Replacing 2-wire Ungrounded Receptacles
Death from electrocution can occur in less than 1/6th of a second from
a touch potential of 50 volts with as little as 50 milliamperes (0.005 amperes) current flow through
the human body. To protect against electrical shock and electrocution, it is critical that overcurrent
protection devices or the switch contacts of GFCI devices open in less than 0.17 of a second, Figure 1.
Opening Circuit Overcurrent Protection Device to Clear Line-to-Ground Fault
The danger of an electric shock from energized metal parts of an electrical system
can be removed by the opening of the circuit’s overcurrent protection device (circuit breaker or fuse).
The time it takes the protection device to open and remove dangerous voltage is inversely proportional
to the magnitude of the fault current. This means that the higher the fault current amperes, the less
time it will takes for the overcurrent device to open and remove dangerous voltage. The voltage of
the fault and the impedance of the fault current path* determines the magnitude of the ground-fault
current. This ampere value can be determined by the formula: I = E/Z
I = fault-current, E = Line-to-ground voltage, Z = Impedance of the fault-current
*The impedance of the fault current path equals the sum of the impedances of the power supply; the
phase conductors, the ground-fault, and the fault-current return path, Figure
To quickly remove dangerous voltage from a line-to-ground fault, the
impedance of the fault current path must allow the fault current amperes to rise to a value of at
least 5 times and preferably 10 times the ampere rating of the circuit overcurrent protection device
[250-2(d)]. For example, a 100 ampere protected load is located 175 feet from the panelboard and it
is wired with No. 3. The equipment grounding conductor for this circuit is No. 8, which meets the
NEC requirements as listed in Table 250-122. Under this condition a line-to-ground fault will quickly
be removed, because the impedance of the fault current path* permits the fault current amperes to
rise to at least 5 times the rating of the 100 ampere overcurrent device, Figure
*For simplicity, the impedance of the power supply, the feeder conductors and the fault are
not included in this example.
For the metal parts of an electrical system to be safe from dangerous touch potential, the electrical
system must be effectively grounded so that the overcurrent protection device will open quickly. This
is accomplished by bonding the metal parts of the electrical system together to form the equipment
grounding conductor [250-118] and then bonding the equipment grounding conductor to the power supply
grounded (neutral) conductor at either the service equipment or at the source of a separately derived
system [250-2(d)], Figure 4.
The earth cannot be used to provide the low impedance path to clear a line-to-ground fault because
its impedance (as it relates to a fault current) will never be less than 50 ohms and likely, it will
be above 100 ohms. If the earth were used for equipment grounding, insufficient fault current would
flow (less than 1 ampere for a 120 volt line-to-ground fault) and the circuit’s overcurrent protection
device would remain closed. The result; metal parts of the circuit would remain energized with dangerous
touch potential [250-2(d)].
Author’s Comment: The resistance of a grounding electrode as determined by the 3-pole fall of potential
method is the resistance of the earth between the grounding electrode and the meters voltage test
stake. This value could be 3 ohms or less, but it is not the impedance of the ground-fault return
path and it cannot be used to determine the amperes of a line-to-ground fault. For more information
on this subject go to: www.mikeholt.com/Newsletters/Newsletters.htm, Figure 5/6.
Ground-Fault Circuit-Interrupter (GFCI)
A ground-fault circuit-interrupter is the only protection device that
can be used to protect against electric shock from an energized conductor. In addition, a GFCI protection
device can be used to protect persons against electric shock from energized metal parts that are not
effectively grounded. GFCI protection can be incorporated into receptacles, circuit breakers, cord
sets, and other types of devices, Figure 6/7.
A GFCI protection device operates on the principle of monitoring the
imbalanced current between the ungrounded (hot) and grounded (neutral) conductors. In a typical 2-wire
circuit, the current in amperes returning to the power supply will be the same as the current leaving
the power supply (except for small leakage). If the difference between the current leaving and returning
through the current transformer of the GFCI protection device is 5 milliamperes (+ or – 1 milliampere),
the solid-state circuitry activates the shunt trip feature to open the switching contacts of the GFCI,
thereby de-energizing the circuit, Figure 7/8.
