Effective System Grounding

8/12/2019 Effective System Grounding

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Effective System Grounding

Presenter:

John DeDad


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Todays Agenda

Why the Concern over Ground Faults?

Electrical Grounding Options

Upgrading from Ungrounded to HighResistance Grounded

Application of Resistance Grounding

Advances in High Resistance Grounding

The Concerns with Solidly GroundedSystems

Controlling Time and Current to MinimizeHazard
http://www.i-gard.com/index.htm


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Why the Concern overGround Faults?


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0

5

10

15

20

25

30

35

40

0-99 100-199 200-299 300-399 400-499 500-599 1m - 1.9m over 2m

Industrial Commercial

Commercial locations such ashotels, universities and shoppingmalls = 72 incidentsManufacturing locations = 156

incidentsTotal occurrences = 228

Industrial Losses:

$ 120,000,000 total$ 769,230 average$ 120,000,000 totalCommercial losses:

$ 60,000,000 total$ 833.300 average

Total losses (industial andcommercial) $ 180,000,000


All figures correspond to losses associated withelectrical ground faults that occurredover a seven year period and were reported byone leading US based Insurance Company .

Losses related to ground faults:


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Business Interruption Costs Cost / Hour 12 Hours

Automotive $15,000 $ 180,000Food and Beverage $ 16,420 $ 197,040Plastic and Moulding $ 7,600 $ 91,200Machinery and Equipment $ 24,700 $ 296,400Metals / Mining $ 24,300 $ 291,600Ticket Reservations $ 72,000 $ 864,000

Property and Equipment Damage

Property Damage from $10,000 to $ 2,000,000

NFPA Equipment Average $ 46,720

Medical Costs and Miscellaneous

Employee Lost Time $ 85,200OSHA Fines $ 20,000 - $100,000

Range of Losses

Minor incident $ 50,000Major incident $ 3,000,000

http://www.mgalive.com/images/workers-compensation-200x200.gif


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Least Effective Most Effective

Protection Prevention

Personal Protection

Wearing of PPE

Administration

Electrical Safety Training

Awareness

Hazard Category Labels

Engineering Controls

Arc Resistant Switchgear

Substitution

Reduction of Time or Fault

Current Available

Elimination

High Resistance Grounding

WebinarFocus


Risk Control of Ground Faults and Arc Faults


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Todays Agenda









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Ungrounded

Solidly Grounded

Resistance Grounded


Industrial Power System Grounding Three Methods


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Popular for LV systems to 1950s

No intentional connection to

ground

Less than 2A ground fault current

=>no shutdown

Feeders must be de-energized to

locate ground fault Susceptible to voltage buildup 6-9

times above ground on

intermittent arcing faults


Ungrounded Systems


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No ground fault:

A

B

C

IC0IC0 IC0

GND

277 V line-to-ground




System Charging Current Definition


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Neutral rises above ground to rated L-N voltage (277V)

Unfaulted phases rise above ground to rated L-L voltage (480V)

N

60

B

C A

N

120

B

C A

G

No Ground Fault

Full Ground Fault on Phase B

VA-G= 277 V

VB-G= 277 V

VC-G= 277 V

VN-G= 0 V

VA-G= 480 V

VB-G

= 0 V

VC-G= 480 V

VN-G= 277 V

N

B

C A

Partial (50%) Ground Fault on Phase B

VA-G= 367 V

VB-G= 138 V

VC-G= 367 V

VN-G= 138 V

G

82


Voltage Rise on Ground Fault


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With bolted ground fault:

Current of 3IC0 flows into fault from un-faulted phases

3IC0 defined as the System Charging Current

A

B

C

480 V line-to-line480 V line-to-ground

0 V line-to-ground


IF= 3IC0IC03 IC03


System Charging Current Definition


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N

Vb

Vc Va

Vbc

Vca

Vab = VagVcb = Vcg

Ib = IaIc= 3Ico

Ia=3Ico60

30

30

Ic=3Ico

60

System Charging Current


System Charging Current


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Current chopping from intermittent arcing ground fault can build up

voltage of system capacitance 6-9 times (2000V)

Ref. Donald Beeman, Industrial Power Systems Handbook,

McGraw-Hill, 1955, pp. 337-338 and 286-289.

