What is GFCI? GFCI is an abbreviation of ground fault circuit interrupter. Some people call it ground fault interrupter (GFI). It is a safety device that protects people from electrical hazards. GFCI works by comparing the current entering to the current returning of an AC circuit. If the difference of these currents (current entering minus current returning) is higher than the pre set value, the GFCI will operate. It will break open the circuit or trigger a breaker to trip.
How this theory applies to a real life? Ideally, the difference of the currents that enters an AC circuit to the current that return is zero. If anybody is accidentally touching an AC voltage after a GFCI connection, there is a current flow from that AC voltage to the person’s body and to the ground. This current will not return to the AC source. However, the current entering will see it. As a result, there is a difference due to the current going to the ground (thus called ground fault). This difference will hit the trip value and the GFCI will do its job and interrupts the circuit or trigger a circuit breaker to trip.
How Exactly the Protection Triggers
A typical GFCI have a current transducer, processor or controller and a disconnecting device. The disconnecting device can be a relay in series to the outlet (for a GFCI outlet) or a circuit breaker that mechanically linked to the GFCI.
The current transducer will measure the difference in currents. The processor or controller is responsible in processing the current difference and give a signal to toggle a relay or activating the mechanical linkage so that the breaker will trip off.
When there is no ground fault, the disconnecting device will stay in the close position or a linked circuit breaker will not trip.
When there is a ground fault, the disconnecting device will cut the circuit open. The reason of a ground fault is anything that connects to the load side that will result to a current to flow to the ground. This is represented by R1 and R2 in the illustration below. These R1 or R2 could be a human body when a person is accidentally made contact to the load side.
How to Implement and Install GFCI
GFCI function is can be incorporated to an outlet and this is called a GFCI outlet. There is a current transducer that will detect the current imbalance between entering and returning to the circuit. There is a controller that process the sampled current and there is a disconnecting device that the controller will disconnect when there is a ground fault. This disconnecting device is could be a relay installed inside an outlet. GFCI outlet is often has low current capacity because the devices needed for high current are too big to fit in a standard outlet.
A GFCI device is could be paired with an external circuit breaker. This is the most common method in higher current application. In this approach, the GFCI will detect the fault current and pull the mechanical linkage between itself and the circuit breaker so that the breaker will trip.
There are lot of reputable makers of GFCI such as Eaton, check their product below
https://www.eaton.com/ph/en-us/catalog/wiring-devices-and-connectivity/gfci-receptacles.html
GFCI must be installed upstream in order to protect everything down stream. In electrical circuit, upstream means the point that is very close to the source. On the other hand, a down stream refers to the point near to final load. If you are looking into your house for instance, upstream is the point where the AC enters your house. On the other hand, your appliances and wall outlets are the down streams.
What Must be the Trip Setting of a GFCI to Effectively Protect People from Electrical Hazard
The UL standard for safety for ground fault circuit interrupters is UL943. The standard defines Class A and Class B GFCI. A class A GFCI means it will trip when there is a current to ground of 6mA or higher. On the other hand, class B GFCI trips when there is a current to ground of 20mA or higher.
UL943 also provided an equation to determine the tripping time of class A and class B GFCI.
For class A,
Time = (20/I)^1.43
For Class B,
Time = (80/I)^1.43
Where Time is expressed in seconds and I is expressed in milliamperes
Example 1: fault current to ground is 6mA, find the time
Time = (20/I)^1.43 = (20/6)^1.43 = 5.6 seconds
Example 2: fault current to ground is 20mA (class A), find the time
Time = (20/I)^1.43 = (20/20)^1.43 = 1 second
Example 3: fault current to ground is 20mA (class B), find the time
Time = (80/I)^1.43 = (80/20)^1.43 = 7.26 seconds
Example 4: fault current to ground is 30mA (class B), find the time
Time = (80/I)^1.43 = (80/30)^1.43 = 4.1 seconds
From the above examples, it showed that when the trip current is high, the time delay is short. For personnel protection, class A GFCI is advisable.