WARNING: Sever electric shock or death can occur if a person touches the energized
(line or hot) and neutral conductor at the same time, even if the circuit is GFCI protected. This
is because the current transformer within the GFCI protection device does not sense an imbalance between
the departing and returning current and the switching contacts remain closed, Figure
WARNING: According to a study (based on data accumulated by the American Society of Home Inspectors)
published in the November/December, 1999 issue of the IAEI News, out of 1,583 GFCI circuit
breakers tested, 21% had failed. Out of 4,585 GFCI receptacles tested, 19% had failed. The failures
were primarily attributed to damage from short circuits and voltage surges (lightning and other transients)
to the metal oxide varistors (MOV) that are used for built-in surge suppression. In areas of high
lighting activity such as Southwest Florida, the failure rate for GFCI circuit breakers was over 57%!
When a GFCI protection device fails, the switching contacts remain closed and the device will continue
to provide power without GFCI protection. A new GFCI receptacle from Leviton Manufactures
de-energized the circuit if the GFCI protection feature fails. This new receptacle, which is listed
by UL, is not yet available in the market place. For more information go to: www.levition.com.
Replacing Receptacles to Meet the NEC
The NEC requires receptacles installed on 15 and 20 ampere branch circuits
to be of the grounding-type and it requires the grounding contacts of those receptacles to be effectively
grounded to the branch circuit equipment grounding conductor [210-7]. However, the Code allows
the installation of any of the following installations when replacing a 2-wire nongrounding-type receptacle
where no ground exists in the outlet box [210-7(d)(3)], Figure 9/10:
(a) Replace the 2-wire receptacle with another 2-wire receptacle.
(b) Replace the 2-wire receptacle with a GFCI-type receptacle and marked
the receptacle with the words “No Equipment Ground.”
(c) Replace the 2-wire receptacle with a grounding-type receptacle
where protected by a GFCI protection device (circuit breaker or receptacle). Since the grounding terminals
for the receptacles are not grounded, the receptacles must be marked with the words “GFCI Protected”
and “No Equipment Ground.”
A grounding-type receptacle that is GFCI protected without an equipment
grounding conductor is a safer installation than a grounding-type receptacle with an equipment
grounding conductor (if GFCI protection is not provided). This is because the GFCI protection device
will clear a ground-fault when the fault-current is 5 milliamperes (+ or – 1 milliampere), which is
less than the current level necessary to cause serious electric shock or electrocution, Figure 10/11.
A grounding-type receptacle without a ground is a safe installation
as long as the GFCI protection circuitry within the device has not failed from shorts and voltage
transients. To insure proper GFCI protection, test the GFCI monthly in accordance with the manufactures
instructions and if the GFCI test does not operate properly, replace the GFCI protection device.
Author’s Comment: The equipment grounding conductor serves no purpose in the operation of a
GFCI protection device, and therefore it has no effect on the function of the GFCI test-button.
Note: When GFCI protection is not provided, Section 250-130(c) allows nongrounding-type
receptacle to be replaced with a grounding-type receptacle at an outlet box that does not contain
an equipment grounding conductor, if the grounding contacts of the receptacle are bonded to any one
of the following locations, Figure 11/12:
(1) Grounding electrode system [250-50]
(2) Grounding electrode conductor
(3) Panelboard equipment grounding terminal
(4) Grounded service conductor
See Section 250-146 on the proper method of grounding receptacles and
see Section 250-148 on the proper method of terminating equipment grounding conductors within receptacle
Author’s Comment: The orientation of the grounding terminal on a receptacle is not specified
in the NEC. The ground terminal can be up, down, right or left. Proposals to the NEC to specify the
mounting position of the grounding terminal were all rejected.