Understanding GFCIs

1
Understanding GFCIs

• Developed by
• NEMA Ground Fault Personnel Protection Section
  (5PP)

2
What Is to Be Covered?

• Electrical shock - why have GFCIs
• How GFCIs Think
• Proper installation of a GFCI
• Wiring Errors
• Grounded Neutral Detection
• Testing GFCIs

3
Electric Shock
4
Occurrences of Electrical Shock
Deaths caused by electrical shock 25 Year
Average (1960-1985)
102 - Street Highway
120 - Farm
120 - Lightning
384 - Industry
474 - Home
5
Electrocutions - 1983 to 1993
6
Effects of Electric Shock
20
4 AMPERES AND OVER Heart Paralysis, Serious
Tissue and Organ Burning
15
.050 AMPS TO 4 AMPS .1 - .2 Certain Ventricular
Fibrillation .05 - .1 Possible Ventricular
Fibrillation
10
4
.050
30 mA - Breathing Difficult, Fibrillation in
small children 15 mA - Muscles freeze in 50 of
the population gt10 mA - Let-Go Threshold 5 mA -
GFCI Trip Level 1 mA - Perception Level
.030
.015
.010
.005
.001
7
Electric Shock Prevention System

• Isolation (Physical)
• Insulation
• Double Insulation
• Equipment Grounding
• GFCI

8
Normal Circuit Operation
6A
N
Equipment
L
6A
Equipment Grounding Conductor (EGC)
9
Ground - Fault (Indirect Contact)
6A
N
L
12A
EGC
5.940A
Ground-Fault
.060 A
10
Ground - Fault (Direct Contact)
5.90A
N
L
6A
Ground-Fault
.1 A
11
How GFCIs Think?
12
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

13
The Current Adding Machine
6A
L
Load
N
6A
If the current out current back, the CT shows
no output.
14
Current Adding Machine
6A
L
240V Load
L
6A
6A
L
120/240V Load
N
L
6A
15
Current Adding MachineUnder Ground Fault
Conditions
Ground- Fault
Load
6A
L
N
5.9A
.1A
16
The Device is really a DCCI not a GFCI

• Dont 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.

17
GFCI Tripping Characteristics
18
Whats in the GFCI?(receptacle)
Solid state circuitry with grounded neutral
detection
Push-to-test button
120V Trip Mechanism
15K resistor
Load terminals
L
N
EGC
Receptacle face on receptacle type GFCIs
Line Terminals
19
Whats in the GFCI?(circuit breaker)
Solid state circuitry with grounded neutral
detection
Push-to-test button
Trip Solenoid
15K resistor
CB Trip Mechanism
Line (breaker jaw)
Load Hot
Load Neutral
Neutral (to panel neutral bar)
20
Whats in the GFCI?(Plug-in)
Push-to-test button
120V Mechanical Latching Device
120V Relay
15K resistor
Load Hot
Load Neutral
Equip.Ground
21
Proper Installation of the GFCI
22
Standard 120V Connection(GFCI Circuit Breaker)
23
Standard 120 Volt Connection(GFCI Receptacle)
120/240Vac Source
N
L1
L2
Line Terminals
Load Terminals
N
24
240Volt Load - No Neutral
25
120/240V Load
26
The Adding Machine?
1A
L
120V
N
1A
27
Multi-Wire Circuits
One leg of multi-wire circuit in use
120W bulb
1A
1A
120V
120V
0A
Switch Open
28
Multi-Wire Circuits
Both legs of multi-wire circuit in use
120W
1A
120W
120V
0A
120V
1A
If what goes out. Comes back.. The GFCI
sees zero total current on the circuit.
29
GFCI CB on Multi-Wire Circuits
30
GFCI Receptacle on Multi-Wire Circuits
N
Ø
Ø
Use two GFCI receptacles
Junction Box
Separate Neutrals
GFCI Receptacles
Downstream receptacles
Downstream receptacles
31
Wiring Errors
32
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
33
Line and Load Reversal on Receptacles
Push-to-test button
Load terminals
Contacts
To Downstream Receptacles
To Panelboard
Line Terminals
Receptacle face
34
GFCI Circuit Breaker Miswiring
Current does not return through the sensor in the
circuit breaker
35
Grounded Neutral Detection
6A
L
?
N
Neutral grounded downstream
EGC
?
.30 A
36
Testing a GFCI
37
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

38
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

39
Push to Test Button

• Test button indicates proper functioning of the
  GFCI
• Does NOT indicate proper installation of the GFCI

Push to test
40
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

41
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?
42
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