Why is domestic light switch rated for AC only?

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> the new Leviton switches are ... AC only."

The old ones were too. Peel back the dust and look.

100 years ago, and into the 1940s, DC lights were a thing. These switches CHUNKed to break a DC arc. But since DC faded from walls, and an AC-only switch is cheaper (and quieter), and price is king, that's all you get today.

As said, you "can" switch DC with an "AC only" switch. Below 25V it may work fine. It may switch _1_ Amp @ 120V with only modest reduction of life. (However it may fail welded-ON, while we normally prefer a switch fails off.)

Same on power tools. A universal motor will eat AC or DC. But the tools are all specced for AC-only. It's the switch. In the past a few toolmakers were honest about it, saying you could run DC (such as a souped-up car generator) but the switch would not last. The High-Service toolmakers will warranty-replace a switch without asking what you were feeding it.
Agree with the above answers.

But one other factor, just because something is not rated for DC does not mean it won't "work" on DC. it is intended for AC use on your wall. It is tested for safety and function at AC, and has all the required safety certifications on AC. They didn't go through all the additional testing to get DC ratings as well because that is not a potential market for their product.
Wondered why light switches in the 50's made a big snap, and starting the late 60's, not. Manhatten had a 115 VDC system downtown, which required fast parting of the contacts to kill the arc. That ended sometime after 1960. AC arc self quenches.

Because quiet light switches were a selling point, and seemed more modern than the older noisy ones. There were even mercury switches available for some time to take quiet to the extreme.
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He lives in the U.S. (California) - actually this was a great question as many people are unaware in general of the issues switching DC.

Stockholm had areas with DC power until the late 1960s, and in the U.S., NYC, Boston and a number of other places had some DC power even later.

NYC used 25Hz rotary converters to power the subway system right up to the turn of the millennium.

War of the currents - Wikipedia

Interesting read, but see the section on remnant and existent DC systems.


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...Where's this bloke from, really???

Says California, and also specifically "120VAC".

I think he is just curious why wall-switches do not have DC ratings. He does not say he has a need to switch DC power.

I hardly believe this is genuine, because Sweden is on a 230Vac grid already long ago....

Sweden surely started with DC. It is fairly wealthy and often dark, so would have lights before development of AC systems. Just as your cars flopped sides, so did your power. Mostly long before any of us were born.

A lot of customers and utilities transitioned to AC, of course. But DC persisted a long long time. AC motors on utility power are essentially fixed-speed; DC motor speed is easy to control. DC motors are(were) better for printing presses, electric railroads, and elevators.

Some cities continued to use DC well into the 20th century. For example, central Helsinki had a DC network until the late 1940s, and Stockholm lost its dwindling DC network as late as the 1970s. ...Parts of Boston, Massachusetts, ...used 110 volts DC in the 1960s, causing the destruction of many small appliances ...used by Boston University students.... New York City's electric utility company, Consolidated Edison, continued to supply direct current to customers who had adopted it early in the twentieth century, mainly for elevators.
In January 1998, Consolidated Edison started to eliminate DC service. At that time there were 4,600 DC customers. By 2006, there were only 60 customers using DC service, and on November 14, 2007, the last direct-current distribution by Con Edison was shut down. Customers still using DC were provided with on-site AC to DC rectifiers. Pacific Gas and Electric Company still provides DC power to some locations in San Francisco, primarily for elevators,...
The Central Electricity Generating Board in the UK maintained a 200 volt DC generating station at Bankside Power Station in London until 1981. It exclusively powered DC printing machinery in Fleet Street,....
War of the currents - Wikipedia

Remnants of DC power distribution kept performing their assigned tasks for decades as the AC grid thickened around them.
One of the best examples is in San Francisco, where 250-volt DC power still flows through underground and overhead cables across the city. ...DC’s perseverance in that neighborhood seems fitting, for it was just a few blocks away that the tiny California Electric Light Co. ..became the first power company in the United States, and possibly the world, to supply electricity to multiple customers from a central generating station. ...a full three years before Edison turned on his famous Pearl Street {DC} generating station in New York City...
You’ll find no reference to DC power distribution in PG&E’s annual reports or on its websites. Even some utility engineers are unaware of its existence...
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 .... But .... elevators were a problem.
DC-driven winding-drum elevators—the leading design until the 1930s—.... 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.
....new installation of winding-drum elevators was banned in the 1940s and...
Existing DC winding-drum elevators, however, have stubbornly resisted exile to the scrap heap, ... 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....
San Francisco’s Secret DC Grid
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Three and four rail trains systems use DC power as skin depth is too small for AC power in a steel rail. The arcing can be spectacular
Yes indeed.
At level crossings the powered bogies draw an 8 foot arc as they cross the road where they can't have a power rail for obvious reasons especially if they are also pulling out of a station. There are a number of crossings like this in the south of the UK.

I am pretty sure that 3 rail trains are equipped for 4 rails so that they can pick up power from the other rail position at points and junctions where they wold otherwise get stuck.
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