Today I picked up a Trav-ler FB73 portable (AC or 6V/90V) for 10 bucks. It's older than I thought it was. Supposedly it was made about 1940, but it has a sort of primitive safety interlock, a spring-loaded switch, so that it shuts down the power if you take the back off. The mechanism could be easily disabled, and it was. But it made me wonder when they started engineering these into radios. I've always associated safety interlocks with the '60s and the '70s.

This one also has a UL "reexamination" sticker on it. When did Underwriters Labs start certifying radios? Didn't think they did this so long ago.

Bob

The Philco 54 series from 1933-35 were AC/DC sets with an sort-of interlock switch ("47" on the schematic). The switch is a momentary pushbutton that is closed when the metal back cover is mounted in place.

http://www.nostalgiaair.org/PagesByModel/788/M0013788.pdf

A question to ask now would be... when was the "modern" interlock design implemented or patented?

Generally, I would say radios started obtaining safety features in the mid 30s. This is also the time when product marketing/engineering developments like the "safety fan" were introduced.


(The Samson SafeFlex)

The 1937 FADA 350 was an ac/dc set with the drop in cabinet. The bottom metal plate was insulated from the chassis.

My mid-1930s Monarch midget has a "floating" ground with an isolated chassis, so that idea is nothing new. On the other hand, many AC-DC radios such as the Fada 1000 and the Setchell-Carlson 416 were still using the hot-chassis design as late as 1946-47; maybe the UL gave everyone a grace period before they all had to comply.

When the first company VP of Marketing got knocked on his butt

Carl

Zenith was still building tube sets with one side of the line connected directly to the chassis in the 1960's. But by then the UL had them using captive knobs, plastic control shafts, plastic blocks for the mounting screws to thread into, and substantial back covers with interlocks so there was no chance of anyone easily contacting the metal chassis.

Safety features appeared to have started in the mid 30's but not all manufacturers adopted them right away.

I guess we went from hot chassis to isolated chassis to safety interlocks to wall warts or batteries as we went from great radios to the cheap plastic throwaways you get today. Great for safety, lousy for radios.

Were there really documented cases of people dying from touching a hot radio chassis? Seems like there would have been lawsuits even then, and even with the attitude that it was your own fault.

The 1950s and 1960s transformerless AM-FM radios did have the "hot chassis" design; evidently, the engineers couldn't get the floating-ground concept to work at VHF.

That's the main reason for the shared attitude of "society's getting stupid and careless".
They really aren't out there.

Back to fans again... .... most fans didn't have restrictive guards by default until the 1970's, a fair century since innovative people got the idea to put a "propeller" on an electric motor.

Most fans, you can fatally injure yourself at a moment's notice. Heavy metal blades spinning at over 1000RPM... not much safer than a giant, dull dado blade.

I believe that that is why many of the early electric fans had exposed power terminals on the back of the motor. As a safety feature. This way after one lost one's fingers in the spinning blade one could cauterize the would on the energized binding posts.
This feature doubtless saved many a carpet.