# Presentation: 507882

## Understanding GFCIs

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

## 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

## Electric Shock

## 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)**

**Notes:**
High rate on farms due primarily to inadequate wiring  - e.g., the farmer does whatever he needs to do to get power back to the milking machine

Lightning deaths - Farmer is the number one casualty.  Number two is the golfer (what do you expect when the guy holds a metal rod in his hands).

## Electrocutions - 1983 to 1993

**Notes:**
Note the significant decline, half the deaths from consumer related products in the 10 year period.

In fact, if we went back to the 1960 time period, there were 600 consumer product related electrocutions... and today 1/3rd that number.

Do GFCI’s save lives.... You bet!

## Effects of Electric Shock

- 20

- 15

- 10

- 4

- .050

- .030

- .015

- .010

- .005

- .001

- 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

## Electric Shock Prevention System

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

**Notes:**
Isolation
Not possible when using electrical equipment
Insulation
Deteriorates with age
accidentally or deliberately circumvented
Double Insulation
doesn’t protect if equipment is submerged
doesn’t protect cord
Equipment Grounding
deteriorates if not maintained
accidentally or deliberately circumvented
GFCI
newest part of the protection system
does not depend on other elements of the system

## Normal Circuit Operation

- L

- N

- Equipment Grounding
- Conductor (EGC)

- 6A

- 6A

- Equipment

**Notes:**
Current flows on the line conductor from line to the load
Current returns from the load on the neutral
Exactly the same amount of current flows from line to load and returns from load to line

## Ground - Fault (Indirect Contact)

- L

- N

- EGC

- Ground-Fault

- .060 A

- 12A

- 6A

- 5.940A

**Notes:**
When a person contacts the grounded enclosure of equipment with a fault to the enclosure it is called “indirect contact”
The equipment grounding conductor (EGC) is effective
Current divides and flows to ground based on the impedance of the two fault paths
EGC has low impedance ... most of the fault current flows on it
person has comparatively higher impedance ... small amount of fault current flows through the person (may or may not “feel a shock”)

## Ground - Fault (Direct Contact)

- L

- N

- Ground-Fault

- .1 A

- 6A

- 5.90A

**Notes:**
If ECG is absent (cut off prong on plug) then ALL the fault current will flow through the person.  This is called direct contact.
Amount of current determined by impedance of the person
Dry hands or large person has higher impedance than wet hands or small person

## How GFCIs “Think”?

## 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

## The Current “Adding Machine”

- Load

- 6A

- 6A

- If the current out = current back, the CT shows no output.

- L

- N

**Notes:**
Current OUT=Current BACK ... differential current is zero

## Current Adding Machine

- 240V
- Load

- 6A

- 6A

- L

- L

- 120/240V
- Load

- 6A

- 6A

- L

- N

- L

**Notes:**
What about a 240V load... current out equals current back... differential current is zero

Same is true for 120/240V systems... current out equals current back.. Differential current is zero

What about 3 phase?  - same principles apply

## Current Adding MachineUnder Ground Fault Conditions

- Current Adding MachineUnder Ground Fault Conditions

- Load

- 6A

- 5.9A

- Ground-
- Fault

- .1A

- L

- N

**Notes:**
When a ground fault is introduced another return path is established

Current out (6A) does not equal current returning (5.9A) because .1A is returning through the ground path

The “differential current transformer” detects the difference... the differential current signals the electronics.

