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#185277 - 03/07/09 03:50 PM Hairdryer with integrated GFCI plugged into a GFCI
JuicemanJim Offline
New Member

Registered: 03/24/07
Posts: 5
Loc: Upstate NY
I have looked on here to see if anyone has posted this, but no luck. A friend of mine had his co-worker demonstrate something to him while on a business trip. Using a hairdryer with and integrated GFCI cord and plugging it into a GFCI receptacle, he filled up the sink with water and dropped the hairdryer into the sink. Neither of the GFCI's tripped. He said he was told two GFCIs cancel eachother out. I have not been able to find a discussion that verifys this, but using my own GFCI plug at home with another GFCI protected 4-gang plug I use on jobs, I could not get either GFCI to trip. Is this correct? Two GFCI's will cancel eachother?? Seems kind of dumb that all hairdryers have a built in GFCI cord, which negates the receptacle you plug into!! Thanks for your input!

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#185278 - 03/07/09 05:15 PM Re: Hairdryer with integrated GFCI plugged into a GFCI [Re: JuicemanJim]
twh Offline
Member

Registered: 03/11/04
Posts: 898
Loc: Regina, Sask.
Perhaps the sinks aren't grounded.

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#185279 - 03/07/09 05:30 PM Re: Hairdryer with integrated GFCI plugged into a GFCI [Re: JuicemanJim]
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
Two GFCI's do not cancel each other out.

If droping it into the water didn't trip the GFCI, there wasn't a ground fault current greater than the GFCI setpoint (40mA?)

For a GFCI to trip, the electric that's supposed to be going through the hair dryer has to be going somewhere else instead...such as through you. It's possible that the sink had a plastic drain line, plastic supply lines, or even a plastic faucet. If so, there's no place for the electric to go to trip the GFCI.

And water might not be conductive enough unless it has some dissolved salts in it to conduct enough electricity to trip one.

Overall, throwing a hair dryer in a sink full of water isn't a very scientific experiment. Too many unknowns and unmeasurables.

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#185281 - 03/07/09 05:42 PM Re: Hairdryer with integrated GFCI plugged into a GFCI [Re: maintenanceguy]
JuicemanJim Offline
New Member

Registered: 03/24/07
Posts: 5
Loc: Upstate NY
The explanation of the sink not being grounded makes sense to me, however, how does the two gfci's in line at home get explained? A GFCI tester will trip a GFCI receptacle individually, so, it should therefore trip either or both of them when I have a GFCI gang box plugged into a GFCI receptacle. The GFCI produces a "leakage" between the hot and neutral through a resistor, no variables needed for that to work. That one still stumps me. Hope someone can explain that one to me.Thanks.

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#185283 - 03/07/09 07:55 PM Re: Hairdryer with integrated GFCI plugged into a GFCI [Re: JuicemanJim]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
First, the big plug on the hair dryer is not a GFCI ... it's what they call an "Immersion detector," and depends on the circuit being completed between two specific points within the hair dryer. A hair dryer, casually tossed into a sink, may float in a manner that keeps these contacts dry.

As for the GFCI receptacle ... it's also quite possible that not enough current flowed to trip the unit. Household water is often surprisingly pure, and thus a poor conductor. I can't help buy speculate that a bit of salt added to the water would have yielded better results.

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#185284 - 03/07/09 08:03 PM Re: Hairdryer with integrated GFCI plugged into a [Re: JuicemanJim]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
A GFCI measures the difference between the current going out one wire and coming back in the other wire. If all the current going out to the appliance is coming back in *the other wire), that difference is zero. If some small amount of current (called the "leakage current") goes through some other path instead of back on the return wire, that amount of current will be seen as a difference of current between the measured wires. A properly wired GFCI device measures this current only on the 2 wires intended for conducting electric current. If a 3rd ground wire is present, the measurement is not done on it, so if the leakage current is going over the 3rd ground wire, it will be detected even though this wire just "goes around" the GFCI device.

If there are 2 GFCI devices, that does not prevent either of them from doing what they do. If one of them is miswired, that could cause either or both of them to trip (and I wouldn't even call that a false trip since it is a hazard to be located and corrected).

GFCI devices will trip and open the circuit when that difference current goes higher than some set level. This level is supposed to be at least as high as 2 to 4 milliamps to avoid false trips from things like capacitance on long cords, but no higher than 6 milliamps (considered potentially dangerous to people in weak health) for GFCI devices intended for "people protection". Home GFCI devices like outlet based GFCI and cord based GFCI are supposed to be for people protection. Industrial GFCI devices intended only for equipment protection have higher set points.

