how to tell if 3 prong plug AC uses is grounded

We have an admiral AC unit. It's pluged into a 3 prong wall socket plug. I assumed the wall socket was grounded as it is 3 prong, but the manual says have an electrician very that. My mother is an old widow and doesn't have alot of money. Is there a way I can determine this ? The unit resets itself after a couple of days, that is it shuts off, the green light on the plug turns off and you have to hit the test and reset switch if I recall correctly, then it makes a noise and the green light is back on and you can run the unit. The test and reset switchs are on the plug that goes into the wall socket. It seems to trip when it's not running. She has been using it since last summer and it seems to work fine otherwise. The manual however seems to indicate it shouldn't trip.
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surf wrote:

That sounds like your AC is plugged into a GFI. Normally I would not suggest doing that, in part due to exactly what you are seeing.
Where is the A/C located? Is it in a kitchen or bath?
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Joseph Meehan

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Joseph Meehan wrote:

Agree it sounds like it's in an external GFCI unit. To OP, reason asked about the kitchen bath is that GFCI is req'd by new code for them. An A/C is heavy load and GFCIs are very sensitive so I'm guessing it's tripping it occasionally when it tries to restart before the compressor has been off long or when something else on the circuit may be on at the same time.
To check the outlet, remove the socket from the wall (carefully or better turn the breaker off first) and see if the third wire is connected to the ground terminal of the plug. One could also go to the service panel and remove the cover and see if there's a ground wire on corresponding to that breaker (again, of course, turn the whole panel off first).
There are some plug-in-the-wall circuit testers but I have to honestly say I've never used one and don't know just how reliable they are for testing for the ground connection.
In general, though, if the house is mid-60s or later, chances are good (and better the newer, pretty rapidly) it was wired w/ ground initially. 50s or earlier, chances diminish rapidly the other way unless it has been upgraded.
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Three-light plug or neon testers are sufficient to tell you that a ground _exists_ (in the former, the two green lights but not the red one light up), but not how "good" the ground it is. If they light up appropriately, the ground is almost guaranteed to be good enough to trip a GFCI. But it's nowhere near a certainty that a hot-ground short would trip the breaker.
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Chris Lewis, Una confibula non set est
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So how do you test how good the ground is? Connect a hair dryer across hot and ground (instead of neutral) and see if it works just as well? Obviously this test would not work on a GFCI receptacle.
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That'd work, but a 100W lightbulb on a pigtail, looking for full brightness is a bit safer and somewhat easier to judge the result.
Either way, pull it out _immediately_ if it's noticably dim or begins to flicker, and keep your eyes and nose open for smoke.
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peter posted for all of us... I don't top post - see either inline or at bottom.

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Joseph Meehan wrote:

The OP clearly stated that the test and reset switches were on the PLUG, like the one's I see used on hair dryers in recent years.
If it IS a CFCI plug, and the receptical it's plugged into DOES have a ground which gets connected to the case of the AC through a ground conductor in the cord, it's entirely possible that after running the AC a while condensation occuring inside the unit could create leakage to ground and trip the GFCI.
Even if the receptical doesn't have a valid ground, it's still possible that condensation and dripping water outside could cause enough leakage to trip the GFCI.
Methinks that problem is going to be a bitch to find and correct, and hiring professional help to do that will prolly cost a lot more than if the OP just purchases a new window AC for his mom.
If it was me, and there was an equipment grounding conductor in the AC's power cord, I'd just verify that the receptical was properly grounded and then chop off that GFCI plug and replace it with a standard grounded plug. But that's me, and I'm not suggesting the OP does it, 'cause his skills are as yet undefined.
HTH,
Jeff
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Jeffry Wisnia
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It might just need a good cleaning.

That'd be a very last resort. I don't like mysterious ground faults, and if the ground is the slightest bit dubious (not able to sink well over 20A), this is a disaster waiting to happen if there's no GFCI.
If I was convinced the GFCI was going bad, I'd hack off the GFCI plug and replace it with a regular plug. _Then_ either put a GFCI outlet in place of the existing receptacle, or, get a (short!) large-gauge wire GFCI extension cord or GFCI'd power bar and plug the A/C into that.
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The wording above implies that the GFI is _part_ of the A/C (a GFI plug). Has anybody seen one of these? I haven't seen one on an A/C (but on other devices certainly). This may be common in "portable" A/Cs that rely on ducting to vent the hot air rather than "thru-the-wall" mounted units.
Which implies that either the AC itself is periodically throwing (probably minor) ground faults or the GFI is getting tired/broken. If it trips when the unit isn't running, a ground fault is likely condensation moisture in the unit (eg: on the switch).
This is likely _entirely_ independent of whether the third prong is really grounded or not.