## 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.

## GFCI Tripping Characteristics

**Notes:**
People are frozen to a circuit at over 10mA and go into fibrillation at over 50mA
UL time/current curve - tripping requirements
at a 6mA fault of SIX seconds or longer
at a 50mA fault of about .4 seconds
NEMA conducted an evaluation of GFCI’s actual tripping characteristics... a typical GFCI will trip
at a 6mA fault of .1 to .04 seconds
at a 50mA fault of less than .02 seconds
Typical GFCI’s are well below the UL requirements

## 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

**Notes:**
Shown here is a GFCI receptacle
Line Terminals where conductors from panelboard are terminated
Double pole mechanical latching trip mechanism actuated by a 120 volt tripping coil
Differential current transformer... detects differential current
Solid state “brain” ask and answers questions
is differential current over 6mA
is duration long enough
screens out momentary surges, RF interference, etc.
closes trip circuit
NOTE the EGC passes through, BUT is NOT involved in the detection circuitry
Grounded Neutral detection circuit to detect a neutral on the load side of the GFCI
Push to test button
test circuit from line through differential transformer
through an 15K or so resister (limits test to about 8mA)
creates a differential current causing the GFCI to “detect and trip”
Receptacle outlets on the LOAD side of the differential transformer
Load side terminals for feed-thru capability

## 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”

- Load
- Neutral

- CB Trip
- Mechanism

**Notes:**
Shown here is a GFCI circuit breaker
Basically the same components as a GFCI receptacle with slightly different configuration

Line- side power derived from breaker connection to the bus
Neutral “pigtail” to connect to the panel neutral bus - this allows the GFCI to be powered as well as pass the load neutral current through the differential transformer
Load terminals for both the “hot” and neutral conductors
Push-to-test button and grounded neutral detection the same as the GFCI receptacle
NOTE NO involvement of the equipment grounding conductor

## What’s in the GFCI?(Plug-in)

- Push-to-test button

- 15K resistor

- Load
- “Hot”

- Load
- Neutral

- Equip.Ground

- 120V Relay

- 120V Mechanical Latching Device

**Notes:**
Principal difference is what is call “open neutral” protection required by UL on plug-in GFCI’s
If you plug a GFCI into a cord or receptacle, it must be de-energized if the circuit into which it is plugged has an open neutral
If this were not required, a fault could exist and the GFCI would not trip
Open neutral protection is accomplished with a normally open 120V relay as the primary trip mechanism or, sometimes in conjunction with a mechanical latching trip device.

## Proper Installation of the GFCI

## Standard 120V Connection(GFCI Circuit Breaker)

- Standard 120V Connection(GFCI Circuit Breaker)

**Notes:**
Breaker “plugged” into the panel for 120V power
Note that the neutral pigtail is connected to the panelboard neutral (S/N).

Load neutral must return to the circuit breaker load neutral terminal

EGC could be unconnected and have no effect on the proper operation of the GFCI

NOTE that the CB performs the standard functions of a breaker as well (overload, short circuit, etc.)

## Standard 120 Volt Connection(GFCI Receptacle)

- Standard 120 Volt Connection(GFCI Receptacle)

- 120/240Vac Source

- N

- N

- L1

- L2

- Line
- Terminals

- Load
- Terminals

**Notes:**
Branch circuit from panelboard connected to the “line” terminals

EGC connected as with any receptacle

Remove label covering “load terminals”

Branch circuit conductors feed other receptacles connected to “load” terminals

Cover downstream receptacles with an appropriately marked plate

## 240Volt Load - No Neutral

**Notes:**
Two pole GFCI Circuit breaker “plugged in” to derive 240V power

NOTE that the breaker pigtail must still be connected to the panelboard neutral (S/N).  The neutral is not used for load current, but must be connected to allow the GFCI to have power and operate

No load connection is made to the breaker load neutral terminal.  GFCI is detecting the differential current between Phase A and Phase B.

Again, the EGC has no effect on the operation of the GFCI

## 120/240V Load

**Notes:**
For a 120/240V load...

The two pole GFCI circuit breaker is connected to the panel buss.

Neutral pigtail is connected to the panelboard neutral

Load neutral is connected to the circuit breaker load terminal

The GFCI looks for differential current between both phases and the neutral.

