i-gard grounding.pdf

8/10/2019 I-Gard Grounding.pdf

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Arc Flash Hazard: HRG Technology can play a role in prevention

Grounding

Why Ground?



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Industrial Power SystemGrounding Methods

Resistance Grounded



Solidly Grounded



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Solidly Grounded



Solidly Grounded



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Ungrounded


What are the main Hazards withUngrounded/Solidly Grounded?

Ungrounded: Method used to ground first powersystems

Very large transient over-voltage conditions may exist.Insulation not rated, therefore, hazard to personnel andequipment.

Very difficult to locate ground fault.Good chance of second ground fault on a different phase due toprolonged ground fault.

Solidly Grounded: Replaced Ungrounded Systems

Very high ground fault currents.Fault must be cleared, shutting down equipment.Generators may not be rated for ground fault .

Tremendous amount of arc flash / blast energy.Equipment and people are not rated for energy.



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Do others agree?To HRG or to not HRG?

IEEE Std 242-2001 (Buff Book)

Recommended Practice for Protection and

Coordination of Industrial and Commercial Power

Systems

8.2.5Ungrounded low-voltage systems employ ground detectors

to indicate a ground fault. These detectors show the

existence of a ground on the system and identify the faulted

phase, but do not locate the ground, which can be

anywhere on the entire system.

One disadvantage of the solidly grounded 480V systeminvolves the high magnitude of destructive, arcing

ground-fault currents that can occur. However, if thesecurrents are promptly interrupted, the equipment

damage is kept to acceptable levels.


Do others agree?To HRG or to not HRG?

IEEE Std 141-1993 (Red Book)

Recommended Practice for Electric Power

Distribution for Industrial Plants

7.2.4The solidly grounded system has the highest probability of

escalating into a phase-to-phase or three-phase arcing fault,

particularly for the 480V and 600V systems. The danger of

sustained arcing for phase-to-ground fault probability is also

high for the 480V and 600V systems, and low for the 208Vsystems. For this reason ground fault protection shall be

required for system over 1000A. A safety hazard existsfor solidly grounded systems from the severe flash, arc

burning, and blast hazard from any phase-to-ground

fault.



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What causes the Hazards inUngrounded Systems?

System Capacitance

Unable to discharge leading to transient over-

voltages

No direct return path for ground fault current

Prolonged fault conditions due to inability to quickly

locate fault.

NEC 250.21(B) Ground Detectors. Ungroundedalternating current systems as permitted in

250.21(A)(1) through (A)(4) operating at not less than

120 volts and not exceeding 1000 volts shall have

ground detectors installed on the system.


Ungrounded Systems

Ungrounded systems do not have an intentional

connection from the source generator or transformer to

ground

Typically a three wire delta system.

Can be a four wire system where the source neutral is

not connected to ground.

BA

C

BA

C



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Ungrounded Systems

Unintentionally grounded through system capacitance Such as cables, transformers, motors, surge suppressors, etc.

Total Capacitive Current

cbI + I + Ia0 =

Iab

IIc

3 Load

480V Delta Source

BA

C

cc c

c c c

caX

cXbccX

l-nV

a,b,cI =Xa,b,c

cc

cX = 277ohms

(typical)

a,b,c

[120 apart]

277V

Ground 0V


C

A B

480V Delta Source

3 Load

IfcI Ib aIc c c

cXccbX

cXa

Ground FaultsUnintentionally grounded through system capacitance

Such as cables, transformers, motors, surge suppressors, etc.

480V

Ground A

a

fI = a(I + I + I )b c

l-lV

bI =X


b cXI =c

Vl-l

c c c

c

cc

cc

I = 0A (short-circuited)



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Ground Faults

Ground Fault voltage distribution (voltage rise)

A

BC

G

C B

A

A

BC

G

a a a

ag

bg

cg

ng

N

N

N

ng

cg

bg

ag ag

bg

cg

ng

V = 277VV = 277VV = 277V

V = 0Va = 120

V = 138VV = 367VV = 367V

V = 138Va = 82

V = 0VV = 480VV = 480V

V = 277Va = 60

(0% Fault) (50% Fault) (100% Fault)


Ground FaultsGround Fault current distribution (current rise)

A

BC

G

C B

A

A

BC

G

a a a

ag

bg

cg

I

cg

bg

ag ag

bg

cg

If

IbgcgI

f

fI

Icg bgI

agI

(50% Fault)(0% Fault) (100% Fault)

I = 0.00AI = 1.73A


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Ground Faults


What Causes the Hazards in SolidlyGrounded Systems?

