I may have misspoke, but it's been a while since I've had my hands inside one of these units (for obvious reasons).
The '3 lights' style of ground detector works on a true ungrounded system such as the ones installed back in the 30's because the phase conductors are only capacitively coupled to ground.
There is no physical connection between any energized conductor and ground in our systems.
http://www.ece.mtu.edu/faculty/bamork/EE5223/EE5223TermProj_Ex3.pdfBTW - Our new installations take advantage of high impedance grounding to maintain system integrity while reducing the potential for catastrophic failure if a second ground fault should occur before the first can be tracked down and cleared.