A

B

C

IC0IC0 IC0

GND

Transient Voltage Escalation on Ungrounded Systems



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Transient Voltage Escalation on Ungrounded Systems


Photos courtesy of Job Garcia IEEE


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Personal Protection

Wearing of PPE

Administration


Awareness




Substitution

Reduction of Time or FaultCurrent Available

Elimination





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System grounding

the connection of earth ground to theneutral points of current carryingconductors such as the neutral pointof a circuit, a transformer, rotatingmachinery, or a system, either solidlyor with a current limiting device.

Equipment grounding

the connection of earth ground to non

current carrying conductive materialssuch as conduit, cable trays, junctionboxes, enclosures and motor frames.

Grounding

What is System Grounding?



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IEEE Std 142-1991 (Green Book)

1.4.3. The reasons for limiting the current by resistance grounding may be one or more

of the following.

1. to reduce burning and melting effects in faulted electric equipment, such asswitchgear, transformers, cables and rotating machines

2. to reduce mechanical stresses in circuits and apparatus carrying fault currents

3. to reduce electric-shock hazards to personnel caused by stray ground fault

currents in the ground return path

4. to reduce arc blast or flash hazard to personnel who may have accidentallycaused or who happen to be in close proximity to the fault current

5. to reduce the momentary line-voltage dip occasioned by the occurrence and

clearing of a ground fault

Why Consider Grounding your System?



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Todays Agenda









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IEEE Std 242-2001 (Buff Book)

8.2.4. High-resistance grounding helps ensure a ground-fault of known magnitude, helpful for

relaying purposes. This makes it possible to identify the faulted feeder with sensitive ground-fault

relays.

IEEE Std 141-1993 (Red Book)

7.2.2. High-resistance grounding provides the same advantages as ungrounded systems yet

limits the steady state and severe transient over-voltages associated with ungrounded systems.

There is no arc flash hazard [for LV ground faults], as there is with a solidly grounded system,

since the fault current is limited to approximately 5A.

IEE Std 242-1986 Recommended Practice for the Protection and Coordination of

Industrial and Commercial Power Systems 7.2.5 Ungrounded systems offer no advantage over high-resistance grounded systems in terms

of continuity of service and have the disadvantages of transient overvoltages, locating the firstfault and burndowns from a second ground fault. For these reasons, they are being used lessfrequently today than high-resistance grounded systems

High Resistance Grounding IEEE Color Books

Upgrading from Ungrounded toHigh Resistance Grounded


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Resistor limits the ground fault to 10 amps or less

Arc flash hazard on ground faults reduced

Faulted feeder remains in service Ground fault pulse locating provides valuable

troubleshooting tool

To prevent voltage escalation on the phase-to-ground

capacitance during intermittent arcing ground faults, theresistor current must exceed system charging current, IR 3IC0


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Step 1 Requirements for Sizing the NeutralGrounding Resistor

Step 2 Determining the System ChargingCurrent

Step 3 Locate and Install the Neutral GroundingResistor


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What are the Requirements for Sizing the Resistor?


The line-to-ground capacitance associated with system componentsdetermines the magnitude of zero-sequence charging current.

The resistor must be sized to ensure that the ground fault current

limit is greater than the system's total capacitance-to-groundcharging current. If not, then transient over-voltages can occur.

The charging current of a system can be calculated by summing thezero-sequence capacitance or determining capacitive reactance of

all the cable and equipment connected to the system.IEEE-32

The resistor must be built with a low coefficient of resistance/Temperature toensure that it carries the rated current during a ground fault condition

Clause 2.2


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Determining System Charging Current


1. 1. Calculation See Application Guide High Res. Grounding

2. 2. Experience < 2 A 480 V and 600 V 2 7 A 2.4 kV and 4.16 kV < 20 A 13.8 kV

3. 3. Rule of Thumb (Conservative) 1 A / 2000 kVA 480 V and 600 V 1 A / 1500 kVA 2400 V 1 A / 1000 kVA 4160 V and higher

4. 4. Measurement


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Measuring the System Capacitive Charging Current.


