GFCI Testing
excerpted from IAEI News Jan/Feb 1994
More on GFCI's

GFCI Receptacle Testing Using the Internal Test Function

Some installers and inspectors question the efficacy of the internal GFCI test circuit either because they mistakenly believe the test is merely a mechanical check or because the internal test and the commercially available GFCI testers may give different results. Their skepticism is unwarranted. Here's why:

In Fig1., note that the internal test circuit connects to the line-side hot conductor; runs through the GFCI sensor, the test button and a resistor; then returns outside the sensor to the line-side neutral. Depressing the test button thus introduces a differential current into the sensor. The GFCI must perform all its functions from sensing to trip. Depressing the test button, therefore, is not merely a mechanical check but a complete test of the entire operations of the GFCI.

Fig. 1

Testing GFCI Receptacles on Two-Wire Circuits

Using GFCI testers to test GFCI receptacles installed on two-wire circuits(permitted by code) may falsely indicate that the GFCI is not functioning. The user may even risk a shock.

The significant difference between a GFCI tester and the integral test circuit of the GFCI is that the external tester's current is introduced on the equipment-grounding conductor. On a two-wire circuit there is no equipment grounding conductor. Thus there is no test current, so the GFCI does not trip. The potential for shock occurs when the person using the tester on the ungrounded circuit is touching a grounded surface and contacts an exposed conductive object connected to the equipment-grounding contact of the receptacle being tested.

Picture an inspector checking an outdoor GFCI receptacle with a metallic cover installed on a two-wire circuit. The inspector, standing on wet ground in soaked leather-soled shoes, holds open the metallic cover, inserts the tester, pushes the test button and promptly gets an uncomfortable shock. Pushing the tester's test button introduced the test current on the receptacle grounding contacts, which were connected to the receptacle strap, which, in turn, was connected to the metallic plate and, through the inspector to ground.

The shocked inspector now wholeheartedly questions the efficacy of the GFCI test button... all because the tester was used incorrectly.

Another potential problem with testers

We recently purchased a $72 tester which appeared to offer a range of test capabilities. It was not UL listed. The tester had a three-light display showing five possible miswiring scenarios and a dial that could be adjusted for 0, 3, 5, 7, 10 and 30mA of test current. The tester was also equipped with a three wire grounding type cord and plug. As with most testers, the test current is impost on the equipment grounding conductor.

The instructions indicated that if the GFCI tripped with 3mA test current it is "oversensitive." In fact, nearly every circuit has some low level of standing leakage. If this standing leakage is 2mA, then imposing a 3mA test current should cause a GFCI with a 5mA trip threshold to trip. Thus, this test can be misleading unless one knows the standing leakage.