ECN Forum

Polarized plugs/recepts.

Posted By: pauluk

Polarized plugs/recepts. - 04/27/02 11:14 AM

Looking back at another thread on this got me thinking.

Can anyone remember exactly when the polarized plug and receptacle with wider neutral prong were introduced?
Posted By: 3rd degree Burns

Re: Polarized plugs/recepts. - 04/28/02 04:42 PM

I can remember in 1962 or 1963 when my parents bought me a Decca record player made of vinyl covered pressboard and cardboard. I also remember getting shocked about half the time. It makes sense now. Amazing that two prong products such as these were allowed on the market back then. Of course such a cheap product wouldn't employ an isolation xrfmr. How many of ya'll reading this post can relate to this?......Burns
Posted By: pauluk

Re: Polarized plugs/recepts. - 04/28/02 07:07 PM

I have two portable record players here that date from a similar period, one with a 2-ire and the other with a 3-wire cord (these are 200 to 250V units).

The ungrounded unit doesn't pose any shock risk, although by gently touching the metal record deck with the back of a finger it's just about possible to feel some minute capacitive leakage.

I've seen a couple of schematics for old U.S. equipment which actually tied the chassis to one side of the line via a 1 meg resistor. To the best of my knowledge, that was never done on British equipment, although some did couple with a low value capacitor, say 0.001 uF or less.
Posted By: NJwirenut

Re: Polarized plugs/recepts. - 04/28/02 07:14 PM

The "hot chassis" electronic products were SUPPOSED to have been constructed in such a way that no part of the energized chassis was accessible in normal operation. Pots with plastic shafts, insulated knobs, plastic hardware, insulated standoffs, etc. were all commonly used for this. Millions of 5-tube "AC/DC" radios and "hot chassis" TV sets were produced (and still are). No safety problem from the end-user perspective, but special precautions (isolation transformer) are required when troubleshooting and repairing them.

If you were getting a shock from such a unit, then something was seriously wrong. An incorrectly installed chassis mounting screw, missing insulator, or improper modifications to the unit could all cause this problem. None of the exposed metal parts (screws, handles, connectors, etc.) should be connected to the chassis ground.

Before the polarized plug was introduced, the instructions for a radio or TV often advised trying the plug in the outlet both ways, to minimise hum. Presumably, the hum would be less when the chassis was connected to the neutral side of the line.
Posted By: pauluk

Re: Polarized plugs/recepts. - 04/29/02 08:36 PM

I second that NJ.

These AC/DC radio sets with "live chassis" were also very common in the U.K. right up until the 1960s. As someone with a great interest in older equipment I still like to work on restoring these old sets (when I get the time that is!). For bench servicework, it's necessary to use a 1:1 isolation xfmr, not just for personal protection but also so that grounded test equipment can be used (e.g. signal generator, oscilloscope).
© 2020 ECN Electrical Forums