Transcript Understanding GFCIs

Understanding GFCIs
Developed by
NEMA Ground Fault Personnel
Protection Section (5PP)
What Is to Be Covered?
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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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Electric Shock
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Occurrences of Electrical Shock
102 - Street & Highway
Deaths caused by electrical shock
25 Year Average (1960-1985)
120 - Farm
120 - Lightning
384 - Industry
474 - Home
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4
Electrocutions - 1983 to 1993
1000
900
800
700
600
500
400
300
200
100
0
1983
1984 1985
1986
1987
Total Electrocutions
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1988 1989
1990
1991 1992
1993
Consumer Product Related
5
Effects of Electric Shock
20
15
10
4
.050
.030
.015
.010
.005
.001
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4 AMPERES AND OVER
Heart 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
children
15 mA - Muscles “freeze” in 50% of the population
>10 mA - Let-Go Threshold
5 mA - GFCI Trip Level
1 mA - Perception Level
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Electric Shock Prevention System
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Isolation (Physical)
Insulation
Double Insulation
Equipment Grounding
GFCI
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Normal Circuit Operation
6A
N
L
Equipment
6A
Equipment Grounding
Conductor (EGC)
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Ground - Fault (Indirect Contact)
6A
N
L
EGC
12A
5.940A
Ground-Fault
.060 A
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Ground - Fault (Direct Contact)
5.90A
N
L
6A
Ground-Fault
.1 A
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How GFCIs “Think”?
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How GFCIs Think
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Knowing how GFCIs “Think” will enable you to
understand
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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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The Current “Adding Machine”
L
6A
Load
N
6A
If the current out = current back,
the CT shows no output.
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Current Adding Machine
L
6A
L
240V
Load
6A
L
6A
N
L
120/240V
Load
6A
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Current Adding Machine
Under Ground Fault Conditions
L
Load
6A
GroundFault
N
5.9A
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.1A
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The Device is really a DCCI not a GFCI
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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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GFCI Tripping Characteristics
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What’s in the GFCI?
(receptacle)
120V Trip
Mechanism
Solid state circuitry
with grounded
neutral detection
Push-to-test button
15K resistor
Load terminals
L
N
EGC
Line Terminals
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Receptacle face
on receptacle
type GFCIs
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What’s in the GFCI?
(circuit breaker)
Trip Solenoid
Solid state circuitry
with grounded
neutral detection
Push-to-test button
15K resistor
CB Trip
Mechanism
Line
(breaker
jaw)
Load
“Hot”
Load
Neutral
Neutral
(to panel neutral bar)
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What’s in the GFCI?
(Plug-in)
Push-to-test button
120V Relay
120V Mechanical
Latching Device
15K resistor
Load
“Hot”
Load
Neutral
Equip.
Ground
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Proper Installation of the GFCI
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Standard 120V Connection
(GFCI Circuit Breaker)
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Standard 120 Volt Connection
(GFCI Receptacle)
120/240Vac Source
N L1 L2
Line
Terminals
Load
Terminals
N
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240Volt Load - No Neutral
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120/240V Load
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The Adding Machine?
1A
L
120V
N
1A
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Multi-Wire Circuits
One leg of multi-wire circuit in use
120W bulb
1A
120V
1A
120V
0A
Switch Open
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Multi-Wire Circuits
Both legs of multi-wire circuit in use
120W
1A
120V
0A
120W
120V
1A
If what goes out…. Comes back.. The GFCI sees
zero total current on the circuit.
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GFCI CB on Multi-Wire Circuits
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GFCI Receptacle on Multi-Wire Circuits
Ø N Ø
Use two GFCI
receptacles
Junction Box
Separate Neutrals
T
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R
Downstream
receptacles
T
R
GFCI Receptacles
Downstream
receptacles
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Wiring Errors
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This “Old House” Problem
GFCI installed on one circuit
6A
L1
N
?
6A
L2
Second circuit installed and neutral
“stolen” from a close-by circuit
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Line and Load Reversal on Receptacles
Push-to-test button
Contacts
Load terminals
To Downstream
Receptacles
To Panelboard
Line Terminals
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Receptacle face
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GFCI Circuit Breaker Miswiring
Current does not
return through the
sensor in the circuit
breaker
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Grounded Neutral Detection
6A
L
N
?
Neutral grounded
downstream
EGC
?
.30 A
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Testing a GFCI
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UL GFCI Tests
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Every GFCI must pass the following “in-line”
manufacturing tests
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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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GFCI Testers
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Why are testers used?
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verify operation of the GFCI
check protection of downstream receptacles
Will not test
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proper GFCI operation
 ALL types of improper installation
 Dangerous on 2-wire circuits
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Will test for some types of improper installation
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line/load reversal
 which outlets are protected by GFCI
 reverse polarity
 presence of the equipment ground
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Push to Test Button
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Test button indicates proper functioning of the
GFCI
Does NOT indicate proper installation of the GFCI
Push to test
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Testing for Line/Load Reversal
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Push the “reset” button on the receptacle
Plug a known “test load” into the GFCI receptacle
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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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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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Summary
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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
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