Very low impedance in ground patho High fault current

High fault energy

Ground Fault Coordinationo Long time delays on upstream devices

High fault energy



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Solidly Grounded Systems

Grounded systems have an intentional connection

from the source generator or transformer to ground

Typically a four wire delta system

Can be a three wire system where the source

neutral is not connected to loads

C

BA

N


Solidly Grounded Systems

Intentionally grounded through ground wire

caI + I + I = 0cb cc


IcacbIIcc

A B

C

3 Load

480V Wye Source

nI~0 ~277



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Arcing Grounded Systems

Arcing ground fault: Lower fault current, so OCPDsmay not clear fault. Delay will cause severe equipment

and personnel damage due to tremendous amount of

energy released.

Arcing ground faults are approximately 38% bolted faults.

No transient

over-voltagesHigh fault

current


Locating Ground Faults

Follow the Smoke!

Direct return to source provides over-current conditions thatallow for OCPD to operate, hence, clearing the fault.

OK, IF the following condition is met (and you like repairwork):

Acceptable Damage

People???

Equipment???

Costs??? Who decides???

Not OK, IF

You do not want to accept damaging people

You pay for equipment repairs



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Coordination Problems

Discussed Over-Voltage and Over-CurrentHazards ...

Now discuss time factor

Energy is also a function of time

E = volts * amps * time

Large radial systems have long time delays forcoordination



IG = Fault Current

(A)

Va = 100V (typical)

t = time (cycles)

Typical Transformer:

2500 kVA, 5% impedance

Ground condition Ig=23kA

KWC = 55,200

Acceptable???

24 Cycles

(0.4 seconds)

12 Cycles

(0.2 seconds)

6 Cycles

(0.1 seconds)



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A) 100 Kilowatt CyclesFault location identifiable at close inspection - spit marks on metal andsome smoke marks.

B) 2000 Kilowatt CyclesEquipment can usually be restored by painting smoke marks andrepairing punctures in insulation.

C) 6000 Kilowatt CyclesMinimal amount of damage, but fault more easily located.

D) 10,000 Kilowatt CyclesFault probably contained by the metal enclosure.

E) 20,000 Kilowatt Cycles

Fault probably burns through single thickness enclosure and spreads

to other sections.

F) Over 20,000 Kilowatt Cycles

Considerable destruction.


Hazards with Ungrounded and SolidlyGrounded

UngroundedSolidly-

Grounded

Transient

Over-VoltageHigh Risk Low Risk

Transient

Over-CurrentLow Risk High Risk

FaultLocation

High Risk(Good Luck)

High Risk(Follow Smoke)

High FaultEnergy

High Risk

(2nd Fault)High Risk



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High Resistance Grounding

How does HRG solve these hazards?

Inserts a resistor between neutral and ground

Eliminates 98% of Arc Flash / Blast Injuries

Source(Wye)

HRG C

BA

N



What if no neutral exists (i.e. delta systems)?A grounding transformer is installed (either a zig zag or a wye-

delta) from all three phases to create an artificial neutral for

grounding purposes only.



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Intentionally grounded through neutral resistor

cccIab

IIc

Ir

A B

C

3 Load

480V Wye Source

HRG

N

Vng0V

277V

Ground0V



cccIabIIcIr fI

A B

C

3 Load

HRG

480V Wye Source

N

Compared to Ungrounded Systems (voltage rise)

VngVan

(277V)

Additionalreturnpath,onlydifference

betweenUngroundedandHRG!

Ground A

480V



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Voltages:Normal Operation

Vag = 277VVbg = 277VVcg = 277VVng = 0V

Fault conditionsVag = 0V (Faulted phase is atground potential)Vbg = 480V

Vcg = 480VVng = 277V



cccIabIIcIr fI

A B

C

3 Load

HRG

480V Wye Source

aC

Resistor(HRG)inlieuofwireaddssignificantamountofresistancetolowergroundfaulttoa

predeterminedvaluepreventingdestructivefaultcurrentsandshutdown!

