GFCI TEST BUTTON

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domnic

Senior Member
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Electrical Contractor
How does test button on a gfci with no EGC work, How does it put a unbalanced load on the sensing coil ?
 

al hildenbrand

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Minnesota
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Electrical Contractor, Electrical Consultant, Electrical Engineer
Basically, a trickle goes from the load side hot to the line side neutral.
 

GoldDigger

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Location
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Through a resistor sized to provide just over 6ma to properly test the detector sensitivity.
I suppose that if the line voltage were low enough the test would fail even on a good unit.
But so would a plug-in tester in that case.

Tapatalk!
 

Jon456

Senior Member
Location
Colorado
I believe it bleeds a tiny amount of current from the hot side (downstream of the sensing coil) to the neutral side (upstream of the sensing coil). Since the net flow through the sensing coil is unbalanced, the GFCI will trip. If the test current was tapped and returned both on the downstream side of the sensing coil, the net current through the coil would still be balanced so the device would not trip.
 

goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
Just an FYI, while the test button on the GFI receptacle will function properly but some plug-in testers may not work if the EGC is not present
 

kwired

Electron manager
Location
NE Nebraska
Just an FYI, while the test button on the GFI receptacle will function properly but some plug-in testers may not work if the EGC is not present
For the reasons others have mentioned - by making the test current flow from one "line side" terminal to the opposing "load side" terminal the current during a test with the unit's test button does not depend on an EGC being present, but still puts an unbalanced "real current" through the sense coil for the protective circuitry to actually respond to what it sees as a real condition and is not a manual activation of the trip mechanism.

That said pretty much all plug in testers will not trip a GFCI if there is no EGC present, as all they can do without more action then simply plugging them and pressing the button is to provide a fault current from a protected conductor to the EGC.


I don't know all the details of AFCI's but it is my understanding pressing the test button on those doesn't necessarily introduce any "real fault current" for the device to react to but rather just manually activates the trip components.
 
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GoldDigger

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It is slightly more than manually operating the trip components.
It tells the microprocessor to do what it would do if an arc signature was detected. So it at least tests that the processor is still running at least part of its control program.
AFAIK it does not test the sensor used by the processor to detect the arc signature.
If the device includes a class B GFCI, I do not think that it separately tests that function.
Not sure about the test button on an AFCI breaker with Class A GFCI.

Tapatalk!
 
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kwired

Electron manager
Location
NE Nebraska
It is slightly more than manually operating the trip components.
It tells the microprocessor to do what it would do if an arc signature was detected. So it at least tests that the processor is still running at least part of its control program.
AFAIK it does not test the sensor used by the processor to detect the arc signature.
If the device includes a class B GFCI, I do not think that it separately tests that function.
Not sure about the test button on an AFCI breaker with Class A GFCI.

Tapatalk!

So how do we really know the test is confirming that we have the "alleged" protection being offered.

Note that to me AFCI protection is "alleged" at this point in time, but is innocent until proven guilty:happyyes:
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
It is slightly more than manually operating the trip components.
It tells the microprocessor to do what it would do if an arc signature was detected. So it at least tests that the processor is still running at least part of its control program.
AFAIK it does not test the sensor used by the processor to detect the arc signature.
If the device includes a class B GFCI, I do not think that it separately tests that function.
Not sure about the test button on an AFCI breaker with Class A GFCI.

Tapatalk!

I found this by searching Yahoo.

View attachment 10184
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
How does test button on a gfci with no EGC work, How does it put a unbalanced load on the sensing coil ?
Domnic,

Here's another variation of the sensing circuitry found in the common GFCI. There's another head scratcher besides what the test button does. The diagram below shows that, even when no load is connected and running, if the load side neutral conductor comes in contact with a ground return path back to the power supply (the power company transformer, in most cases), then a trick second sensing circuit causes the GFCI to trip.

Think about it. The grounded circuit conductor ( zero volts ) comes in contact with the equipment grounding conductor, or something like it, which is also at zero volts, and that second "grounded neutral transformer" causes current to flow and trips the GFCI.

GFCIOperation_zpse4a67f2e.jpg
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
140426-0904 EDT

In a Leviton GFCI receptacle the sensing current transformer is at the input, and only two conductors pass thru the current transformer. The test resistor, 15 k, is connected directly to hot on the output side of the current transformer, and the other end of the test resistor is connected thru the test switch to the neutral conductor on the input side of the current transformer. The breaker contacts break both hot and neutral on the output side of the current transformer. Also when the breaker is open the load side output terminals are disconnected from the front face receptacle sockets.

I doubt that any receptacle type GFCIs have a third wire pass thru the current transformer because it makes no sense from a manufacturing perspective. Thus, I see no reason to show the schematics as they are drawn.

You can see internal photos of the Leviton GFCI at my website page beta-a2.com/GFCI.html

.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
I doubt that any receptacle type GFCIs have a third wire pass thru the current transformer because it makes no sense from a manufacturing perspective. Thus, I see no reason to show the schematics as they are drawn.
You may well be correct that no OEM has assembled the TEST circuit as a discrete conductor passing through the current sensing transformer as the diagrams illustrate.

However, from a teaching perspective to someone who is working with a beginning of Kirchoff's Laws, and who isn't going to spend time with Thevenin Equivalents, having a graphic image of current path helps to de-mystify the "Black Box" of a GFCI.

Nice photo study of the Leviton 7899. Thank you for the careful exposition.
 

kwired

Electron manager
Location
NE Nebraska
You may well be correct that no OEM has assembled the TEST circuit as a discrete conductor passing through the current sensing transformer as the diagrams illustrate.

However, from a teaching perspective to someone who is working with a beginning of Kirchoff's Laws, and who isn't going to spend time with Thevenin Equivalents, having a graphic image of current path helps to de-mystify the "Black Box" of a GFCI.

Nice photo study of the Leviton 7899. Thank you for the careful exposition.
But whether via one wire or two, same current still passes through the CT.
 
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