What are the Equivalent Devices of GFCI
GFCI is popularly used in the US. In other countries like Europe, it has its own device that will protect people from electric shock. It is called residual current device (RCD) or residual current circuit breaker (RCCB). RCD is a generic term but RCCB is more specific. It is an RCD with its own breaker to trip the circuit when there is ground fault, functioning like a GFCI outlet does.
RCD or RCCB work the same as GFCI. Both cannot provide protection for over current. Over current happens in between line to neutral, but GFCI or RCD only monitors the current imbalance between entering and returning lines.
There is also a device in Europe that will provide both residual current and overload or short circuit protection. This is called RCBO (residual current circuit breaker with overload protection).
In Europe, the usual fault current trip for RCD, RCCB and RCBO is 30mA. This is higher compared to GFCI class A of 6mA.
How to Check if a GFCI is Working Good
Method 1 – using a trip test button
GFCI device has a feature that can be used for testing itself. This is called trip test or self-test. In the GFCI body, there is a button that can be pressed to deactivate the GFCI. This method is safe and good enough for ensuring a GFCI is working. This test method is also the same for RCD. This is the safest way to test household GFCI or RCD.
Method 2 – using a test resistor
For engineering purposes or further technical demonstration, a GFCI an be tested by putting a known resistor value to either the live or neutral side (in the load side) going to ground. This resistor value must result to a current higher than the set trip point, for a GFCI to protect.
Example 1 for method 2:
For a line voltage of 120Vac and a Class A GFCI, the test resistor from live or neutral must be
Rtest = 120 x 1.42 / 0.006 = 28.4 kohm
Example 2 for method 2:
For a line voltage of 120Vac and a Class B GFCI with 20mA trip level, the test resistor from live or neutral must be
Rtest = 120 x 1.42 / 0.020 = 8.52 kohm
Take note, method 2 is not safe for non-technical persons. Do not use this method if you are not expert in electrical circuits. Observe safety precautions all the times.
Asymmetrical and Symmetrical Ground Faults
There are two classifications of a ground fault. It could be asymmetrical and symmetrical faults. Asymmetrical fault could mean;
(a) only either of the live or neutral has fault on a single-phase system or 1 or 2 lines has ground fault for a three-phase system
(b) both the live or neutral has fault (single phase) or all three lines in a three-phase system has fault but differ in fault current level
On the other hand, a symmetrical fault means that both the live and neutral or all the three lines in a three-phase system is shorted to ground. This is also called a balance fault. Asymmetrical and symmetrical faults are performed in method 2 by engineers during the design verification. It is a requirement that a GFCI or RCD must protect to any fault classifications.
Where GFCI is a Must to Install?
According to NEC 2023, areas that are prone to wet must have GFCI. These include kitchen, bathrooms, outdoor receptacles, basements, garages and the likes. For a typical household, installing a single GFCI in the AC source (where the AC enters the house) is enough to protect all the circuit branches. Circuit branches are wall outlets, lamps, air conditions and so on.
How to Install GFCI or RCD to Become Effective in Protecting from Ground Faults and Electric Shock?
For a GFCI or RCD to work, the fault must happen on the load side. This means that all connections must be made after the GFCI or RCD. Any connections made before the GFCI or RCD will not cause the device to trip.
What is the Best Location to Install GFCI or RCD in Household?
In your house, ensure that the GFCI or RCD must be installed at the main AC line coming into your house. All the branch circuits such as wall outlets, lamps, air conditions and so on must be connected to the load side of the GFCI.
Any connection made in the input side of the GFCI or RCD will not be protected during a ground fault. When a person accidentally touches the AC line in the load side of the GFCI or RCD, he will be saved. However, if he touches the AC line before the GFCI or RCD or in the input side of the device, he will be electrocuted!
Installing a GFCI or RCD in your house is a basic requirement for safety. It is not a significant cost adder. Do not save the amount in exchange to you and your family’s safety!