Importance of additional path versus Solidly Grounded



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Compared with Solidly Grounded (current rise)

cccIabIIcIr fI

A B

C

3 Load

HRG

480V Wye Source

Resistorinreturnpath,onlydifference

betweenSolidlyGroundedandHRG! 5.83A3.00A+5.00A

5.83A

Ground A

277

1.73A

55.4

5.00A



Currents:

Normal Operation:

Fault conditions;

AIII c

c

c

b

c

a 0)(

AV

R

VI

r

ng

r 0

4.55

0

AIIIII c

c

c

b

c

arf 0)( 22

AIII cccbca

9000.3)12073.16073.10()(

AV

R

VI

r

ng

r

000.54.55

277

AIIIII c

c

c

b

c

arf 83.5)( 22

AIf

3183.5000.59000.3



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ccbIIcIr

fI

A B

C

3 Load

HRG

480V Wye Source

aCc

Ia

Contactorshortsoutpartoftheresistorchangingtheresistance,hence,changingthecurrent.

Groundfaultcurrentnowisapulsesignalthatallowsfordetection!

Another advantage of return path: ground fault

location



ZSCT

Meter

ZSCT

MeterMeter

ZSCT

0A

55A

50A

50A80A

80A

50A 50A 50A

50A50A55A30A 30A 30A

30A30A30A

MotorMotor

5A

5A0A

5A

HRG

5A

480V Wye Source85A

C

BA

55.4ohms

Method to quickly locate ground faults.

Meter reading willalternate from 5Ato 10A every 2seconds.



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Ground Faults

Damage to Power System Components:

Thermal Damage (Irms)2 * t

Mechanical Damage (Ip)2

Comparison between S-G example and HRG

System Grounding

HRG

S-G

Ground Fault (A)

5

22,800

Damage to Equipment (1 sec)

1 per unit

(22,800 / 5)2 = 20.8x106 p.u.

SolidlyGroundedSystemshave20.8milliontimesmoredamagethanHRG!!!


Do Others Agree?To HRG or not HRG?

IEEE Std 142-1991 (Green Book)

Recommended Practice for Grounding of Industrial and

Commercial Power Systems

1.4.2 Numerous advantages are attributed to grounded

systems, including greater safety, freedom from

excessive system over-voltages that can occur on

ungrounded systems during arcing, resonant or near-

resonant ground faults, and easier detection and location of

ground faults when they do occur.

1.4.3 A system properly grounded by resistance is not

subject to destructive transient over-voltages.



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IEEE Std 142-1991 (Green Book)Recommended Practice for Grounding of Industrial and CommercialPower Systems

1.4.3 The reasons for limiting the current by resistancegrounding may be one or more of the following.

1) To reduce burning and melting effects in faulted electricequipment, such as switchgear, transformers, cables, androtating machines.

2) To reduce mechanical stresses in circuits and apparatuscarrying fault currents.

3) To reduce electric-shock hazards to personnel caused

by stray ground-fault currents in the ground return path.



IEEE Std 142-1991 (Green Book)Recommended Practice for Grounding of Industrial and CommercialPower Systems

1.4.3 The reasons for limiting the current by resistancegrounding may be one or more of the following.

4) To reduce the arc blast or flash hazard to personnel whomay have accidentally caused or who happen to be in closeproximity to the ground fault.

5) To reduce the momentary line-voltage dip occasioned bythe clearing of a ground fault.

6) To secure control of transient over-voltages while at thesame time avoiding the shutdown of a faulty circuit onthe occurrence of the first ground fault (high resistancegrounding).



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IEEE Std 141-1993 (Red Book)

Recommended Practice for Electric Power Distribution for

Industrial Plants

7.2.2 There is no arc flash hazard, as there is with solidly

grounded systems, since the fault current is limited to

approximately 5A.

Another benefit of high-resistance grounded systems is the

limitation of ground fault current to prevent damage toequipment. High values of ground faults on solidly groundedsystems can destroy the magnetic core of rotating machinery.




IEEE Std 242-2001 (Buff Book)

Recommended Practice for Electric Power Distribution for

Industrial Plants

8.2.5 Once the system is high-resistance grounded, over-

voltages are reduced; and modern, highly sensitive ground-fault protective equipment can identify the faulted feeder on

the first fault and open one or both feeders on the secondfault before arcing burndown does serious damage.



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Advanced High Resistance Grounding

SENTINEL/OHMNI DSP

The only SMART HRG

Selective instantaneous feeder

isolation 2nd fault

Mitigate 95-98% of arc flash

incidents - on 1st phase to

ground fault

Assisted fault location

Resistor-integrity monitoring

Time-selective feeder isolation

For more information pick up a product brochure or visit us at www.i-gard.com


Advanced High Resistance Grounding

Avoiding second ground fault



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Advanced High Resistance Grounding:Assisted Fault Location

SENTINEL/OHMNI DSP

The only SMART HRG

Phase Indication

MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG

. . . Several Feeders . . .

