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Understanding GFCIsUnderstanding GFCIs
Developed byNEMA Ground Fault Personnel
Protection Section (5PP)
Developed byNEMA Ground Fault Personnel
Protection Section (5PP)
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2March 1999
What Is to Be Covered?
Electrical shock - why have GFCIs How GFCI’s “Think” Proper installation of a GFCI Wiring Errors Grounded Neutral Detection Testing GFCIs
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3March 1999
Electric Shock
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4March 1999
Occurrences of Electrical Shock
102 - Street & Highway
120 - Farm
120 - Lightning
384 - Industry
474 - Home
Deaths caused by electrical shock 25 Year Average (1960-1985)
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5March 1999
Electrocutions - 1983 to 1993
0
100
200
300
400
500
600
700
800
900
1000
1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993
Total Electrocutions Consumer Product Related
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6March 1999
Effects of Electric Shock
20
15
10
4
.050
.030
.015
.010
.005
.001
4 AMPERES AND OVERHeart Paralysis, Serious Tissue and Organ Burning
.050 AMPS TO 4 AMPS
.1 - .2 Certain Ventricular Fibrillation
.05 - .1 Possible Ventricular Fibrillation
30 mA - Breathing Difficult, Fibrillation in small children15 mA - Muscles “freeze” in 50% of the population>10 mA - Let-Go Threshold5 mA - GFCI Trip Level1 mA - Perception Level
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7March 1999
Electric Shock Prevention System
Isolation (Physical) Insulation Double Insulation Equipment Grounding GFCI
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8March 1999
Normal Circuit Operation
L
N
Equipment GroundingConductor (EGC)
6A
6AEquipment
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9March 1999
Ground - Fault (Indirect Contact)
L
N
EGC
Ground-Fault
.060 A
12A
6A
5.940A
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10March 1999
Ground - Fault (Direct Contact)
L
N
Ground-Fault
.1 A
6A
5.90A
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11March 1999
How GFCIs “Think”?
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12March 1999
How GFCIs Think
Knowing how GFCIs “Think” will enable you to understand why GFCIs must be installed a certain way why GFCIs trip under various circumstances how to logically explain what appears to be illogical
tripping
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13March 1999
The Current “Adding Machine”
Load6A
6A
If the current out = current back, the CT shows no output.
If the current out = current back, the CT shows no output.
L
N
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14March 1999
Current Adding Machine
240VLoad
6A
6A
L
L
120/240VLoad
6A
6A
LN
L
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15March 1999
Current Adding MachineUnder Ground Fault Conditions
Load6A
5.9A
Ground-Fault
.1A
L
N
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16March 1999
The Device is really a DCCI not a GFCI
Don’t change the GFCI acronym… Change how you think about GFCIs
GFCIs are really “Differential Current Circuit Interrupters” not “Ground Fault circuit Interrupters”
Yes… they trip on ground fault caused differential current, but they also trip on other types of differential current as well.
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17March 1999
GFCI Tripping Characteristics
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18March 1999
What’s in the GFCI?(receptacle)
120V Trip Mechanism
Solid state circuitry with
grounded neutral detection
Receptacle face on receptacle type GFCIs
Load terminals
Line Terminals
Push-to-test button
15K resistor
EGC
L
N
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19March 1999
What’s in the GFCI?(circuit breaker)
Trip Solenoid
Solid state circuitry with
grounded neutral detection
Push-to-test button
15K resistor
Line(breaker
jaw)
Neutral(to panel neutral bar)
Load“Hot”
LoadNeutral
CB Trip Mechanism
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20March 1999
What’s in the GFCI?(Plug-in)
Push-to-test button
15K resistor
Load“Hot”
LoadNeutral
Equip.Ground
120V Relay120V Mechanical Latching Device
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21March 1999
Proper Installation of the GFCI
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22March 1999
Standard 120V Connection(GFCI Circuit Breaker)
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23March 1999
Standard 120 Volt Connection(GFCI Receptacle)
120/240Vac Source
N
N L1 L2
Line Terminals Load
Terminals
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24March 1999
240Volt Load - No Neutral
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25March 1999
120/240V Load
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26March 1999
The Adding Machine?
1A
1A
L
N
120V
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27March 1999
Multi-Wire Circuits
1A
1A
Switch Open
0A
One leg of multi-wire circuit in use
120V
120V
120W bulb
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28March 1999
Multi-Wire Circuits
1A
0A
Both legs of multi-wire circuit in use
120V
120V
120W
120W
1A
If what goes out…. Comes back.. The GFCI seeszero total current on the circuit.
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29March 1999
GFCI CB on Multi-Wire Circuits
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30March 1999
GFCI Receptacle on Multi-Wire Circuits
Downstreamreceptacles
N
Downstreamreceptacles
Ø Ø
GFCI Receptacles
Use two GFCI receptacles
T RT R
Separate Neutrals
Junction Box
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31March 1999
Wiring Errors
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32March 1999
This “Old House” Problem
N
GFCI installed on one circuit
Second circuit installed and neutral“stolen” from a close-by circuit
6A
?
6A
L1
L2
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33March 1999
Line and Load Reversal on Receptacles
Receptacle face
Load terminals
Line Terminals
Push-to-test buttonContacts
To DownstreamReceptacles
To Panelboard
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34March 1999
GFCI Circuit Breaker Miswiring
Current does not return through the sensor in the circuit breaker
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35March 1999
Grounded Neutral Detection
L
N
EGC
.30 A
6A
?
?
Neutral groundeddownstream
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36March 1999
Testing a GFCI
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37March 1999
UL GFCI Tests
Every GFCI must pass the following “in-line” manufacturing tests no trip below 4mA (no load) must trip at 6mA (no load) no trip below 4mA (with load) must trip at 6mA (with load) must trip with 2 ohm grounded neutral must trip within 25 ms with a 500 ohm fault must trip with “test button” must not trip with “noise” calibration test at 102V test button at 132V 1500V hi-pot
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38March 1999
GFCI Testers
Why are testers used? verify operation of the GFCI check protection of downstream receptacles
Will not test proper GFCI operation ALL types of improper installation Dangerous on 2-wire circuits
Will test for some types of improper installation line/load reversal which outlets are protected by GFCI reverse polarity presence of the equipment ground
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39March 1999
Push to Test Button
Test button indicates proper functioning of the GFCI
Does NOT indicate proper installation of the GFCI
Push to test
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40March 1999
Testing for Line/Load Reversal
Push the “reset” button on the receptacle Plug a known “test load” into the GFCI receptacle
load could be a nightlight, GFCI tester, circuit tester, etc.
Push the “test” button (if GFCI trips - then the GFCI is properly functioning)
If the “test load” is energized, the GFCI receptacle is improperly installed
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41March 1999
GFCI Testers
Plug-in testers divert current to the equipment grounding conductor
What if there is not equipment ground....? Such as in a 210-7(d)(3) application?
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42March 1999
Summary
GFCIs have contributed to a reduction in the number of deaths due to electric shock
GFCIs look at the current going out and compare it to the current coming back
Avoid common wiring errors - “Think like the GFCI”
Remember that GFCIs detect grounded neutrals downstream - possible source of “nuisance tripping”
Test the GFCI by using the test button and a load