These special electrical devices (GFCI’s) shut the power off to a circuit when as little as .005 amp is leaking. Under normal circumstances, the current flowing through a circuit is the same at any point. That is, if there is a 5 amp flowing through the white wire coming back.

If there is a flaw in the system, some electricity may be flowing to a dangerous spot, but there may not be enough flowing to blow a fuse or trip a breaker. For example, if the electrical insulation in an appliance is defective, there may be a small current leaking to the case of the appliance.

Under normal circumstances, this would be detected. However, this can become very dangerous if a person (particularly one who is not well insulated with rubber gloves/shoes) touches the case. A potentially fatal electrical current can flow through the person to ground. This creates an electrical shock hazard.

A GFCI prevents this from happening by comparing the electricity going out through the black wire against that coming in through the white wire. If the difference is more than .005 amp, the system will shut off.

Sometimes it is difficult to know how many electrical components in a house are protected by a GFCI. It is possible to have several electrical outlets all protected by one GFCI.


These devices may be incorporated into a circuit breaker, or into an electrical outlet. Most codes require their use on outdoor outlets and bathroom outlets. They do make sense wherever water and electricity may be brought close together. For example, GFCI’s are also required on swimming pool and whirlpool electrical systems.


 Common Misconceptions of GFCI’s

The first misconception of GFCI’s is what they actually do. GFCI devices do not detect gound faults! They detect a difference in current. While this difference can be created by a fault or ground, the device does not directly test this. The GFCI device detects the resulting condition, i.e., the difference in current between the hot and neutral conductors. While an external tester uses the grounding conductor in the detection device within the receptacle itself, the grounding conductor is not involved at all.

Another common myth is that the test button only tests the mechanical action of the GFCI device. This is not true. Pressing the test button creates a difference in current between the hot and neutral by the use of a resistor. Circuitry in the receptacle detects the difference and activates the trip mechanism.

Pushing the test button is a complete test of the GFCI device, but will not tell you if the receptacle is properly wired. This is where the external tester is useful. If the device is miswired, it can remain hot even after the internal mechanism has tripped.

A common situation that home inspectors face is a GFCI receptacle that trips using its test button, but does not trip using the inspector’s external tester. This generally indicates a wiring problem such as:

  • ŸReversal of line and load conductors (Connected to wrong terminals on receptacle)
  • ŸOpen grounding conductor
  • ŸReversed polarity

Combination testers will detect some of these wiring faults, but single function GFCI testers will not.

If the line and load connections are reversed at the receptacle, the receptacle will remain energized after the device trips. This is a common wiring error. According to GFCI tester literature, a reversed ground and neutral will produce the same result; but actual tests performed by home inspectors differ from this information. An external tester used by an inspector does create an actual ground fault by diverting current to the grounding conductor. If there is no grounding conductor present in the circuit, or if its disconnected, the external tester will indicate that the device is not working, when in fact, it may be.


Risk for the Inspector                                   

Testing ungrounded receptacles does create some risk for the inspector. Here’s why: The GFCI tester operates by momentarily connecting the hot and grounding conductors. The current will pass to anything grounded, such as the receptacle cover or metal box (as commonly found outdoors). If the inspector is touching the grounded metal box, or perhaps holding open the metal cover over an outdoor receptacle, he will receive a shock when using the tester.

More GFCI Facts

Another little known fact is that GFCI devices are required to detect a grounded neutral so as to assure that there is no return path on the neutral in the event of a ground fault. Current returning on the neutral could reduce the difference in current being detected, and prevent the device from operating.

It is permissible to install a GFCI receptacle in an existing 2 wire ungrounded circuit. Many electricians are unaware that this is acceptable. They will incorrectly insist that a grounding conductor is necessary for the device to operate.