MotorMotor


Advanced High Resistance Grounding:Assisted Fault Location

SENTINEL/OHMNI DSP

The only SMART HRG

Feeder Indication

MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor



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Advanced High Resistance Grounding:Avoiding 2nd Ground Fault

SENTINEL/OHMNI DSP

The only SMART HRG

Options for Faulted

Feeder:

1) Alarm Only (No Trip)

OR

2) Trip with Time DelayMODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor


Advanced High Resistance Grounding:Avoiding 2nd Ground Fault

SENTINEL/OHMNI DSP

The only SMART HRG

MODBUS

TRIP TRIP

ZSCTZSCTDSP HRG


MotorMotor

2nd Ground Fault:

Prioritize Feeders

Trips least important,maintaining operation on

most important

Up to 50 Feeders



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Advanced High Resistance Grounding:Neutral Path

SENTINEL/OHMNI DSP

The only SMART HRG

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


Advanced High Resistance Grounding:Neutral Path

SENTINEL/OHMNI DSP

The only SMART HRG

Remote Monitoring:

Tie into Internet

Monitor plant anywherein world

Notify maintenance orlocal qualified electrical

contractor to locateground fault

MODBUS

TRIP TRIP

ZSCTZSCT

DSP HRG


MotorMotor



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Minimizing Second Simultaneous GroundFault


DSP Relay Double Ended Unit SubApplication



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Design Considerations when applyingHRG Systems

HRG is the best Grounding Method available todayoFirst developed with resistor and pulsing contactor (Analog)

oLeast Hazards of all grounding methods, but some still exist

Elevated Voltages

Trained Personnel

Cables, TVSSs, VFDs Insulation

Line-to-Neutral Loads

Phase-to-ground-to-phase Faults

Bypasses neutral grounding resistor

Single-poling circuit breakers

HRG Systems Resolve these Hazards


ASD, UPS



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ASD, UPS

-800

-600

-400

-200

0

200

400

600

800

0 200 400 600 800

AMPLITUDE

TIME

A-G B-G C-G N-G


ASD, UPS



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Design Considerations when ApplyingHRG Systems

NFPA 70: National Electrical Code (2005)

250.36/186 High-impedance grounded neutral systems in which agrounding impedance, usually a resistor, limits the ground-fault currentto a low value shall be permitted for 3-phase ac systems of 480 voltsto 1000 volts where all the following conditions are met:

1) The conditions of maintenance and supervisionensure that only qualified persons service theinstallation.2) Ground detectors are installed on the system.3) Line-to-neutral loads are not served .

Continuity of power is required.(Removed 2008).


Elevated Voltage Hazard

A B

C

3 Load

HRG

480V Wye Source

N

Maintenancemustbeawareofelevatedvoltagesandmethodtolocatefault. IFNOT,DO

NOTHAVETOMAINTAINPOWER. Allowedtotrip(sameasSG)butwithoutthehazards.

277V

0V

0V

480V

480V

Ground A

Properly rated equipment prevents Hazards.



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Elevated Voltage Hazard

A B

C

3 Load

HRG

480V Wye Source

N277V

0V

0V

480V

480V

Ground A

Properly rated equipment prevents Hazards.

Cables,TVSSs,VFDs,etc.andother

equipmentmustberatedforelevated

voltages(UngroundedSystems).


Resolve Cable Insulation Issue

600V CablesInsulation thickness based on mechanical strength, not electricalExtra thickness exceeds 600V electrical ratingTherefore, should be used on 600V systems (HRG)

1000V CablesOnly CSA listed, not UL

5000V CablesNon-shielded: Should be used on 2400V systems (HRG)Shielded: Should be used on 4160V systems (HRG)

8000V CablesNon-shielded: Should be used on 4160V systems (HRG)



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Resolve NEC Requirement

Add small 1:1transformer and solidlyground secondary for 1loads (i.e. lighting).


Resolve NEC Requirement

Advantages of 1:1 transformer

oAbility to retrofit HRG Systems

oOnly ~20% of facility / plant load is 1No neutral required from main source and mainswitchgear (cost savings,)

oSignificantly reduced risk of Arc Blast / Flash Hazard

Only small portion of power system is solidlygroundedLighting Ballasts



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Phase-to-Ground-to-Phase Fault

A B

C

3 Load

HRG

N

480V Wye Source 2000A/3P/65kAIC

Single-poling circuit breaker

During phase-ground-phase fault, single-pole of MCB has to clear the 480Vfault at 65kA. However, per UL 489, single-pole interrupting rating is only at20kAIC. HAZARDOUS?


