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.

 

Update: Adding an image to help visualise my explanation. I used a toaster so I could “draw” the electrical internals (a heating element in this case), but you can pretend it’s a computer or anything else with a metal case if you prefer.

(click for a larger version)

3 prong outlet jumpered neutral to ground example

Since my drawing skills aren’t exactly going to win any awards, to make it easier to follow start by identifing the:

  1. TOASTER, OUTLET (plug), and FUSE PANEL.
  2. WIRES

…note that the jumper wire you hopefully didn’t add is shown at the bottom-right of the outlet (plug).

If you remove the jumper wire, the flow of power stops here instead of being routed to the toasters case via the toasters ground wire. The toaster doesn’t work, but touching the case doesn’t zap you… unless of course you’re really unlucky and the toaster has an internal short to the case that has gone unnoticed up till now… at which point hopefully you have a working GFCI.