It is preferable to measure the magnitude of the charging current on existingpower systems for correct grounding equipment selection. The measured valuesmust be adjusted, to obtain the maximum current, if not all system componentswere in operation during the tests.


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Locating the Resistor


Once we have determined the size requirement for the resistor the next step typicallywould be to connect the current limiting resistor into the system. On a wye-connectedsystem the neutral grounding resistor is connected between the wye-point of thetransformer and ground as shown below.

A

B

C

Neutral

Grounding

Resistor(NGR) NGRGNGR

CG

C

NGR

G

NGR

RIW

II

XR

I

ER

2

0

0

33

3

Where IG= Maximum

Ground Current (A)

Ohms

Ohms

Amperes

Watts


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Locating the Resistor


On a delta-connected system, an artificial neutral is required since no star point existsthis can be achieved by use of a zig-zag transformer as shown

A

BC

N

ZIG-ZAG

Transformer

RNGR

NGRGNGR

CG

C

NGR

G

NGR

G

RIW

II

XR

I

ER

EIVA

2

0

0

3

3

3

VA

Ohms

Ohms

Amperes

Watts


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Todays AgendaWhy the Concern over Ground Faults?








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Where service continuity is vital and where anorderly shutdown is essential

Where arc flash hazard reduction desired

Where Line to Neutral loads can be separated frombalanced phase to phase loads with isolationtransformers.

Where Maintenance personnel proactively locateand repair ground faults

High Resistance Grounding When to Choose

Application of ResistanceGrounding


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System charging current less than resistor current rating.

Rule of thumb charging current 0.5A/1000kVA Choose 5A continuous-duty resistor, tapped 2.5/5A or

5A/10A for ground fault pulse locating

Alarm pick-up level typically 50% resistor let-thru current

Optional individual motor feeder alarm relays set at 10%

pre-alarm setting (monitor winding insulation)

High Resistance Grounding - Low Voltage



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High Resistance Grounding - Medium Voltage


Typically in 5kV and below, but available for systems up to

13.8 kV.

To avoid ground fault escalation into a phase-to-phase fault,system charging current should be 5.5A

(J.R. Dunki-Jacobs)

5A-10A resistor typical (IR 3IC0)

Alarm pick-up level typically 50% resistor let-through current

Optional individual motor feeder alarm relays set at 10% pre-

alarm setting (monitor winding insulation)


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NEMA Std MG 1 Motors and Generators


32.13

A synchronous generator shall be capable of withstanding, without damage, a 30-

second, three-phase short circuit at its terminals

32.34 The neutral of a generator should not be solidly grounded unless the generator has

been specifically designed for such operation. With the neutral solidly grounded, the

maximum line-to-ground fault current may be excessive, and in parallel systems

excessive circulating harmonic currents may be present in the neutrals.


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Generator Grounding IEEE Color Books



Page 452: Solid grounding of a generator neutral is not recommended because this

practice can result in high mechanical stresses and excessive fault damage to the

machines.


1.8.1.

Generators have low zero sequence impedance compared to transformers.

Thus a generator will have higher ground fault current than 3-phase fault current if

solidly grounded.


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Low Resistance Grounding of Parallel Generators


GENERATORS 4160V

100A10 sec

40A

51G

40A1 Sec

G

52

52

51N

52

51N

51G

20A0.6 sec

10A0.25 sec

87G87GD

5A1 min

G

52

51G

87G87GD

5A1 min

G

52

51G

87G87GD

5A1 min


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High Resistance Grounding of Parallel Generators


Must not solidly parallel the neutrals of generators otherwise excessive

triplen harmonic circulating currents

Use zig-zag grounding transformerGENERATORS 600V

600V

VOLTAGE

SENSING

MULTI-FEEDER

ALARM RELAY

15-20A, 3P

100 kAIC

5A

G G G G

To DCS

To DCS


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Todays Agenda





Advances in High ResistanceGrounding




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Safety Concerns with Traditional HRG Systems


Unable to locate the ground fault in a timely manner resulting inexcessive damage

Arc Flash hazard when opening the main switchboard to trace thefault

Second ground fault resulting in destructive phase-to-phase faults

Loss of the Neutral Path resulting in loss of protection

Closing a main-tie onto a ground fault

Intermittent Faults


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MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG

. . . Several Feeders . . .

