Please help an clueless dad who has been asked yet another stumper from his curious 8 year old.
We live in a house built in the 20’s that has undergone some partial remodeling. As a result some of the outlets are of the “2-prong” variety while others are “3-prongers”. I know the third prong is for grounding so is there some safety advantage to these outlets? If I use an 3-to-2-prong adapter am I circumventing some safety feature?
Not really. The 3-prong thingies are just an offshoot of the “safety first” mindset of the latter half of the paranoid 20th century.
IANA electrician, but if you’re bad, then so am I. I’ve got a 1920s house, too, and, “No, Mr. Electrician, I am not going to pay you megabucks to come rewire my entire house with nifty modern 3-prong outlets.”
To look at my house, you’d think I owned stock in Acme Adapters, Inc. 
The grounding is the safety feature, so yes you are circumventing it with the adaptor. The grounding gives the current a better route than running thru your body. Water seeks its lowest level and electricity seeks its easiest route.
The two to three pronged adapter actually has a metal loop in place of a prong for the ground. You are supposed to ground that with a wire to a ground point.
Yes, electric current is more dangerous when YOU are the ground, instead of the third prong.
Kniz is correct.
Three prongs are better…but you might never find yourself in a situation to find out. That’s called luck.
And ground fault interrupters are better to, but you might never find yourself in a situation to find out.
Capish?
Yes, we all grew up in houses without grounded circuits and lived, so we shouldn’t need them now. It’s overkill, paranoia …
Sorry, but that argument doesn’t wash.
The electical demand of the modern home now bears no resemblance to that of a pre-1970’s home. (This is approximately when grounding all circuits in a dwelling became the norm.)
Consider the additional load on the electrical system with devices like:
microwaves
hair dryers
stereos
multiple TVs
hair dryers
convection ovens
security & alarm systems
garage door openers
yard lighting
swimming pools
whirlpool tubs
This stuff either was not in general use then, or existed as humble versions of the modern ones. In total they present more opportunities for failure and danger.
Additionally, some devices require a ground to work properly. The computer you’re using to access this board is presumably plugged into a surge protector. Without the surge protector being grounded, you have no surge protection at all.
(Apologies if this sounds too “parental”, but I stand by my points.)
Related question,
If you suffer some sort of injury (life, limb, or property) due to the improper use of a 2-3 prong adapter, would your insurance company still pay your claim?
Duck Duck Goose, I’d like to see some cites that the Electric Code is a product of feel-good policies rather than a genuine effort to save life and property.
My personal policy is to replace 2 prong plugs with 3 prong GFCI plugs, appropriately labeled with “No Equipment Ground,” and not worry about electric shock. To nitpickers, I install one GFCI as close to the breaker as possible and label the down circuit plugs “GFCI Protected.” I also write the breaker number on the lower plug, but that’s me.
Some equipment, such as monitors and phonographs benefit from having a good equipment ground.
It’s really is a question of safety.
You’re usually on the ball with most stuff so I’m surprised by your post. 3 prong outlets are not a fussy affectation. They have real safety benefits. Take it from personal experience - You only need to get shocked badly once to made them worth while.
The Danger of Electrical Shock
http://www.tva.gov/power/homesafety.htm
“Three-Prong Plugs. You may have noticed that the electrical outlets in most homes now have three-wire receptacles that accommodate electrical cords with three-prong plugs. The third prong provides a path to ground along which the electric current travels. Most major appliances, such as stoves, refrigerators, and computers, have three-prong plugs, meaning they are grounded. Most older homes do not have three-wire receptacles. If yours does not, you should have an electrician rewire the home to accommodate the three-prong plugs. Although three-prong adapters can be purchased, they are not recommended for permanent use. Also remember never to clip the third prong off a plug to make it fit a two-hole outlet”.
If you will not convert to three prong outlets at least make sure the adapter is gounded properly.
http://www.geocities.com/PicketFence/3124/articles/esafety.html
“I know the screw in the center of a two hole outlet is often connected to ground, but many times an adapter is used to convert a three prong plug to a two prong plug. I can count on one hand the number of times I’ve seen a three prong to two prong adapter that had the grounding lug properly attached to the ground screw on the outlet. You just know you’re not going to bother to make sure that appliance is grounded, just like most people don’t.”
