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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.

 

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.

10 Comments

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  1. 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

  2. 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.

    • Deep thought! Thanks for contribtuing.

  3. 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.

  4. Internetgent

    This whole thread was exactly what I needed to know. Thank you very much for getting on the internet

  5. The ground and neut have a bridge in the breaker box, so what’s the concern

    • I just added an image to help show the issue.

      However, the main differences between joining-in-panel vs joining-at-outlet are that:

      • A lot of extra points of failure doing it after all the household (wall) wiring at an outlet.
      • The bonding in panels doesn’t just bond your household outlet end… it also bonds the incoming neutral (from carrier) and ground rod. The panel has effectively has 2 really good paths to ground. The “equivalent” at an outlet would be bonding neutral and ground in a 3-wire incoming household feed (L+N+G). Though you’re not allowed to do it, and it wouldn’t really make sense anyway.
      • Panel failures are much less likely than any household wiring after the panel. Panel failures are also much more likely to be really catastrophic. The chance that a panel failure results in a shock when touching a toaster is quite a bit lower than the chance a panel failure results in a big spark show as the opening event to burning a house down.

      Really, jumping live to neutral to convert a 2-prong outlet to 3-prong is just not at all a good idea.

      If wanting a working ground for safety reasons, you’re better off using a GFCI (if you don’t have a real ground wire, just don’t connect anything to the ground screw – only connect live + neutral). The GFCI will generally trip (and save you from shocks) if the device shorts to the case internally, falls in a sink full of water, etc. Using a GFCI outlet in this fashion (to upgrade a 2-prong outlet) is allowed by the electrical code in a number of areas.

      If wanting a working ground for what I’ll call “electronics” reasons (electrical noise, power bar warning lights, surge protection, etc), really your only good option is to run some new 3-wire (L+N+G, often 14/2 romex in US/Canada for a 15A circuit). If that absolutely isn’t an option, you could feasibly see if the metal outlet box is grounded. Grounding to the box when you don’t know *how* the box is grounded isn’t exactly a great idea since whatever is grounding it might-not-be-capable-of-carrying-15A-to-ground and hotspots/fires become a risk if there’s a short in the device and the conductor isn’t up to the task. Examples:

      • Old BX cable (metal clad/shielded without a ground strip) used the spiral metal shielding as the ground. But the metal shielding has higher resistance (it’s not a good conductor like copper or aluminum), and thus can glow red if it’s actually asked to pass a lot of current, especially in rusty sections, which could start a fire. It’s no longer allowed in new installations by most electrical code. Good news is that it probably wouldn’t have trouble handling the current of typical electrical noise. Bad news is the resistance is so high it might not be great at reducing electrical noise much anyway.
      • Properly gauged, properly grounded copper wire has sometimes been fished through walls/ceilings to each box to retrofit existing 2 wire. This is probably the best case scenario since it’s a good conductor.
      • A small run of metal clad cable that isn’t grounded might be rubbing up against something that actually *is* grounded (water pipe etc). Bad scenario because this is not a solid, reliable ground.
      • A small run of un-grounded metal clad cable might be joining 2 or more outlets. One of the things plugged into that outlet might have it’s case grounded (metal case on concrete basement floor for example) and is proving the “ground”. This is quite a bad scenerio.

      Even the “intended” grounds in the list may not be code-approved in your area. And it’s not always easy to figure out what’s actually grounding your box. But take it for what it’s worth.

  6. it’s all an safety concern. it’s to eliminate the possibility of it happening. if their is just a chance of it happening aka [actcident] its better to fault on the safe side. then take the chance.not saying it would happen. but the potential for it happening is their. that’ s why. safety first. products are in our house from all over the country. god knows how they are wired. or if inspected. to late after it happens to say op ps. and the danger of a person doing his own wiring. and then selling the house. and someone else buys and don’t know what he did. makes it dangerous. go the extra mile to be safe. its time well spent. Bill

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