bottom terminals when the pushbutton is pressed; higher voltage ā brighter, near zero ā dark or very dim