Well, here’s the deal. If all your electrical equipment is functioning properly, then the ground isn’t really necessary (any more than the seat belts of your car are necessary if you’re not currently crashing into something).
Now, consider the case of a damaged or malfunctioning appliance where, for some reason, the “hot” wire touches the case or some metal part of the appliance. In a system with a ground connected to the case, the current will flow harmlessly to the ground. The worst that can happen is you set off a circuit breaker. In a non-grounded system, the case is now “hot” and will happily ground through you next time you touch the appliance.
Note that the ground wire isn’t going to help you if you actually touch the “hot” wire or drop a hairdryer in the sink. For this, you need a GFCI breaker that detects if there is unequal current through the hot wire and the grounding wire.
A GFCI breaker will protect you if you only have a two-wire circuit with no ground.
If I may add to astro’s post, Just because your outlet is installed in an outlet box, that does not guarantee that the outlet box is grounded. Unless your house is wired with 3-wire grounded wiring, metal conduit, or BX, your two-conductor wiring will not be grounded, and screwing the 2-3 into the center nut will do nothing.
Really, if your appliance has three prongs, that third one should be grounded.
You can purchase a simple plug-in device that divines how things are wired.
For kicks, you guys might want to take a multimeter to your outlets who are harping on the three prongs.
Measure the voltage between each receptacle. When you find the two receptacles that have no potential (the “neutral” and “earth ground”) go ahead and measure the resistance between them.
I expect a full report.
astro, Finagle and NotMrKnowItAll are correct.
Here’s Cecil’s take on a related question:
I can’t plug in these new plugs! What’s the deal?
The polarized plug is better than the old, unpolarized plug, in that it keeps the hot and neutral wires from getting swapped. The 3-prong receptacle is even better.
[Duck Duck Goose**, it might be worth it to have an electrician check out your house. He wouldn’t have to rewire it, but it would be much safer if he replaced any unpolarized receptacles with 2-prong polarized receptacles, and made sure hot and neutral were on the correct blades. It really can be dangerous if it’s not correct. You can even buy a cheapo outlet tester at a hardware store and just plug it in and check.
Arjuna34
May I refer all to the great web site http://www.codecheck.com . They have very nice descriptions and refers to relavent code entries. Here’s a link to conversion of two prong to three two prong to three prong
Oh, and erislover, here’s a link that talks about how the ground and common are bonded together at the breaker box. As it goes on to say, the two leads serve different purposes: The common provides a path for current back to the transformer common tap, the ground provided a low impedance faulty current path back to the transformer common tap.
http://www.codecheck.com/grounding.htm
So to answer, there would be a very low resistance between the two, but I sure don’t always trust the white wire to be neutral.
For the the record, I’m the GM of an electrical contracting company. I’d just like to echo the good advice of the prior postings from NotMrKnowItAll, finagle, & astro.
Ground prongs are normally unnecessary on a double-insulated device (something with a plastic case) because a fault cannot be transmitted through the case.
The ground connection is there to provide a safe return path for fault current. This allows circuit breakers to trip properly and reduces the likelihood of your body being a lower impedance path for the circuit.
Polarization is a very important safety issue. One common household danger of polarity reversal is electrical shock from lightbulbs - the neutral wire is at zero volts potential to earth ground and is connected to the screw shell. Polarity reversal puts your finger in contact with the “hot” wire while you’re changing the bulb.
NotMrKnowItAll is correct: If there is no conductive return path to the transformer, an adapter is potentially dangerous.
finagle makes a good point with ground fault circuit interruptors(GFCI’s) - these devices are designed to open the circuit if an imbalance(fault) is detected. They respond quickly and at very low current levels and are definitely much safer. Be sure to test them often. Ask an electrician in your area about replacement of your existing 2-wire receptacles with GFCI receps.
just FYI - the National Electrical Code is copyrighted material and is unavailable online, AFAIK.
