Understanding GFCIsUnderstanding GFCIs

Developed byNEMA Ground Fault Personnel

Protection Section (5PP)

Developed byNEMA Ground Fault Personnel

Protection Section (5PP)

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

3March 1999

Electric Shock

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)

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

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

7March 1999

Electric Shock Prevention System

Isolation (Physical) Insulation Double Insulation Equipment Grounding GFCI

8March 1999

Normal Circuit Operation

L

N

Equipment GroundingConductor (EGC)

6A

6AEquipment

9March 1999

Ground - Fault (Indirect Contact)

L

N

EGC

Ground-Fault

.060 A

12A

6A

5.940A

10March 1999

Ground - Fault (Direct Contact)

L

N

Ground-Fault

.1 A

6A

5.90A

11March 1999

How GFCIs “Think”?

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

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

14March 1999

Current Adding Machine

240VLoad

6A

6A

L

L

120/240VLoad

6A

6A

LN

L

15March 1999

Current Adding MachineUnder Ground Fault Conditions

Load6A

5.9A

Ground-Fault

.1A

L

N

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.

17March 1999

GFCI Tripping Characteristics

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

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

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

21March 1999

Proper Installation of the GFCI

22March 1999

Standard 120V Connection(GFCI Circuit Breaker)

23March 1999

Standard 120 Volt Connection(GFCI Receptacle)

120/240Vac Source

N

N L1 L2

Line Terminals Load

Terminals

24March 1999

240Volt Load - No Neutral

25March 1999

120/240V Load

26March 1999

The Adding Machine?

1A

1A

L

N

120V

27March 1999

Multi-Wire Circuits

1A

1A

Switch Open

0A

One leg of multi-wire circuit in use

120V

120V

120W bulb

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.

29March 1999

GFCI CB on Multi-Wire Circuits

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

31March 1999

Wiring Errors

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

33March 1999

Line and Load Reversal on Receptacles

Receptacle face

Load terminals

Line Terminals

Push-to-test buttonContacts

To DownstreamReceptacles

To Panelboard

34March 1999

GFCI Circuit Breaker Miswiring

Current does not return through the sensor in the circuit breaker

35March 1999

Grounded Neutral Detection

L

N

EGC

.30 A

6A

?

?

Neutral groundeddownstream

36March 1999

Testing a GFCI

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

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

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

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

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?

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