Prongs on a plug?

Discussion in 'General Electronics Chat' started by speedster239, Jan 24, 2010.

  1. speedster239

    Thread Starter New Member

    Nov 24, 2009
    This might seem simple to somebody else, but it is a serious question. I thought AC didn't have a negative and a positive because the direction of current flow simply switched back and forth 60 times a second. However, when using a plug, one of the prongs is always bigger than the other, thus allowing the connection in only one way. Is this done on purpose in order for respective wires to line up? What is the purpose of this if there is no difference between each wire since the direction of current flow is infinitely changing?

    I'd be really grateful if somebody could help me out with this question.

    Thank you,
  2. radiohead

    Distinguished Member

    May 28, 2009
    In Alternating Current, the black wire is hot, the white wire is neutral and green (or bare wire) is ground. The reason (ungrounded) plugs, such as a toaster, have one prong larger than the other is to ensure the hot wire is tapped correctly. The smaller prong is hot and the larger one is neutral.

    And yes, current is changing direction (swinging from positive to negative) 60 times per second (a sine wave), and it cannot do that on only one wire, it needs the neutral line to complete the circuit.

    If you have a multimeter, put the setting on AC Volts. Put the negative lead in the ground socket (the (D) shaped hole) and measure the voltage in the small slot then the big slot and you will see that the big slot has zero volts AC and the small slot has line voltage (110-120 VAC).
  3. RobbJohnson

    Well-Known Member

    Dec 15, 2009
    120v house power has 3 wires

    white= nutual
    black= hot
    green= ground

    white and green are tied togather at the breaker box.
    Some cords have only two prongs so this is
    the reason for the two sizes of prongs hot connected to hot.
  4. 3ldon

    Active Member

    Jan 9, 2010
    there is a lot of miss information about polarized plugs.

    basically if you have a lamp, the switch is on the hot side, so when you turn it off you can't shock yourself.
    Or if the toaster is "off" then sticking a knife in it won't shock you.

    No you can't wire up the neutral to the chassie, because there is a chance that the outlet was wired backwards.
    Also, some equipment like small transformers, the "neutral" will be wired up on the inside coil closest to the core, to minimize capacitive coupling between the "hot" and the low voltage output.

    If filling off the larger plug creates a safety hazard, then it should have been grounded with a proper 3 prong plug in the first place, or the manufacturer should have used a DPST switch to turn it off. example being the toaster and an outlet wired backwards.
  5. jpanhalt


    Jan 18, 2008
    You shouldn't do that even if the outlet is correctly wired. It is
    against building code and dangerous. Please get a copy of the current code book (about $10 or less) at any building center.

    Filing off the large plug so you can insert it reversed is a very dangerous suggestion, particularly if someone has connected the neutral to chassis. Consider what would happen if both both legs of the 220V supply are wired to different devices in the room. Or, what if there is steam heat and someone touches the radiator while touching the device . AGAIN, get a code book and follow it. Anything that is outside of code is potentially dangerous. Cross wiring (black to neutral) the American system is particularly so.

  6. hgmjr

    Retired Moderator

    Jan 28, 2005
    Like jpanhalt, I understand that the connection of AC power needs to be consistent throughout the resident so that a potential difference does not exist between any two devices. If an difference occurs then there is a shock hazard if you touch the two miswired devices at the same time.

  7. Wendy


    Mar 24, 2008
    When I was a kid working on some AC wiring for a light in a chicken coop I accidentally left the power on. When the hot wire touched the metal tin on the coop is sparked, while the neutral didn't. I didn't understand it then, but it is why AC is polarized, there is a difference between the wires.

    And before you fuss about my unsafe behavior, this was around 40 years ago.
    Last edited: Jan 24, 2010
  8. VoodooMojo

    Active Member

    Nov 28, 2009
    interesting way to get the chickens to lay more eggs?
  9. Wendy


    Mar 24, 2008
    This was during daylight hours, they were eating their share of bugs. When I realized the power was on I took care of it.

    This was a pretty large coop. I remember getting in a shooting contest with a skunk, I was aiming a 22 rifle through the chicken wire window.

    We both lost. I suspect the chickens weren't too happy either.