On an electrified railroad, a phase break is needed when sections of the overhead receive power from different power stations or power grids whose power may get out of synchronization with respect to each other. It is undesirable for such unsynchronised alternating current to be connected to each other, even by the momentary joining that might occur as a locomotive's pantograph crosses a section break, so a more elaborate scheme is used.
A phase break consists of two section breaks in sequence, as close together as can be achieved. This gives us two long pieces of wire, going off in both directions from the phase break, and a short length of contact wire that is strung only between the two section breaks. When the power supplies on both sides of the phase break are in synchronisation, this center piece of contact wire is energized (by one supply or the other) and no action is required on the part of the locomotive's engineer. Should they become de-synchronised, however, automatic equipment senses this and disconnects the center piece of contact wire, leaving it electrically dead and thus making sure no electrical connection between the two power sources is possible.
The sudden loss of power would jar the locomotive, so when the phase break is 'dead', the engineer is informed by signals. When these signals are lit, the locomotive's controller (throttle) should be placed into a neutral, no power position before the locomotive hits the phase break and only opened up again once it is past. The pantograph does not need to be lowered.
Phase breaks are positioned in places where locomotives should always be running at high enough speeds to coast through the dead section with no problems. It's very rare for a locomotive to become stranded, since the dead section is very short.
Occasionally, a phase break is permanently 'dead' if the two sections on either side of it are energized by power supplies that are not intended to be synchronised.
On the Pennsylvania Railroad, phase breaks are identified by a metal sign hung in the catenary with the letters "PB" on it, created by holes being drilled in the metal. If the phase break center section is 'dead', that is shown by an illuminated signal consisting of eight lights arranged in a circular pattern, all of which are lit.
With help from Mark Bej's The Broad Way at http://broadway.pennsyrr.com/.