San Francisco's Secret DC Grid:

Nikola Tesla's alternating current may have "won" the War of Currents at the end of the 19th Century, but the defeated incumbent -- direct-current distribution, aggressively championed by Thomas Edison -- endured. [...] Remnants of DC power distribution kept performing their assigned tasks for decades as the AC grid thickened around them.

In fact, a few live on to this day. One of the best examples is in San Francisco, where 250-volt DC power still flows through underground and overhead cables across the city. These DC lines peacefully coexist with their AC counterparts; you can see this mix of currents straddling utility poles in the city's South of Market district. DC's perseverance in that neighborhood seems fitting, for it was just a few blocks away that the tiny California Electric Light Co. -- a forebear to California's dominant Pacific Gas and Electric (PG&E) -- became the first power company in the United States, and possibly the world, to supply electricity to multiple customers from a central generating station. It was in September 1879 -- a full three years before Edison turned on his famous Pearl Street generating station in New York City -- that California Electric began burning coal, raising steam, and driving dynamos in a wooden shack at the corner of Fourth and Market streets to feed current to its customers' electric lights. [...]

DC endures in San Francisco because more than 900 of PG&E's customers still need it. Most of the utility's customers transitioned to AC lightbulbs and appliances easily enough as competing power distributors coalesced within PG&E and harmonized their equipment around AC. But for some of these building owners, however, elevators were a problem.

DC-driven winding-drum elevators -- the leading design until the 1930s -- use a DC motor in the basement that winds and unwinds the elevator's steel cable on a steel drum, thus lifting and lowering the car from pulleys atop the elevator shaft. DC drive was the only way to go at the time for a speedy elevator, because only DC could deliver variable-speed operation for smooth starts and stops. The DC motors were also energy efficient, capable of something that has only recently become possible with modern elevator designs: regenerating power when the elevator descends.

However, safety was a weak point. If a winding drum's control system fails, its motor can drive the elevator through the roof, according to San Francisco -- based elevator consultant Richard Blaska. As a result, says Blaska, new installation of winding-drum elevators was banned in the 1940s and 1950s in favor of traction elevators, whose cable will simply slip and hold the car at the top floor if the control system fails. Traction elevators can be engineered for either AC or DC operation.

Existing DC winding-drum elevators, however, have stubbornly resisted exile to the scrap heap, in no small part with support from local elevator repair firms such as Erik Bleyle's. Bleyle Elevator makes replacement parts, rebuilds DC motors, and designs custom circuits to sustain these machines from a bygone era. Bleyle admits that repairs can be pricey, especially hand-rewinding a DC motor, which can run between US $30 000 and $40 000. But he says even a refurbished motor looks cheap compared with the $500 000 cost of replacing the elevator, not to mention the months of involuntary stair climbing during the upgrade.

"Usually people just go for the motor," says Bleyle. [...]

The DC grid was also always difficult to troubleshoot because faults are hard to localize on a single large circuit -- a challenge that Austin says is compounded by the scant support this forgotten technology gets from equipment vendors. Austin adapted a circa-1990 AC/DC hammer drill to create his own diagnostic tool for so-called phantom voltage -- tiny dribbles of DC flowing across blown fuses that can hoodwink unsuspecting "troublemen" and their trusty voltmeters. Austin knows he's found a phantom when he clips his modified Black & Decker Macho III hammer drill onto a circuit, pulls its trigger, and gets a whimper instead of a roar. [...]

"When you had a failure out there like a fire in a manhole, the DC grid saw it as a load and just kept on pumping power at it," says Austin. The Tenderloin fire provided fuel for critics of PG&E's maintenance record and prompted the utility to accelerate and complete an ongoing redesign of its DC supply system. PG&E finished the job and shut down its two old rectifiers at the end of 2010.

I had no idea that this lunacy was all around us here! What the!

It reminds me of the fact that our fair city also has... unique... notions about how sewers should work. (After all, no discussion of electricity is complete without a plumbing analogy.) It's one of the few cities in the world that uses the same pipes for sewage and rain drainage. Doing it that way fell out of favor some time between the Romans and the London cholera die-offs in the 1800s.

San Francisco Combined Sewers:

San Francisco collects both sewage and stormwater in the same network of pipes, then treats and discharges the combined flows to San Francisco Bay or the Pacific Ocean. Except for portions of Old Sacramento, all other cities in California have separate sewer systems, which means there are two sets of pipes in the ground. One set of pipes takes sanitary waste to the treatment plant while a second set carries stormwater runoff from street drains directly into creeks, lakes, or the ocean. [...]

San Francisco's combined system holds these large volumes of water in underground storage vaults called transport/storage (T/S) structures, which encircle the city. San Francisco built the T/S structures in the 1980s and 1990s to prevent pollution of the bay and ocean during large storms. All combined flows pass into and through these structures on their way to the treatment plants. This upgrade greatly reduced the number of sewage overflows. The current system is designed such that overflows to the bay or ocean now occur on average one to ten times per year, depending on the rainfall and the watershed.

WHY A COMBINED SYSTEM? Many United States cities built prior to 1900 had combined sewer systems. At that time, sewage treatment was not available and sewers simply directed sewage into local water bodies. When sewage treatment became necessary to protect public health, newer cities built separate systems to save on the costs of treating stormwater. Some of the older cities opted to separate their combined systems. San Francisco, already a dense urban environment, decided that separation was too costly and disruptive to the residents.

Also: A Rare Look at the Tunnels Under San Francisco:

In the early '90s my friends and I used to tape flashlights to the handlebars of our bikes and go riding around in underground storm drain tunnels. There was a whole network of these tunnels under the city that sat empty for most of the year. We would go for miles snaking up and down the sides of the tubes, clapping and yelling to see how far our echoes would carry, eventually popping out in some other part of the city covered in cobwebs and bat guano. When the tubes got too small, we laid down on skateboards and kept going. If we found a flooded part, we taped garbage bags around our legs and crossed our fingers.

Previously, previously, previously, previously, previously, previously, previously, previously.