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Selective CoordinationStreamlining System Design

Make the most of your energySM

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22

Smart design and expert guidance for

meeting regulatory requirements

Schneider Electric offers an online Selective Coordination Analysis Tool

that assists users in selecting coordinated breakers on two- or three-tier

systems. Visit www.schneider-electric.us/go/consultantsand look for

the Selective Coordination Analysis Tool under Resources.

Achieving Selective CoordinationDesigning a coordinated electrical distribution system requires knowledge

about the application and circuit breakers being specied. Considering selective

coordination early in the design phase will make the process easier and more

cost-eective.

Conducting a comprehensive short-circuit current study and an overcurrent

coordination study o the electrical power system is the only true method o

achieving selective coordination and equipment protection. The ollowing actions

help simpliy the creation o compliant systems that meet NEC requirements orselective coordination.

Step One: Short-Circuit Current StudyShort-circuit current studies consist o determining the prospective short-circuit currents available at all points in the system, when ed rom both

the normal and alternate power sources. It is helpul to label the nodes othe system with prospective current levels to aid in proper selection o the

circuit breakers.

Step Two: OvercurrentCoordination StudyThe overcurrent coordination study is a comparison o the time it takes the

individual devices to operate when certain levels o normal or abnormal current

pass through the protective devices. The objective o a coordination study isto determine the characteristics, ratings, and settings o overcurrent protective

devices that help ensure the minimum unaulted load is interrupted when

the protective devices isolate a ault or overload anywhere in the system. At

the same time, the devices and settings selected should provide satisactory

protection against overloads on the equipment and interrupt short circuits asrapidly as possible.

Step Three: Short-CircuitCoordination TablesTime-current curves graphically represent the trip time compared to currentlevels or a si ngle protective device. In an electrical system, each circuit is ed by

two or more circuit breakers rom the source to the branch (load). During a ault

which causes a circuit breaker to operate in its instantaneous tripping region,

the circuit breaker adds a dynamic impedance to the circuit. This can aect the

tripping times o upstream circuit breakers and it is impossible to chart theseinteractions on a single time-current curve.

Much like the use manuacturers use ratio tables, Schneider Electric publishes

short-circuit coordination tables to provide advanced inormation on the

interaction o circuit breakers to allow appropriate selection.

These tables should be consulted when ove

the instantaneous trip characteristics o twobreakers is observed on their time-current c

Short-circuit coordination tables provide the

levels o coordination between two circuit b

in a series. Taking into account dynamic imp

where circuit breakers limit the prospective fcurrent that upstream circuit breakers regist

downstream ault, the level o coordination b

two circuit breakers in series may be greater

what their individual time-current characterist

indicate. By using our coordination tables, ahigher levels o selective coordination is pos

Additional Design Resources

From the expertise o our ConsultingEngineer Specialists to downloadable w

papers rom our Design Resource Cente

www.schneider-electric.us/go/consulta

Schneider Electric can provide expert

technical advice to aid in the developmo compliant system designs. Additiona

we provide our circuit breaker tested

combinations to electrical engineer

sotware manuacturers, making them

available on time-current curve overlayseasier device selection and system des

FPO

Dynamic Impedance Effect Between Circuit Breakers

FPO

Certain electrical power system

designs need to ensure that the

selection and setting o protective

devices be done in such a manner as

o cause the smallest possible portion

o the system to be de-energized

due to an abnormal condition. In

a selectively coordinated system,

only the circuit breaker that directly

supplies the overloaded/aulted part

o the system will trip, limiting the

service interruption to only the circuitexperiencing the problem and not

shutting down the larger portion o the

power system.

CB B1, CB PM1Coordination through 2.0 kA

21.6 kA

1CB PM

CB B1

Actual Tested Combination

CB B1 and

CB PM1coordinate

up to theavailable

fault currentof 25 kA,

despite whattime-currentcurves show,

due todynamic

impedanceeffects.

10 100

1K

10K

100K

TIMEINSECONDS

100

1K

10

K

100K10

0.01

0.10

1

10

100

1000

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

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Meeting Selective Coordination

Code RequirementsSchneider Electric delivers the industry experience, technical knowledge,application expertise, market-ocused products, and expert servicesupport to meet your selective coordination design needs.

For more information

Contact us today or your next selective coordination design project at

www.schneider-electric.us/go/consultants

November 2012

2 0 1 2 S c h n e i d e r E l e c t r i c

A l l R i g h t s R e s e r v e d

S c h n e i d e r E l e c t r i c a n d M a k e t h e m o s t o f y o u r e n e

r g y a r e t r a d e m a r k s o w n e d b y S c h n e i d e r E l e c t r i c I n d u s t r i e s S A S o r i t s a f f i l i a t e d

Document Number 0100BR0801R10/12

Schneider Electric USA

1415 S. Roselle Road

Palatine, IL 60067

Tel: 847-397-2600

Fax: 847-925-7500

www.schneider-electric.com/us