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 visualize 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)
Since my drawing skills aren’t exactly going to win any awards, to make it easier to follow start by identifying the:
- TOASTER, OUTLET (plug), and FUSE PANEL.
- 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.
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
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.
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.
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.In any case, generally an electrician will do things according to local code. If uncertain, you may want to consider running it by another electrician in your area.
That said, I could easily be missing something, and it's very possible that things are done very differently in your country. Again if unsure I'd really consider running it by an electrician there.
Can I just change all the outlets to gfci and be done with it?
- GFCI + keeping the ground/neutral jumper: Adds the standard GFI shock protections (5-20mA), does not protect against the specific failure mode above (it occurs post-GFCI).
- GFCI + removing the ground/neutral jumper: Similar to a 2-wire to GFCI upgrade and is the "approved" method of converting a 2-prong outlet to 3-prong outlet in some places. Some GFCIs even come with "not grounded" stickers that can be placed on the wall plate. Provides GFCI shock protecion (5-20mA). Removing the jumper mitigates the specific failure mode above.
I should be clear that removing the neutral/ground jumper does present a trade-off. An unconnected ground has its own set of possible issues: some devices really like to be grounded. GFCIs also don't protect against painful high voltage shocks if they occur at low current - there's a current threshold that has to be met (roughly 5-20mA of current as I just mentioned). If you have 3 prong devices that actually *do* cause the case to become live at a low current but normally dump it to ground... remove the jumper wire and you might find you're getting some new shocks or fuzzy-feels when you touch the device whether or not a GFCI is involved (though the GFCI should trip before the current gets to typically lethal levels).As to the house burning down... not sure if there were other factors at play..? The jumped ground/neutral on its own shouldn't have any bearing on fire risk unless there is something wonky about it (undersized, not well fastened, resulted in too many wires for the box, other stuff that applies to all electrical installations, etc). The risk is shock if the neutral in the wall becomes disconnected and a person touches the case while they are grounded. It has nothing to do with fire.
This is an approved method per the NEC, isn't it? (We would be using the "not grounded" stickers, of course.) We just don't want the inspector coming back on us to replace all the GFCIs with 2-prong outlets. (I realize the ground isn't connected, but I feel it's safer than the cheater plugs dangling from the walls!)
However, local codes can vary, and from what I gather in some locations inspectors have been known to refuse certain things that they don't like despite meeting the code. So you're really best to talk to a local inspector or electrician.
What if the ground cable also breaks? Maybe we should add more ground cables to lower this close to zero risk?
This problem could easily be solved anyway by not grounding the neutral, and keep it hot.
Here the issue:
i don't have ground wire at all, and my PC case always have a current in it, if ever i was bare footed & touch the case i got shocked, we have 220V not 110V, also i recently bought a laser machine which needed to be grounded.
after some search online i came up with a solution, not permanent, but it did the job.
i made a jumper between the neutral & the ground at the wall outlet and it worked like a charm!
before i took that step i asked many electricians in my circle if i can do this, but i got surprised they don't know anything about that!
of course this way provides a solution for devices already have a leakage, and work as a temp solution for devices which needs a ground connection in situations like mine, no grounding infra structure at all.
Thanks again for the brainstorming :)
In regards to what happens if there's a disconnect in the outlet itself, in the 3 scenarios you presented: (1) item stops working (2) item not grounded (3) item not working and not grounded.
As for using the uninsulated wire as a neutral, not a good practise in my view as it the reason it gets to be bare because it's not expected to be energized - if someone goes this route (which I suspect will increase in likeliness due to the deteriorating economic climate in many countries putting people in positions where costly rewiring would come at the expense of feeding the family), hopefully both ends at least get taped/marked so that a future homeowner or electrician can tell at a glance that this circuit is non-standard.
I'm not trying to flex any arcane electrical circuits knowledge, I failed circuits 2 twice, and electricity is baffling, but please don't tell people that Neutral ~= Ground, because NEUTRAL == LIVE and this could be dangerous information.
Also in relation to the main discussion, if instead of using neutral/ground jumper, you just run a separate wire from outlet ground to neutral in the main panel, will that solve the problem of the jumper? Thanks
Ultimately, you're probably best to have a local electrician take a look at things.