Forty plus years ago I don’t recall any consumer electronic equipment that used an EGC, (Equipment Grounding Conductor). Late 1970s and earlier consumer electronics came with a two wire non polarized plug. (Though the home you were living in may have had polarized 3 prong grounding type wall outlets).
Go back to the late 1950s and earlier the wall outlet found in our homes was just a 2 wire non polarized wall outlet. NO EGC back then...
Back in those days for a person to receive and electrical shock from a piece of equipment with a hot chassis when touching the HOT chassis of the equipment you had to come into contact with a grounded object with another part of your body. What were the odds? What was close enough for some other part of your body to come into contact with a grounded object? Now if you were standing on a basement concrete floor in you bare feet you would definitely know the chassis was hot!. Or if you had a radio in the kitchen next to a sink and you were washing dishes and reached over and touched a metal screw reaching for the volume control on the radio, and the chassis was hot, you would find out instantly the chassis was hot. Would it kill you, probably not, but you might receive a wake up notice...
Radios and TVs built in the 1950s didn’t need an internal fault to make the metal chassis of the equipment HOT. There was a 50/50 chance of it being hot. It was the way it was wired. One AC mains conductor was connected to the chassis...
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Back then Radios and TVs metal chassis were enclosed in a non conductive enclosure, case, made of plastic or wood. Knobs were made of plastic or wood. Mounting screws were still made from steel or brass. So the screws could be hot.
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Returning back to the 1960s and especially the 1970s where separate pieces of equipment were being made. Like tuners, tape players, and such. They were connected to an amp or receiver using wire interconnects. The power cords were 2 wire with a 2 prong non polarize plug. You still had a 50/50 chance of plugging the piece(s) of equipment into the wall outlet so all the equipment AC plug polarity orientation would be the same. Good thing the equipment did not have one of the AC mains conductors connected directly to the chassis.
Sill no EGC to contend with. And the equipment was fed from the same wall outlet, same circuit. Things were still pretty electrically safe in the 1970s. Even with all the metal cases and metal face plates. A person still might want to avoid standing in you bare feet on a basement concrete floor. And of course never use an AC powered radio near a bath tub. That could prove fatal... No GFCI protection in the 1970s.
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And then some ya-hoo decided audio equipment needed to be grounded. (Though not the Japanese. The first 2 wire cord with a polarized plug on audio equipment I seen was made in Japan.)
So when audio equipment started being built having a 3 wire grounded plug is when things got interesting. And of course back then there were not IEC inlet connectors on equipment. Cords were captive, solidly, connected to the equipment.
Well for those of you that can remember the dam plug wouldn’t plug into the 2 wire wall outlet... That’s why the 3 wire to two wire adapter was in invented. Not to cheat the ground. Hey everything still worked the same. None of the audio equipment was grounded.
Problem?
It’s the mix of manufactured grounded equipment, where the 3 wire grounding plug is plugged into the wall outlet, and a piece of manufactured grounded equipment that has the EGC lifted from the wall outlet, that can be a problem. In the event the piece of equipment, with the lifted EGC, has a HOT to chassis fault. You no longer have an arms length safety factor.
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