Wow, Sirsonic; great
information! Thank you for filling in all the gaps; no pun intended!
You mentioned the green catenary poles as a reminder to E60 engineers to manually trip the MCB at the gap. I had not heard that story before, but I do remember they had a terrible time at first with them blowing fuses. Everybody was desperately looking for Bussmann KAB-200 fuses. NJT must have called every electrical distributor in the state for them. They were "semiconductor fuses" specially designed to protect the power devices (SCR's and such) in high power controls. I thought at the time that there might be a connection between blowing fuses and the phase gaps.
timz wrote: Strictly speaking, does the term "phase gap" imply a change of phase only? If so, what are we supposed to call a voltage-change gap?
The term phase gap or phase break applies wherever there is an electrical "discontinuity" in the catenary system. When high voltage AC traction was in its' infancy, there was only one voltage and frequency: 11kV and 25Hz. Larger systems had more than one generating source, and although the different sources were usually kept in synchronism, they were not as "tightly" interconnected back then as our utilities are today... (Forgetting the summer 2003 northeast blackout brought to you by our friends at First Energy, of course...)
If the different generating stations serving the railroad lost sync but remained connected to opposite ends of the same catenary, damaging circulating currents back and forth would cause even greater instability possibly leading to both plants being tripped off line. So the phase gap was created. If the power systems remained in sync, there was no voltage or phase difference across the gap. If the systems lost sync, the voltage and frequency would remain the same on each side, but the "phase" (or "timing") of the voltage between the two systems could differ, hence the term "phase gap".
Today, we not only can still have differences in phase (...as in using different utility companies on opposite sides; although they should
be in sync...), but differences in voltage and/or frequency as well. They all represent an electrical "discontinuity" at the gap, so the terms "phase gap" or "phase break" to represent the point of an electrical discontinuity are still used.