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1) Typically, it means that the equipment grounding conductor (bare copper or green insulated ground wire) is open in either the entire circuit or at that receptacle or the next upstream receptacle or no ground is present. If the circuit was installed in EMT metal conduit, this can also mean that either a coupling or a box connector came loose on the conduit and no longer continuity. EMT metal conduit also acts as an equipment grounding conductor.
2) However, when using 3 prong receptacle testers, these can provide a false reading depending upon the hot or neutral conductor terminations.
3) The best and recommended testing tool to check for a properly wired receptacle is an AC voltmeter. Plug-in cube testers are not the most reliable and accurate electrical testers as they have been know to provide false indications.
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