GFCI protect people from electric shocks. If a faulty appliance is plugged into a receptacle outlet that is protected by a GFCI device, the protection disconnects the circuit when someone touches it and some of the current flows through the person.
How GFCI works
Ground Fault Circuit Interrupters (GFCI) monitors the difference between the current flowing in the ungrounded (hot) conductor and the grounded (neutral) conductor. Even relatively small (4 to 6 mA) difference between the two current values trips the protection device thus turning the circuit off. The unbalance is caused by some of the current going to the load returns following a different path. This mechanism prevents even relatively small amount of current possible going through a human body. That is how the GFCI protects from electric shock.
The reason for some of the current returning at a different path to ground than the neutral conductor can be that the electrical equipment is faulty or some water gets into the conductive parts of the device.
Where GFCI is required
The NEC code (210.8 A) requires that personnel ground fault protection be provided for all 125 V, 15 A and 20 A receptacle outlets in dwellings as follows:
- in bathrooms
- in kitchens,for all receptacles that serve countertop surfaces (including islands and peninsulas)
- ~not for outlets only serving a clock
- ~not for outlets inside an upper cabinet for plugging in microwave
- ~not for receptacle installed hidden in the lower cabinet for disposer, trash compactor and dishwasher
- within 6 feet of a laundry, utility or wet bar sink
- in garages
- ~not for outlets without easy access (ceiling outlet for garage door opener)
- in other buildings (storage buildings, sheds, workshops)
- ~not for outlets without easy access
- ~not for outlets that have dedicated appliances plugged in all the time (freezer, washer, gas dryer)
- in crawl places below grade
- in unfinished basements
- do not use GFCI for sump pumps
- not required for permanently installed burglar or fire alarm
- swimming pools
- circuits supplying heated floors
- underground branch circuits if buried at 12 inches, without protection 24 inch required
How GFCI protection is implemented
There are different ways to implement GFCI protection. The NEC code does not regulate the exact implementation.
The most common way is to install Ground Fault Circuit Interrupter is with devices that are built into receptacle outlets. These devices have five terminals. One terminal is the ground connection. Two connections are connected to the power coming in (one hot and one neutral). The other two are outputs (one hot and one neutral). The built-in GFCI protection protects not only the receptacles in the device but any circuit that is connected to the output terminals. Usually circuits are wired such that the devices are daisy chained as the wiring goes from the distribution panel to the last device. It is customary to install the device with the GFCI protection first after the breaker and connect the rest of the outlets to the output of the GFCI. As a result all of them are protected.
There is no reason to connect any more protection after the first device. The “quality” of the protection does not increase only the inconvenience does. When there is a ground fault event the protective device trips. If there are multiple protection devices installed, they all trip at once. When the fault is repaired and the device can be reset, the process has to be done in just the correct order. One has to start with the device closest to the source and go down the chain in the correct order. If the user is not familiar with the exact order he cannot reset the devices successfully.
The GFCI protection device is set to trip at a very low level of current imbalance. On one hand it makes it very safe. A few mAmps (one thousands of an Amp of current) of current is at a safe level. However, detection of false positive can happen with enough moisture in the air, if the wiring is very long or some other mechanism.
Appliances that are properly installed and UL listed have very small allowed leakage current. It should not cause nuisance tripping under normal circumstances. If an appliance found to cause issues it should be checked out by a qualified repairman.
In most cases when nuisance tripping is an issue, the cause is either water getting into the protected outlets or the device is getting old. Sprinklers, rain, etc. are a common cause. Proper in use covers (bubble covers) can prevent the issue from repeating a lot of times. A lot of devices can be misbehaving as early as five years after they were installed. Devices that are exposed to weather usually give in sooner than GFCI devices in a protected environment.
Limitations of GFCI Protection
GFCI protection is a very useful tool that increases the safety of the electrical system. However, it only protects from one kind of electrical fault, ground fault. It is important to understand what GFCI does not do, so users do not have a false sense of security.
- GFCI does not protect against electrical shock, if the person touches both the neutral and the hot at the same time.
- does not limit the amount of ground fault current until the device disengages (~25 milli second)
GFCI does not replace the traditional fuse or circuit breaker in the distribution panel, it augmants their protection. The following are situation that are protected by the circuit breaker but not the GFCI.
- short between the hot and a neutral
- short circuit between hot wires on different phases
- overload protection
Electrical arching due to damaged insulation or wire is not corrected neither by the circuit breaker nor by the GFCI. An AFCI breaker is necessary to disconnect power in case of arching.
Some faults that cannot be detected by GFCI devices and cannot be detected by any other existing standard protection methods either. These faults would require some intelligent outlet technology to be isolated.
Also, AFCIs provide no specific protection against “glowing” connections, excess current, high line voltages or low line voltages. AFCI circuit breakers for use in a panelboard include a standard inverse-time circuit breaker. Glowing connections occur when relatively high electric current exists in a relatively large resistance object. Heat comes from power dissipation. This energy, when dissipated in a small junction area, can generate temperatures above 1000 degrees Celsius and can ignite most flammable materials.
Bad wiring junctions can occur in utilization equipment, cords or in-situ wiring and especially in a defective switch, socket, plug, wiring conneArc-fault circuit interrupterction and even at the circuit breaker or fuse panels. Terminal screws loosened by vibration, improper tightening or other causes offer increased resistance to the current, with consequent heating and potential thermal creep, which will cause the termination to loosen further and exacerbate the heating effect. In North America, high resistance junctions are sometimes observed at the terminations of aluminum wire circuits, where oxidation has caused increased resistance, resulting in thermal creep. No technology located in a circuit breaker or fuse panel could detect a high-resistance wiring fault as no measurable characteristic exists that differentiates a glow fault from normal branch circuit operation. Power Fault Circuit Interrupters (PFCI) located in receptacles are designed to prevent fires caused by glowing connections in premise wiring or panels. From the receptacle a PFCI can detect the voltage drop when high current exists in a high resistance junction. In a properly designed and maintained circuit substantial voltage drops should never occur. Proper wire terminations inside utilization equipment, such as appliances, and cords prevent high-resistance connections that could lead to fires.
An AFCI does not detect high line voltage due to an open neutral in a multiwire branch circuit. A multiwire branch circuit has both energized wires of a 120-240V split phase service. If the neutral is broken, devices connected from a 120 V leg to the neutral may experience excess voltage, up to twice normal.
AFCIs do not detect low line voltage. Low line voltage can cause electro-mechanical relays to repeatedly turn off (relay opens) and on (relay closes again). If current is flowing through the load contacts it will cause arcing across the contacts when they open. The arcing can oxidize, pit and melt the contacts. This process can increase the contact resistance, superheat the relay and lead to fires. Power fault circuit interrupters are designed to prevent fires from low voltage across loads.
Neutral wiring issues
It is important to avoid certain wiring mistakes in order to ensure proper functionality.
How to operate and test GFCI
The following are the features that are important to look for in a GFCI device
Buttons and how they work.
Testing in a regular bases.