Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.
Reversed polarity has no effect on equipment. The “polarity” flips every 8.3 ms anyway. The issue is which conductor is grounded. This can make the shell of lamp holders hot exposing the user to added danger on lamp changes. It also affects which conductor the switch opens, making an “off” appliance still “hot”. Back in the olden days there were also things with the chassis connected directly to one side of the line but these won’t have a polarized plug anyway so it is somewhat moot. That is why they are called “hot chassis”. 50% of the time they might be hot, depending on how you plug them in … feel lucky?
Originally Posted By: rbennett This post was automatically imported from our archived forum.
Charles
You have had two answers one sort of yes and one sort of no
Reversed polarity on AC is confusing since AC does not have a constant polarity. If what we refer to as HOT and NEUTRAL are reversed equipment can be damaged.
The one that I like is - hot and neutral are reversed on a wall outlet. A powerstrip with an internal ckt breaker is plugged into said outlet. A low resistance develops in something connected to the powerstrip to some external ground. The ckt breaker in the powerstrip will not open since it is electrically in the neutral side of the ckt. The current might not be high enough to trip the main breaker. Result - additional damage to the equipment. While the above is a LARGE "what if" type of failure one can see that we can have equipment damage when hot and neutral are reversed.
Now for reversed polarity on DC equipment. Depending on the equipment maybe damage maybe not. Some motors will run backwards some will not.
Be it AC or DC proper wiring is required for many reasons. Safety is the big one. Switches are to disconnect the hot side of ckts. If they disconnect the neutral side, the equipment still has power that can kill.
When working on electrical equipment never assume that a switch will kill the power. Test in respect to ground all leads. Equipment and people can be damaged.
Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.
I am starting to feel pedantic but the failure of a “strip” breaker to open should have no effect on “equipment” plugged into it. That equipment was certified assuming it was plugged into a 20a breaker (in the US). That breaker is to protect the line cord of the strip.
Originally Posted By: rbennett This post was automatically imported from our archived forum.
Charles
When I do training in this area one of the first questions I get is "why do some DC motors reverse and some don't"
I understand that this is not an HI subject but here is the real simple answer
Motors turn because there are TWO magnetic fields that relate to each other (attracting or opposing is not important) if the DC is reversed it reverses both fields (if both fields are electro magnets) thus the relationship stays the same and thus the rotation stays the same
If one of the magnetic fields is a permanent magnet it will not reverse if the DC is change but the other field which is an electro magnet will change direction thus the relationship between the two fields changes thus the rotation of the motor is reversed.
Sort of easier to see if you can see a diagram
Three phase is sort of the same just explained a little different
Charles
I know that the above really did not pertain to your question but I just had to try to "show off a little" I had to have some fun - thank you for the question that allowed me to get on the stage.
RLB
Greg
The powerstrip's failure to trip is sort of BS in the example. (good that your caught it) The same problem would have existed for additional equipment damage without the strip. It is just an example to show that equipment damage is possible if the hot and the neutral are reversed. I do agree that this is a stretch but the safety issues are the real point.
A point not stated but probably will be ask - "what equipment damage would result with a reversal of hot and neutral?" To be real honest - I am looking a better example. Anyone got one??
Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.
I really can’t think of a legal situation where polarity reversal could affect equipment on an AC circuit. I do have a true strip story that could kill a baby.
True
Last weekend our 2 year old and the 1 year old conspired to get behind a laser printer and pull out the power cord. The first time anyone really figured it out the 1 year old had the IEC C-13 in her mouth.
The strip was off so no harm done but if the polarity was reversed it would have been ugly.
Originally Posted By: jmurphy1 This post was automatically imported from our archived forum.
I remodel old houses and try to make a profit reselling them. Light fixtures are so cheap I buy new fixtures and pretty much replace every light fixture in each house I remodel.
When I change a ceiling light fixture I go to the light switch, turn it off, remove the old fixture, and install the new one.
Nine times out of ten this procedure works just fine, however, I have gotten shocked from doing this. The first few times were in older houses.
About a year ago I had this happen in a home built in the late 1970's.
I believe the reason I get shocked is because the neutral wire (rahter than the hot wire) goes to the wall switch.
I guess I could go to the main breaker and shut everything down, but I do a lot of work at night and so that won't work.
The other alternative is to find the specific circuit for the light fixture and turn it off and I will probably do so in the future.
Most people probably don't shut down the breaker because they are in a hurry.
I remember a year or so ago I was changing a light fixture in the kitchen of the above mentioned house and I found a plastic electrical box in the ceiling. The existing light fixture apparently had put out a lot of heat. The two mounting screws in the plastic box were fried and just disentegrated as I removed the mounting screws with the old fixture.
I ended up cutting out the drywall around the box, removing the old plastic box, installing a new one, installing the new fixture etc. I was not a fan of plastic boxes after that.