MotorMotor

Phase Indication


Unable to locate the ground fault in a timely mannerresulting in excessive damage.


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MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor

Feeder Identification


Unable to locate the ground fault in a timely mannerresulting in excessive damage.

Ad i Hi h R i t


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Options for Faulted Feeder:

1) Alarm Only (No Trip)

OR

2) Trip with Time Delay

MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor


Second ground fault resulting in destructivephase-to-phase faults

Ad i Hi h R i t


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Second ground fault resulting in destructivephase-to-phase faults

Photos courtesy of Schneider Electric Chile

Advances in High Resistance


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MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor

2nd Ground Fault:

Prioritize Feeders

Trips least important,

maintaining operation onmost important

Up to 50 Feeders


Second ground fault resulting in destructive

phase-to-phase faults



43/80

System Ground Monitor:

Continually monitors

circuit from Neutral to

Ground

Alarms if OPEN circuit

Alarms if SHORT circuit

MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor


Loss of the Neutral Path resulting in loss of protection



44/80

MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor

Remote Monitoring:

Tie into Internet

Monitor plant anywhere

in world

Notify maintenance or

local qualified electrical

contractor to locate

ground fault


Intermittent Faults


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Todays AgendaWhy the Concern over Ground Faults?







The Concerns with Solidly


46/80

Low impedance bonding system

Bolted Fault --low impedanceat point of fault

277V

High fault current quickly trips breaker

Solidly Grounded Systems

The Concerns with SolidlyGrounded Systems



47/80

Popular for 3-wire LV systemssince 1950s

Solved overvoltage problem

System intentionally grounded(usually neutral)

Faults easy to locate

Permits line-to-neutral lightingloads





48/80

Fault path has two parts:

1. Impedance of the fault, betweenthe live conductor and bonding

system (unpredictable)

2. Impedance of the bondingsystem (low)

BOLTED FAULTS (low impedance)quickly isolate faulted circuit

ARCING FAULTS (high impedance)do not quickly trip the breaker





49/80

Potential for severedamage at point of faultdue to intense heat energyof the arc Arcing groundfaults are more common

than bolted faults



Switchboard of a solidly grounded system at an

amusement park in Ontario Canada



50/80

Low level arcing groundfaults not detected byphase relays or fuses,until fault escalates

Sustained arcing faultscan release intense heat

and mechanical energy,causing severe damageand injury



Switchboard of a solidly grounded system at an

amusement park in Ontario Canada



51/80

2000

10,000 KWC Acceptable

IG = Amperes

Va = 100V

t cycles

Arcing Fault Damage


A) 100 Kilowatt Cycles

Fault location identifiable at close inspection - spit marks on metal

and some smoke marks.

B) 2000 Kilowatt Cycles

Equipment can usually be restored by painting smoke marks and

repairing punctures in insulation.



52/80

C) 6000 Kilowatt Cycles

Minimal amount of damage, but fault more easily located.

D) 10,000 Kilowatt Cycles

Fault probably contained by the metal enclosure.

E) 20,000 Kilowatt Cycles

Fault probably burns through single thickness enclosure andspreads to other sections.

F) Over 20,000 Kilowatt Cycles

Considerable destruction.

Arcing Fault Damage




53/80


8.2.2. One disadvantage of the solidly

grounded system involves the high magnitude

of destructive, arcing ground-fault currents

that can occur.

IEEE Std 141-1993 (Red Book)

7.2.4. The solidly grounded system has the

high probability of escalating into a phase-to-

phase or three-phase arcing fault, particularlyfor the 480V and 600V systems. The danger

of sustained arcing for phase-to-ground

faultis also high for the 480V and 600V

systems, and low or near zero for the 208V

system.