Earth ground is not useless, I hope I didn’t imply that. They are useful when the electrical device is made to accept them.
They are commonly used to ground the frame which otherwise serves no operational purpose.
Sometimes the frame is grounded for operational purposes, in which case the neutral and the earth ground are likely tied together.
Not having a ground is not inherently dangerous, but if you have grounded plugs you may want to check how the earth ground is used, if it is used at all, in order to better understand why they became part of the building code in the first place.
Polarized plugs with earth ground are a good idea. I don’t know how prohibative the cost is, however. I would likely take a multimeter to all my plugs and mark which side was hot with reference to an earth ground I could locate (say, a cold water pipe under the sink). This would account for polarity, but if a house isn’t up to code with respect to earth-grounded outlets, then the circuit breakers aren’t made to accept them anyway. Right?
A question for electricians (never did any code-specific elecrician-style work… they did the outlets for me ;)): do old wirings even have a ground wire there? It would seem the boxes themselves which hold the outlet in place would be grounded anyway… aren’t they? I honestly don’t know how older wiring was done. If that is the case, anyone who knew how to use a screwdriver and self-tapping screws could buy the correct plates and modify their house (with caution regarding live wires, of course).
But are there grounded boxes at the outlets?
As a litigator who has dealt with a number of electric shock cases might I recommend three prong GFI’s, I installed mine myself.
The assumption that you won’t die from normal household electric shocks is usually correct. (Haven’t seen a hair dryer in bathtub death yet). But I’ve seen a couple of falls from ladders injuring other parts of body severely due to unexpected shocks.
For the OP, while much has been said of grounding, the thing that strikes me the most as an electrical engineer is the fact that much of the house wiring dates from the 20’s. I would be more concerned about corroded wires possibly catching fire than whether or not the 3rd prong is in the circuit. As wires and connections degrade over time, you are left with less copper for the current to flow through, and this means a higher resistance which then translates into more heat generated. I’d recommend getting an electrican out there to inspect your old wiring.
As for erislover’s suggestion to measure the resistance between the nuetral and ground, keep in mind that the nuetral carries current. A multimeter measures resistance by running a current through the connection and measuring how much current flows. If you have a significant amount of current running through that particular circuit, you could get a false reading or worse, potentially damage your multimeter. If you turn the breaker box off then you should measure the resistance of the copper wire between the outlet and where the ground and nuetral are bonded together.
Old wiring usually only has the two conductors (hot and nuetral). There is likely not a ground wire in the 2 prong outlet boxes.
erislover: “then the circuit breakers aren’t made to accept them anyway. Right?”
No, the circuit breaker (fuse) doesn’t care where the current is flowing, only how much is going through it.
erislover:
“do old wirings even have a ground in there?”
Not all of them do, Knob and tube doesn’t. amoung others.
erislover:
“When you find the two receptacles that have no potential (the “neutral” and “earth ground”) go ahead and measure the resistance between them. I expect a full report.”
If your multimeter “thinks” these wires have no resistance, ya be needing a better meter. Just by the rule that 1000’ of 12Ga wire has 1 Ohm resistance, you will read something. Might be better expressed in Mho’s rather than Ohm’s though.
Heck nowdays Dryers need 4 prong plugs. Hot-Hot-Neutral-Ground. (1998 NEC)
This is definatialy not a case where three’s a crowd.
Ground for you and Ground for me, can be at quite different potentials.
To the OP, Install GFCI’s.
Quoth erislover:
It’s a couple of bucks for the outlet itself, plus a minimal service charge (probably an hour or a half-hour’s worth… Ten bucks?) for the electrician to install it, if there’s a ground available to connect to. If there’s not a ground readily availible, then it’ll depend on how far the electrician has to run a wire to reach a ground. Even then, it’s almost certainly worth the price: If the increased safety isn’t good enough, I suspect that you’d also get a better rate on your insurance.
Lot of good info here, and some misinfo, so I’ll see if I can clarify a few things.