Phase-to-Ground-to-Phase Fault

For condition to occur, all of the following must be true:1) One fault must be on line side of MCB

Very uncommon2) Low impedance per ground fault

Very uncommonGround faults are usually arcing faults (high impedance faultsper IEEE Std 241, 9.2.5)

3) Faults on different phases4) No other over-current protective devices in fault path

Very uncommonIf so, they will open: eliminating the single-pole interruption

Although remote, HAZARD may still exists:Should be considered during coordination studyDetect ground faults per NEC 250-36



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Resolving Hazard via New Technology

M M M M

DFM DFMDFM DFM DSM

First fault: Sound Alarm

Send signal

Second fault: Open feeder with lower

priority

Third fault: Open feeder with lower

priority


Additional Advancements in HRGSystems


Communications

RS232 (Serial) / RS485 (Modbus, Profibus) / TCP/IP

(Ethernet)

Control and monitor relay remotely via existing

SCADA system

Data Logging & Trending

Most ground faults are intermittent, so when you goto locate via pulse, fault may have cleared

Data log can link ground faults with equipment

starting or running


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Additional Advancements in HRGSystems


Filters Harmonics / Noise / RFMonitors fundamental voltage and current for ground faults

Avoids nuisance trippingMonitors 3rd harmonic voltage and current

High Harmonics may require de-rating resistorLow Harmonics may indicate ground fault near generatorneutral

Zone Selective InterlockingAllows coordination between interlocked protection relays on thesame system

Continuously measures system impedanceElectrical systems are perpetual systemsSystem capacitance may increase causing grounding resistor tobe incorrectly sized

Undesirable, higher fault current may flowTransient over-voltage may occur

To Summarize


Hazards with Ungrounded Systems

Severe transient over-voltages

Cannot efficiently locate ground faults

Hazards with Solidly-Grounded Systems

Very high fault currents and time delays

Causing severe arc blast / flash conditions

Ground fault coordination problems


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To Summarize


High-Resistance Grounded Systems

Best Grounding Method todayResolves Ungrounded hazards

Resolves Solidly-Grounded hazards

Technology continues to make HRG Systemssafer than any other grounding method, but needhelp

Continue to educate and train personnel (engr and maint.)NETA

Update standards and guideline that hinder HRGNEC

NFPA 70E and IEEE 1584

Optical or Pressure Detection


Which one is faster?

Speed of light = 300 x 106 meters/sec

Speed of sound =350 meters/sec

Pressure travels at the speed of sound


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Optical Arc Fault Detection


Relies on detecting a high- intensity light

Fastest method of arc detection

Can issue a command to trip in 1 ms to 7 ms

Detectors are light sensors.

Sensors local to the relay, rely on Fibre optic to

conduct light to the relay

Sensor remote to the relay, are hard wired to the

relay

Pressure Detection


Pressure wave of an arc travels at the speed of

sound

Ralph Lees research shows that arcing faults can

produce 20-1000 psi

Depending on placement of pressure sensors,

detection can be as fast as 8-18 ms

Can still trip in less than 100 ms

Commercially available


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Temperature Detection


Temperature detection is a slower technique to

detect the presence of an arc

This method is more effective as an early detection

system for series faults

Can easily detect loose connections by monitoring

the outgases of products produced by overheating

Distance


Effect on Personal Protective Equipment (PPE)

requirements

Remote operation inherently increases distance

from arc source


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Reduce Incident Energy


Reduce Time

Reduce Bolted Fault Current

Do both.



Activate a current limiting device that will quickly

insert an impedance into the circuit on the

occurrence of an arc fault;

Can reduce the fault current from 50 kA to 0.4 kA in

approximately cycle.


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This conceptual unit will reduce the incident energy

from 5.7 cal/cm2 to 0.9 cal/cm2

Thank You!

Questions?


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I-Gard CorporationHead Office7615 Kimbel St., Unit 1 Phone: 905.673.1553 Toll Free 1.888.737.4787Mississauga, Ontario Canada L5S 1A8 Fax: 905.673.8472 E-mail: [email protected]

Our business hours are Monday to Friday 9:00 a.m. to 5:00 p.m. (EST), if during thosehours you are not able to call us, simply send us an e-mail or leave us a message.

The following members of our inside sales team will be pleased to assist you:

Mr. Doug Gonyou [email protected] Mr. Edmundo Perich [email protected] Mrs. Kavita Raghunathan [email protected] Mr. Upul Herath [email protected]

www.i-gard.com

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