IEEE Color BooksArcing Faults




54/80



Personal Protection

Wearing of PPE

Administration


Awareness




Substitution

Reduction of Time or FaultCurrent Available

Elimination





55/80


56/80

Limit the fault current Limit the time

NGRs limit the fault magnitude.Ground fault relays trip breakersand limit how long a fault lasts

Mitigating Factors

Controlling Time and Current toMinimize Hazard

Controlling Time and Current to


57/80

Low Resistance Grounding: 2.4kV 25kV

Low Resistance Grounding




58/80

2.4kV to 25kV

When system charging current is high and requires high

current rated resistor not suitable for continuous operaion. Limits ground fault current to safe level

Faulted feeder trips in 0.25 1 sec

Resistor sized 50A to 200A

Provides sufficient ground fault current to selectively tripground fault relays

Arc flash hazard on ground faults reduced

Low Resistance Grounding on MV




59/80


1.4.3. The reasons for limiting the current by resistance grounding may be one

or more of the following.

1. to reduce burning and melting effects in faulted electric equipment, such

as switchgear, transformers, cables and rotating machines

2. to reduce mechanical stresses in circuits and apparatus carrying fault

currents

3. to reduce electric-shock hazards to personnel caused by stray groundfault currents in the ground return path

4. to reduce arc blast or flash hazard to personnel who may haveaccidentally caused or who happen to be in close proximity to the fault

current

5. to reduce the momentary line-voltage dip occasioned by the occurrence

and clearing of a ground fault

Low Resistance Grounding IEEE Color Books


Controlling Time and C rrent to


60/80

For ground fault coordination:

Trip settings range from 10% - 40% of resistor let-thru

current Time delay settings range from 0.2 1 sec with 0.3

sec coordination interval

Minimum trip setting (10%) must exceed the systemcharging current to avoid sympathetic tripping of un-faulted feeders

Low Resistance Grounding: Relay Trip Settings




61/80

Designed and tested to IEEE Standard 32

CSA approved for Canadian applications

UL listed for US applications Installed in Canada to CEC rules 10-1100 thru 10-1108

Installed in the US to NEC articles 250.36, 250.186, and 450.5(B)

Resistors rated for :

Line-to-neutral voltage, let-thru current, allowable on time Grounding transformers rated for :

Line-to-line voltage, let-thru current, allowable on time

Codes and Standards for Neutral Grounding Devices


Todays Agenda


62/80

Today s Agenda










63/80



Personal Protection

Wearing of PPE

Administration


Awareness




Substitution

Reduction of Time or Fault

Current Available

Elimination






64/80

Total Clearing Time is Critical

Reduce the Time, Reduce the Damage, Reduce the Incident Energy

-35 ms: no significant damage to persons or 1.27 Cal /cm2

Switchgear, which can often be returnedto use after checking the insulation resistances

- 100ms: small damage, requires cleaning and possibly 3.23 Cal/cm2

some minor repair likely

- 500ms: large damage both for persons and the 18.1 Cal/cm2

switchgear, which must be partly replaced.

The arc burning time is the sum of the time to detect the arc and the time to open the correct breaker.

*Based on 50kA maximum bolted fault current on a 480 volt solidly grounded system.


Arc Detection and Mitigation



65/80

0 100 200 300 400 500 600 700

o

1

2

3

4

5

6

7

8

9

Current kA

An arc is developed within milli-

seconds and leads to the

discharge of enormous amounts

of destructive energy. The

energy in the arc is directly

proportional to the square of theshort-circuit current and the

time the arc takes to develop.

Reduce the Time,

Reduce the Damage,

Reduce the Incident Energy.


Arc damage curve showing arc current versus arc time



66/80

Coordination for

ground faults

difficult unless

branch breakers

have ground fault

relays

NEUTRAL

PHASES

ZERO SEQUENCE CT

OVERLOAD GROUND FAULT

RELAY1200 A PICKUP

1 SECOND DELAY

FEEDER TRIPS ON

OVERLOAD

MAIN BREAKER TRIPS ON

GROUND FAULT!