Each GFCI device may have a slightly different set point. A slight level of leakage current could trip one device but not another. Even if the leakage current is much higher than the set point for 2 or more GFCI devices connected in cascade, it is quite possible for one device to act so fast that another device does not have the time to act, or the electrical energy to act.

Connecting 2 GFCI devices in cascade may well result in one of them tripping but not the other, in a circumstance where both of them should trip. This is the likely source of people believing that 2 GFCI devices in cascade will somehow cancel out. One could trip faster than the other.

Connecting 2 GFCI devices in cascade will NOT cause both of them to be ineffective. They are measuring line current and as long as the line current flows through the GFCI devices, they can do their job. The only way to defeat a GFCI device is to entirely reroute all of the line current on both conductive wires around the GFCI device.

One catch with GFCI design is that the test button usually does NOT create a false leakage. A badly designed test button might have a 20K ohm resistor connected from the hot wire to the grounding (EGC) wire. That kind of test button would fail to test the GFCI device if used in an ungrounded circuit. GFCI devices are intended to operate on both grounded and ungrounded circuits. So a different means is used to perform the test. The 20K ohm resistor would be wired between one wire after the current measurement point and the other wire before the current measurement point. This would successfully simulate a current difference for both grounded and ungrounded circuits, but it does not simulate a leakage. That test is intended to verify that the current measurement component and trip action components are functional in that device alone. The test button of one GFCI device wired as just described will NOT trip an upstream GFCI device. If the downstream GFCI device is defective and fails to trip when tested, the upstream device will not trip under this test. There are separate external GFCI testing devices that must use a ground as the leakage path to carry out the test. These devices will create a true controlled leakage, and should trip at least one of a series of GFCI devices connected in cascade.

If you find an installed GFCI device that fails to open a circuit when its own test button is operated, or when an electrical shock between a wire and something else like a water faucet is felt, you should contact a professional licensed electrician to investigate and correct the problem. If an external GFCI device, like found on a cord, fails, that device should be returned where it was purchased from if it is still eligible for returning as defective, or it should be disabled (cut the cord in half to prevent someone from using it) and disposed of.

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#185286 - 03/07/09 11:04 PM Re: Hairdryer with integrated GFCI plugged into a [Re: pdh]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Joe Tedesco has a video of this test. It is something he did when he was bored in a hotel room when he was doing his IAEI road show.
I say the immersion detector was bad/missing and the sink wasn't grounded (plastic trap in the drain and no direct connection to the supply pipes, plastic sink) so the GFCI didn't trip.
I have taken several hair dryers apart. The immersion detector is a metal ring in the discharge chute connected by the 3d wire to the circuit in the plug. I assume it looks for current on that wire.
_________________________
Greg Fretwell

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#185287 - 03/07/09 11:08 PM Re: Hairdryer with integrated GFCI plugged into a [Re: gfretwell]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
BTW a way to duplicate the GFCI part of this test is to drop a 2 wire extension cord into a plastic bucket full of salt water. Without a ground path, this just becomes a crude water heater popping and bubbling away. Sort of like that old "Hot Dogger" that electrocuted hot dogs.
_________________________
Greg Fretwell

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#185288 - 03/08/09 09:56 AM Re: Hairdryer with integrated GFCI plugged into a [Re: gfretwell]
KJay Offline
Member

Registered: 11/27/07
Posts: 763
Loc: MA, USA
Juiceman,
You didn't mention if you friend tried placing anything conductive between the energized water and the sink faucet.
I would think that if either the hot or cold supply lines were metallic and the plumbing was bonded, if you stuck your hand or a jumper wire in the water and then touch a metallic faucet, the GFCI device would most likely have tripped if it was functioning properly.

Since this was sort of an experiment, my guess is that if you measured the voltage from the water to that grounded faucet with the test leads of a high impedance voltmeter, it would probably read the approximate available voltage but not trip the GFCI. If you tried reading this voltage with a low impedance meter or solenoid type tester it would probably would trip.
Seems like it might be a pointless risk though, but I would keep the video recorder running anyway. grin

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#185293 - 03/08/09 03:42 PM Re: Hairdryer with integrated GFCI plugged into a [Re: pdh]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Just to be picky.
Originally Posted By: pdh
Industrial GFCI devices intended only for equipment protection have higher set points.


GFCI stands for Ground Fault Circuit Interrupter, it is a very specific item built for people protection. The only currently available ones are Class A which must operate when the current imbalance exceeds 6mA based on a specific time-current curve. There used to be a Class B device but it was only for swimming pool lights. GFCI are only available up to 240V.

Any other Ground Fault protective device should not be called a GFCI. There is no universal abbreviation for these other devices. Common terms include GFPE, for 30mA trip ranges, and GFI or simply GF, for service entrance types in the range 200-1200A.

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