Polarity Danger

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beanland

beanland

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OK, I know what is to code and what is not and how to wire a receptacle, but, what are the specific adverse consequences of reversing the line and grounded conductor (not the ground) at a receptacle? The only reason for this to be an issue seems to me to be if you have an old metal tool what has the "neutral" attached to the metal case. Then a reversal is fatal. But what about for modern double-insulated tools? What about a computer with a switch-mode power supply? What about a TV? How about a lamp? If I have a 3-wire cord and the hot and grounded conductor (not ground) are reversed, where is the danger? I still believe there is a "correct" way to wire it but what happens if it isn't done right? (Other than the inspector will write it up.)
 
Screw Shell Danger

Screw Shell Danger

Very good point! However, I have seen lots of lamps with non-polarized cord plugs. And even polarized ones can be plugged in backwards to some cheap extension cords. So far, 1 excellent reason. Any more out there?
 
beanland said:
Very good point! However, I have seen lots of lamps with non-polarized cord plugs. And even polarized ones can be plugged in backwards to some cheap extension cords. So far, 1 excellent reason. Any more out there?
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There are some appliances that reversing the 2 might cause shock problems or even equipment damage.Simply wire them correctly and test with a plug checker.If that cord is polarized it is for a reason.
 
With the circuit wired with correct, normal polarity, and with some "plug and cord" item in your hand, and with the switch of that item turned to "off," there will be no power available internal to the item, or at least no deeper into the item than the location of the switch.

With the circuit polarity reversed, everything internal to the item up to, and stopping at, the switch, will be energized.

This would make very little difference if nobody decides to open the item up while it is still plugged in. I suspect that the polarization of receptacles and cords came into being as a method of protecting idiots who open stuff up without first pulling out the plug.
 
If the wiring is reversed on a lamp the power will be on the side edge of the socket instead of the small tab down inside the socket on the bottom. this could bite you when changing a lamp. Also like CharleB said it would be live when the switch is off.
 
Beanland,
Many modern pieces of consumer electronics such as TVs, VCRs, etc. have what are known as "hot chassis" in which there is a low resistance between the nuetral side of the line and the chassis "ground". These units typically use a polarized plug with no ground. The chassis ground is basically tied to nuetral and this "ground" is exposed at external connectors such as coax, line in and out, etc. As far as why this is done is a rather deep subject, but it basically allows for a much simpler design and lower cost to the consumer. Regardless of whether or not we consider this a safe practice it is common practice in todays world. If the polarity were reversed, hazardous voltages may be exposed to the consumer and if connected to another piece of equipment with the correct polarity, one or both units will more than likely spill the "magic smoke". The bottom line is: right or wrong, many modern pieces are counting on the wide blade to be nuetral and the narrow to be "hot" and it's our job to wire the receptacles correctly.

Hope this was helpfull, Altis;)
 
Altis said:
Many modern pieces of consumer electronics such as TVs, VCRs, etc. have what are known as "hot chassis" in which there is a low resistance between the nuetral side of the line and the chassis "ground".
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I may be wrong, but I don't think that anybody's made a "hot chassis" device in 25+ years (since "double-insulated" started to hit the market?). I recall that even in my high-school electronics shop we didn't see that many of them, and that was over 30 years ago.
 
zbang said:
I may be wrong, but I don't think that anybody's made a "hot chassis" device in 25+ years (since "double-insulated" started to hit the market?). I recall that even in my high-school electronics shop we didn't see that many of them, and that was over 30 years ago.
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zbang,
There may be some truth to your claim and I may be using the term "hot chassis" in error, however, there still seems to be a measure of comonality between the nuetral and the chassis ground on many modern units. It may be capacitive rather than resistive, but I just now hooked my TV up in reverse polarity with everything else disconnected, (it's about 3 years old). With the TV on, I measured 150 volts from my receptacle ground to the exposed connectors on the TV using an analog meter set to the 250 VAC scale. Some or all of this may be a product of the switching supply rather than the line as it doesn't match up with the 120 volt line, but there was definately a potential there. I wasn't brave enough to test it for current potential :rolleyes: as I don't want to smoke my TV.
I too do not recall seeing this while I was studying electronics 26 years ago, but I did consumer electronic repairs for a living for 14 years. I've been out of the consumer side for about ten years, but I know it was still important then to keep the nuetral side nuetral on many units. Incidentally, I did the same test on my TV with the correct polarity and measured about 10 volts AC, go figure! :-?
There still seems to be a reason manufacturers are using polarized plugs.
 
Altis said:
...Many modern pieces of consumer electronics such as TVs, VCRs, etc. have what are known as "hot chassis" in which there is a low resistance between the nuetral side of the line and the chassis "ground"....
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I'm not saying this isn't so, but wouldn't this "hot chassis" trip every gfci or afci?

carl
 
coulter said:
I'm not saying this isn't so, but wouldn't this "hot chassis" trip every gfci or afci?

carl
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You have a very valid point there! I'll have to check into that, but I would certainly think it would indeed cause a ground fault device to trip if it had any other path to ground, such as the shield of a coax. :confused:
 
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