## The Adding Machine?

- 1A

- 1A

- L

- N

- 120V

**Notes:**
Remember - what goes out must equal what comes back

If it doesn’t there is differential current

A DCCI

## Multi-Wire Circuits

- 1A

- 1A

- Switch Open

- 0A

- One leg of multi-wire circuit in use

- 120V

- 120V

- 120W bulb

**Notes:**
Many are confused about GFCI’s applied on multi-wire circuits.  Remember that as long as what goes out ... comes back, the differential current is zero.

In this drawing with one leg of the circuit open, the current flows out on the “hot” conductor and back on the neutral

## 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 sees
- zero total current on the circuit.

**Notes:**
When the switch is closed.  If the load is balanced the “neutral” has zero and the differential is between the two “hot conductors”

If the load is not balanced, there is some current on the neutral, but the differential current sensor still sees a total of what goes out versus what comes back regardless of the amounts of current in each wire.

## GFCI CB on Multi-Wire Circuits

**Notes:**
The GFCI CB on a multi-wire circuit works in the same fashion.

A two pole GFCI circuit breaker must be used in order to sense the current across both phases and the neutral.  
The “common neutral” comes back to the GFCI circuit breaker load neutral terminal.

The pigtail of the breaker is connected to the panel neutral.

Note that regardless of how the receptacles are loaded, the differential transformer sees the “net” effect of all the current.

## GFCI Receptacle on Multi-Wire Circuits

- Downstream
- receptacles

- N

- Downstream
- receptacles

- Ø

- Ø

- GFCI Receptacles

- Use two GFCI receptacles

- Separate Neutrals

- Junction Box

**Notes:**
Run the multi-wire branch circuit to a junction box
Feed the circuits with separate neutrals from there
Install GFCI receptacle as if it was on a conventional 2-wire grounded circuit
One or both halves of the multi-wire GFCI circuit can be protected

## Wiring Errors

## 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

**Notes:**
GFCI installed on a circuit with a neutral “stolen” from another circuit
Let’s think like a GFCI
6A on L1 switch on L2 open... What does the “adding machine” see going back on the neutral
Since it sees 6A, what does the GFCI do?  Nothing.
Let’s close the switch on L2... with 6A on L1, but what is going back on the neutral?   0A is the correct answer (provided the loads are the same)
What happens?  The GFCI trips
Every time the switch is closed, the GFCI trips
Additionally, this multi-wire circuit doesn’t meet the code rules in 300-3 (keep all the circuit conductors together)

## Line and Load Reversal on Receptacles

- Receptacle face

- Load terminals

- Line Terminals

- Push-to-test button

- Contacts

- To Downstream
- Receptacles

- To Panelboard

**Notes:**
If the branch circuit conductors from the panelboard are connected to the GFCI “load” side terminals and downstream receptacles are connected to the “line” terminals ...  what happens?
When the test button is pushed ... it trips.
When a tester is plugged into the downstream receptacles after the trip... they are de-energized.
When a tester is plugged into the GFCI receptacle... it is NOT de-energized.. Even though the GFCI has tripped.
Downstream receptacles are protected/GFCI receptacles are not protected
This is why there is a label covering the load terminals.

## GFCI Circuit Breaker Miswiring

- Current does not return through the sensor in the circuit breaker

**Notes:**
The most common wiring error on the GFCI circuit breaker...

Not bringing the load neutral back to the breaker load neutral terminal.

If the neutral is brought back directly to the panelboard neutral.  GFCI trips every time the circuit is energized and loaded.

## Grounded Neutral Detection

- L

- N

- EGC

- .30 A

- 6A

- ?

- ?

- Neutral grounded
- downstream

**Notes:**
GFCI will trip if neutral is connected on the load side of the GFCI (receptacle or CB)

Such a connection sets up a parallel neutral/equipment grounding conductor path which, under some circumstances will prevent the GFCI from tripping at over 6mA.

## Testing a GFCI

## 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

## 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

## Push to Test Button

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

- Push to test

## 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

## 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?

## 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