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Why can’t you connect neutral to ground to convert an outlet from 2-prong to 3-prong?

Not everything I put in the riddles, puzzles, and brain teasers section involves a king and some sort of puzzle that has to be solved with fancy logic. Sometimes it’s a real life question, and this is one of those times.

The question of course is if you have a 2-prong outlet (live + neutral) and want to convert it to a 3-prong (live + neutral + ground), why can’t you simply use a little jumper-wire on the new outlet and hook up ground to neutral? After all, they both go to ground eventually. This would mean that your surge protector that needs ground to operate correctly would work (surge protectors simply dump surges to ground before they hit your equipment). The problem is, by connecting neutral to ground in the outlet, there’s a situation that could happen that could kill you. No, it doesn’t have to do with the surge protector or power surges. Want to guess what it is?

Answer

If the neutral wire in the walls were ever to become disconnected/broken/etc, any 3-prong devices that were connected to the outlet (for example, a computer) would now have a live case. If you touched the case, you could be electrocuted.

You see, the metal case on 3-prong devices is always connected to the ground prong, mainly because if there were a problem in the device and the case somehow became live, that power would have a clean path to ground and would hopefully blow a circuit breaker before any harm could be done.

Since you connected the neutral prong directly to the ground prong, the case itself now has the potential to carry the return current. If the neutral connection were ever broken, it would, and if you touched the case (and were yourself grounded), your body would be completing the circuit. The device might even power on for as long as you were touching it. Unfortunately, all that current would be running through your body. For those of you who have seen the “call before you dig” commercials, you’ve probably got an idea of what would happen next… Never a good thing.

Note for those who haven’t seen the “call before you dig” commercials…

You die.

  • Tyler

    I’m a mechanical person without much electrical experience, so please correct me if my thinking is off. If you put a jumper between ground and neutral, and the neutral line somehow is disconnected, is that not the same as the neutral connection failing in a two prong outlet? Either way, you could be shocked? Or is there some extra protection within electronics with the two prong? I appreciate the help.

    • Tyler:

      The easiest way to explain this is probably to have you take a gander at the Wikipedia entry on Appliance Classes http://en.wikipedia.org/wiki/Appliance_classes .

      Basically, 2-prong devices are supposed to be designed in such a way that a single failure inside the case won’t bring dangerous voltage to the conductive casing. If the neutral becomes disconnected, the device just plain shouldn’t work. You shouldn’t receive a shock by touching the case because the case should be well-insulated from any high-voltage internals. Or… they might have just used a plastic casing which won’t conduct electricity anyway. Either way, the case should always be “dead” in a 2-prong device unless you have multiple failures going on within.

  • sky jumper

    I guess the problem with this explanation (not just yours, but the electrical code as well) is that it requires BOTH a fault in the device that shorts the chassis to AC “hot” AND an open ground/neutral feed for the danger to exist. To me this seems very unlikely. Further, you’d have to assume that whatever caused the neutral to open circuit would not also cause the ground to open – since the wires are all run together I think it’s likely that any disruptive event that would open the neutral (e.g. a contractor’s recip saw) would also cut through the ground and hot wires. I’ll admit that I’m not an electrical inspector or fire investigator that has seen many cases of improper grounds causing problems — but the logic behind it still escapes me.

    • sky jumper: I’m about 9 months late on the response here (sorry).

      But no, it doesn’t require a fault that shorts the chassis to hot – just the open neutral. A picture would probably have been the easiest way of showing this, but if the ground prong is wired to the neutral prong via a jumper and you get a broken neutral, essentially this is the route the power takes:
      -“hot” wire
      -“hot” prong
      -through device internals
      -neutral prong
      -“jumper wire” you put in
      -ground prong
      -device-case

      The moment you touch the device-case, your body will complete the circuit if you’re grounded.

    • Jalen

      Deep thought! Thanks for contribtuing.

  • Steve Gergetz

    Matthew,

    Thanks for a concise and very clear explanation. I’ve heard it before but couldn’t remember the explanation. Your version of it was stated so simply I’m sure it will stick in my head this time.

    Steve