As many others have mentioned, the “third prong” is definitely important, as it is a safety ground. If you have an appliance with a 3-prong plug, the ground prong must be connected to the center tap of the transformer, and it’s usually done for safety’s sake. In other words, there must be a low impedance path between the appliance’s ground prong and the “common” bus bar at the breaker box.
Why?
Three reasons:
Safety. Appliances that have a ground plug usually have an external metal chassis, or internal components with metal chassis (e.g. motor, compressor). If a fault occurs somewhere in the appliance, wherein there is a low impedance path between hot and chassis, the breaker or fuse will blow if the chassis is properly grounded. If a fault occurs and the chassis is not grounded, the entire chassis will float at 120 VAC. If you happen to be grounded to earth ground and you touch a “hot” chassis, you’ll get zapped.
Shielding. Some sensitive instruments utilize a shield to attenuate EMI from getting into the instrument and/or from being radiated from the instrument. This shield needs to be at ground potential, thus it is electrically connected to the infamous “third prong.”
Transient protection. As NutMagnet already mentioned, surge protectors need an earth ground. But a slight nit-pick: A surge protector needs an earth ground for normal mode protection. Even without an earth ground, you’ll still have common mode protection.
So don’t try to defeat it with those little orange adapters! Unless, of course, you ground the little tab. As NotMrKnowItAll pointed out, do not assume the plate cover screw is grounded! Perhaps it is, perhaps it isn’t. Check it by turning off the breaker and performing a continuity check between neutral and the screw head. (This is actually a “quick and dirty” method, but it’s better than nothing. A better method would be to measure continuity between the breaker box and the outlet.)
Someone mentioned that “the neutral is at earth ground.” This is a true statement if there is no current on the neutral line. If there is current, then there will be a potential. The voltage on the neutral line, which is usually very small even when a device is on, will be proportional to the current on the neutral line and the resistance of the neutral line. So why doesn’t the neutral do “double duty” as a safety ground? Well, if you were using an appliance where the neutral was connected to the chassis, and there was an open circuit anywhere on the neutral line, the chassis will become hot (even though current will no longer flow).
Now a word about polarized plugs. Many 2-prong (i.e. non-grounded) plugs have these, and for good reason – the hot conductor is, well, hot. As atypicalcarl pointed out, a lamp has a polarized plug to ensure the screw part of the bulb is connected to neutral. Also, if a non-grounded device has a power switch, the switch should be connected to the hot conductor, thus a polarized 2-prong is used. If a non-grounded device does not have a power switch, and/or the hot and neutral directly connect to a transformer, then a polarized plug is usually not needed.
A few tidbits about GFCI outlets. The NEC does allow you to replace a 2-prong outlet with either: a) a GFCI outlet, or b) a regular 3-prong outlet that is fed from a GFCI outlet. But you must do two things in order to make it “legal":
i) If you’re replacing a 2-prong outlet with a GFCI outlet, you must affix a sticker on the outlet cover that says “No earth ground.” Why? Primarily for reasons #2 and #3 above, i.e. some sensitive instruments need their shields connected to earth ground, and surge protectors need an earth ground for normal mode protection.
ii) If you’re replacing a 2-prong outlet with a regular 3-prong outlet that is fed from a GFCI outlet, then you must have two stickers: one that says “No earth ground” and another that says “GFCI protected.”
Keep in mind, however, that the “best” arrangement is a properly grounded GFCI outlet. Why? Let’s say you plug a device (with 3-prong plug) into an ungrounded GFCI outlet. The chassis will not be tied to earth ground. Now let’s say something breaks inside the device in the middle of the night, and a hot conductor makes contact with the chassis. What happens? I’ll tell you what: nothing. The GFCI will not blow, and the breaker will not blow. The next morning your barefoot 3 year old daughter goes up and touches it. Assuming she’s grounded, the GFCI outlet will (hopefully!) sense the current imbalance and blow. But this sucks, for the following reasons:
I’m not saying GFCI outlets are bad, I’m saying that a properly grounded 3-prong outlet offers better protection for internal ground faults over an ungrounded GFCI outlet. In the latter situation, it is often the case that the GFCI has to “wait” for you to blow it via current running through your body. Kinda makes me wonder why the NEC allows it…