That'd suggest moisture problems in the unit. Checking to ensure that the unit is "sloped" properly towards the drain is a good idea. Also check to make sure that air flow inside the unit thru where the wires/switches/controls are isn't restricted.
The OP wants to test the ground. It probably has no bearing on the GFI issue. But if the OP still wants to test it, a cheap neon bulb tester between the ground socket and the hot side of the outlet (mostly likely the narrow blade side) will light up as a first approximation of a useful ground, and only costs a few dollars.
I say "first approximation". If it lights up, it's almost definately good enough to trip the GFI on a fault in the AC. But it may not be good enough to trip the breaker if the fault occurs ahead of the GFI (or the GFI refuses to trip). Testing that isn't for the inexperienced (or those not possessing much more expensive test gear).
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The unit is in the master bedroom, which is maybe a bit over 200 square feet. I didn't realize this was this complicated. I replaced an old unit. That unit was plugged into a different type of wall plug that has the slits going horizontal instead of vertical. I'm refering to the 2 plug slots other than the ground plug part. The admiral unit I bought her last summer doesn't have the same kind of plug, so it's in a regular 3 prong plug. She has been running it alot and it seemed to work OK except I guess it trips every few days when it's not on. Iooked at the manual more closely, it's says if the unit is more than 7.5 amps and 115 volts it needs it's own circuit, but it's on the circuit with the small TV, cordless phone, and bathroom. It says 11 amp/115 volts on the side of the unit. If it remains this way, is there a possible serious problem or what could happen ? The older unit I think cooled 1/2 the house, but this one is just to keep her room cool, so I think it's rated lower, though it's much newer. She thinks the house is 25-30 years old.
How much money would it cost to fix or could the other wall plug be used via an adapter or changed ? The manual says not to use adapters etc. I think the other wall plug has a higher rating because I see a section in the manual that seems to rate that to 250 volts.
Thanks, sorry for the ignorance.
Joseph Meehan wrote:

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It isn't, really. Main problem is that we can't _see_ the situation, and we have to grill you for details you didn't think to include.

That means the old unit was 240 volts (almost a 100% certainty). You _cannot_ use the circuit for the new A/C as it is now. It'd go boom!
You _could_ get the 240V circuit converted to 120V. It's easy - many of us here could do it in 20 minutes for about $1-3 in parts (a new receptacle).
However, given your apparent level of expertise, you should hire someone to do this. It involves replacing the receptacle with a standard 120V one, and making a slight change in the breaker panel.
If you can find a electrician _nearby_ willing to do this for less than $150 or $200, I'd jump at the opportunity if I were you. Most electricians wouldn't find a job that small worth their travel time.
Handymen would do it for less, but I'd be worried that they wouldn't know anymore on how to do it than you...
If you have a _good_ DIY friend familiar with electrical work, a case of beer might do it, but who knows how much they know?

Very remote possibility. If the A/C cycles while she's using a hair dryer, it should trip the breaker. Repeated trips may lead to a risk of fire.
The breaker isn't tripping. It's remotely possible something else on that circuit is triggering the GFCI, despite not being "downstream" of the GFCI.
Converting that 240V circuit would give you the dedicated 120V circuit you should have for the A/C and better isolate the GFCI from everything else.
This should be done whether or not the A/C turns out to be going bad and needs to be replaced.
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From the description, you have a GFCI receptacle. It is needed for certain uses, normally outdoor, kitchen, bathrooms where electric appliances are used near water. They sometimes trip out with appliances, even if there is not a ground fault.
To answer your first question, if it is that type of receptacle, yes, it is grounded. You can buy inexpensive plug in test unit that will tell you that it is grounded.
The tripping you are getting my be from another receptacle, downstream, on the same circuit. In my house, the bathrooms and an outside receptacle were all protected by the same GFCI, If you have a similar situation, the problem may well be at another location. With a little investigation on your part, you may be able to find the problem and eliminate it without an electrician. First, find out what all is on that circuit. With a helper, plug a light in and have someone shut the breaker off to see if the light goes out. Then find out what other receptacles are on that circuit the same way. Could be mom has a defective hair dryer plugged in someplace and it tripping it.
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Actually, a GFCI _plug_ on the A/C cord.

There does not seem to be much evidence of that happening with appliances unless they really do have a fault.

Uh, not really. GFCI outlets are a legal substitute for grounding with old circuits that have no ground. The GFCI doesn't create one - it provides alternate protection.
If the house was constructed prior to about the mid-late 60s, it probably doesn't have a ground (or at least, not a very good one).
Since this isn't a GFCI receptacle, the only "downstream" is the A/C itself.
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