Common
industry term for those large power
transformers on
utility poles. They are so named because they reside on poles and because they are large and extremely heavy (over 200 pounds).
Pole pigs reduce the high distribution voltage (usually over 10,000 volts) to the standard 120/240V, or for commercial use, sometimes 240/480V, to be distributed directly to utility customers. Three-phase power is commonly supplied with multiple pole pigs conntected in a delta or WYE configuration. They generally start at 10KVA (Kilo-Volt Amps) and go up from there. In the most common design, the high voltage primary terminals are on the top of the unit, with large insulators to shield them from the pole pig's grounded chassis, and the low voltage secondary terminals are on the side of the unit. They are usually filled with oil, which provides both insulation and cooling to the actual transformer inside.
Pole pigs sometimes fail in spectacular ways when faults occur. An energized pole pig has a lot of potential energy stored as a result of its inductance; fault current can heat up the secondary windings so fast that they melt and short out completely. The result is a powerful explosion and a bright blue flash. You can sometimes see these flashes in the distance during a really nasty hurricane as the wind blows power line conductors together.
Some people take surplus pole pigs and use them as step-up transformers; they make great voltage sources for tesla coils and Jacob's ladders. It is inadvisable to attempt this unless you know what you're doing, though; you don't want to be anywhere near an exploding pole pig. Not only that, but pole pigs are completely unregulated; unless you have a current limiting device, a pole pig will likely instantly trip the branch circuit you're feeding it from the moment you try to do anything with its high voltage primary "outputs".