At the time this photo was taken, Bluffton, Indiana was still using the old 6.6A series street lighting loop, which only a few cities and towns were using at the time. The street lighting circuits were changed over to modern parallel circuit system a few years after the photo was taken.

This is a very close up shot of a U-399B loop insulator. A great example of what the U-399B loop insulator was designed for. The metal "cannister" device is a mercury vapor lamp isolation transformer. Its primary side is connected to the 6.6A series circuit on each side of the U-399B insulator. The secondary of this transformer connects directly to the mercury lamp inside the fixture. In the event of bulb failure the isolation transformer's primary coils maintain continuity within the series loop without interruption. The secondary leads going to the lamp are low-voltage, even during lamp removal or failure (secondary-side open circuit). Much thanks to Joe Maurath for that extra info.

Please note = I have posted this shot a few years back, but decided to add it to this series with better post processing and a higher resolution.

November 18, 2012