175 A 400 A


Coordination on Solidly Grounded Systems



67/80

System Charging Current 5 A

100 A10 sec

40 A

1 sec

40 A1 sec

20 A0.6 sec

10 A

0.25 sec

4.16 kV

51G

51N

52

52

52

51N

51N

80 A

1 sec

System Charging Current 10 A

200 A

10 sec

80 A

1 sec

40 A

0.6 sec

20 A

0.25 sec

13.8 kV

51G

51N

52

52

52

51N

51N

Examples of Coordinated Relay Settings




68/80

ZSI offers an excellent solution to this problem. Itimproves arc flash safety upstream in the plantdistribution system without affecting servicecontinuity. ZSI is applied both to phase overcurrentdevices (on the short-timeprotection function), and to ground fault protectivedevices. It is available on electronic trip units and

relays of circuit breakers.

With ZSI, a breaker that senses a fault will trip withno intentional time delay unless it receives a restraintsignal from the breaker immediately downstream. Ifso restrained, the breaker will wait to time out beforetripping. The downstream breaker only sends a

restraint signal upstream if it also senses the fault,i.e. only for faults located downstream of bothbreakers.For the fault at point Y, the Sub-Feeder breaker willrestrain the Feeder breaker; and the Feeder breakerwill restrain the Main breaker. Hence the Main andFeeder will wait to time out. In the meantime, theSub-Feeder breaker will clear the fault.


Zone Selective Interlocking (ZSI),



69/80

The final option for solidly groundedsystems is to employ arc detectiontechnology.

An arc is accompanied by radiation in theform of light, sound, and heat. Therefore,

the presence of an arc can be detectedbyanalyzing visible light, sound waves, andtemperature change.

To avoid erroneous trips, it is normal touse a short-circuit current detector along

with one of the aforementioned arcindicators.

The most common pairing inNorth America is current and light.

gMinimize Hazard

Arc Detection Technology



70/80

Arcing is accompanied by radiation in the form of light, sound, heatand electromagnetic waves as well as an associated pressurewave.





71/80

Two Direct Detection Methods

Pressure Arc Detector

Light Arc Detector

Detecting the pressure wavegenerated by the arc

Detection time 8ms

Detecting the arc flash throughoptical arc detection

Detection time 1ms

gMinimize Hazard




72/80

gMinimize Hazard

Arc Detection and Mitigation Current and Light Schematic



73/80

Optical Sensor Schematic

Minimize Hazard



74/80

Ground Fault Protection, Zone Interlocking Protection (ZSIP) Remote Monitoring and Arc FlashMitigation all in one relay

Ground Fault, ZSIP and Arc Detection Example

Minimize Hazard



75/80

The combined use of high resistance grounding for protection from ground faults and its ability to prohibitthe escalation of the fault, the use ZSI to eliminate the delays associated with time and currentcoordination, and arc mitigation technology including pressure sensors and optical arc detection for phase-to-phase and three-phase arcing faults is an effective engineering approach to minimizing the impact ofground faults and the arc-flash hazard and to establish an effective and safe electrical grounding system.

80%

20%

Arc Flash Mitigation

Prevent - HRG Technology

Protect - ZSIP and OpticalDetection

gMinimize Hazard

Hazard Prevention and Protection



76/80

Arc Detection and MitigationProtection Type Clearance Time Incident Energy

MCGG Over-Current 3.1 seconds 37 Cal / cm2

MCGG Instantaneous 0.45 seconds 5.4 Cal / cm2

Pressure sensor 0.058 seconds 1.3 Cal / cm2

Optical Arc Detection 0.051 seconds 1.2 Cal / cm2

Assumes circuit breaker interrupting time of 0.05 seconds

gMinimize Hazard



77/80



78/80

What type ofgrounding system doyou employ?

Ungrounded

ResistanceGrounded

SolidlyGrounded

Upgrade to HRGwith first fault

time delay,second fault tripand feederidentification

Upgradeto relaywith ZSIPand ArcDetection

Add OpticalArcDetectionRelay

High Low

Add NGRMonitoring

Relay

HighResistanceGrounded

Eliminate Risk

Eliminate Risk

Substitute Risk

Eliminate Risk

HighResistanceGrounded




79/80

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http://www.i-gard.com/roi.asp

Want to view our entire product range, then go to

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Want to read more on Effective System Grounding, then

download the White Paper at

http://ecmweb.com/whitepapers/effective-system-grounding/

Got a question, then Ask the Expert

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80/80

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Effective System Grounding