After pushing the test pushbutton of the illustrated high voltage ground detection system, the ground lamps indicate as shown in example #3 where lamps A and B are brighter than normal and lamp C is darker than normal. What does this indicate? See illustration EL-0009.
• Three-phase ground detection circuits and how lamps are connected to each phase • What happens to voltage-to-ground on each phase when one phase becomes grounded in an ungrounded or impedance-grounded system • How the test pushbutton changes the circuit and causes one or more lamps to get brighter or dimmer
• Before the pushbutton is pressed, think about the normal voltage of each phase to ground if there is a ground on one phase of the bus • When you press the test pushbutton, which lamps will now see more voltage and which will see less voltage if a particular phase is grounded? • Match the pattern in example #3 (A and B brighter, C darker) to the phase that must already be at or near ground potential
• Verify which lamp is connected to which phase (A, B, C) in the simplified circuit • Be sure you understand that a grounded phase will have almost zero volts to ground, while the healthy phases will have higher-than-normal line-to-ground voltage • Confirm that the example #3 pattern (A and B brighter, C darker) can only occur with one specific phase grounded, not with multiple grounds at once
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