How the Test Button Works on a GFCI Receptacle

Q. Mike, I spoken to a rep from ______, and he has told me that the TEST button on most GFCI receptacles does not actually test the protective circuit. Rather, it merely exercises the mechanical trip mechanism. If this is the case, then pushing the test button does not assure adequate GFCI protection. I was told that one good way to test the GFCI protective circuit is with one of those plug-in testers with the GFCI tester built into them. Apparently, they actually cause an imbalance between the grounded and ungrounded conductors, thus verifying true GFCI protection. I though you might like to pass this along to your readers.

Great job on the emails and the website. Many thanks!
Michael Hofkin, President IAEI, Eastern Pennsylvania Chapter Stan, Mike,

A. From Leviton Mfg. Co., Inc.
Mike, I hope the following explanation helps clear things up: On all traditional GFCI receptacles, when you press the test button you are creating an actual imbalance within the current transformers in the GFCI, exactly the same as if there was a ground fault present. This is done by connecting the load side hot terminal to the line side neutral terminal through a resistor calibrated to conduct a small amount of current at 120V (approx. 8-10 ma). When you press the test button you complete the circuit through the resistor; the GFCI detects the imbalance and trips.

An external tester cannot do what is described above because both the hot and neutral blades of a plug-in tester make contact with the load side contacts of a GFCI. The imbalance is created by dumping the small amount of current from hot to ground. This is why an external tester will not work on an ungrounded circuit but the GFCI's internal test button will still work.

The only exception to the above is Leviton's SmartLock GFCI. In this case, the test button mechanically trips the GFCI by simply pushing the latch which holds the contacts closed. We satisfy the UL requirements for testing the electronics when you press the reset button. When you press the reset button the test circuit described above is invoked and creates the current imbalance. If the GFCI is operating properly, it will sense this and fire the solenoid used to trip the GFCI. We use the firing of the solenoid to move shutters blocking the latching mechanism for the contacts. The result is, if the GFCI does not sense the ground fault and fire the solenoid correctly, you will not be able to reset the GFCI - no power without protection. An added benefit is that the SmartLock GFCI will also block the reset button if the GFCI is wired incorrectly.

Let me know if you need any additional info.

Regards, Bill Grande Product Manager, Safety Products
Leviton Mfg. Co., Inc.
59-25 Little Neck Pkwy
Little Neck, NY 11362
ph: (718)281-6605
fax: (718)281-6136