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Table of Contents
Introduction...............................................................................2
Need.for.Circuit.Protection........................................................4
Types.of.Overcurrent.Protective.Devices..................................8
Circuit.Breaker.Design.............................................................
Types.of.Circuit.Breakers.........................................................23
Circuit.Breaker.Ratings............................................................27
Time-Current.Curves...............................................................29
Selective.Coordination.............................................................32
Series-Connected.Systems.....................................................34
Siemens.Circuit.Breakers........................................................37
Residential.Circuit.Breakers.....................................................38
Panelboard.Circuit.Breakers....................................................46
General.Purpose.Thermal-Magnetic.Circuit.Breakers..............49
Solid-State.Trip.Unit.Circuit.Breakers.......................................56
Internal.Accessories................................................................62
External.Accessories...............................................................65
VL.Circuit.Breakers.................................................................. 7
WL.Circuit.Breakers................................................................79
Review.Answers.....................................................................84
Final.Exam...............................................................................85
quickSTEP.Online.Courses......................................................88
Introduction
Welcome.to.another.course.in.the.STEP.series,.
S
iemens.
T
echnical.
E
ducation.
P
rogram,.designed.to.prepare.our.
distributors.to.sell.Siemens.Energy.&.Automation.products.
more.effectively..This.course.covers.
Basics of Circuit Breakers
.
and.related.products..
Upon.completion.of.
Basics of Circuit Breakers
.you.will.be.able.
to:
•. Explain.the.need.for.circuit.protection
•. Identify.various.types.of.overcurrent.protective.devices
•. Explain.the.basic.electro-mechanical.operation.of.a.circuit.
breaker
•. Identify.various.types.of.Siemens.circuit.breakers
•. Identify.circuit.protection.ratings.for.various.types.of.
Siemens.circuit.breakers
•. Describe.time-current.characteristics.on.a.time-current.
curve
•. Explain.the.benefits.and.function.of.circuit.breaker.
coordination
•. Identify.internal.and.external.circuit.breaker.accessories
2
This.knowledge.will.help.you.better.understand.customer.
applications..In.addition,.you.will.be.better.able.to.describe.
products.to.customers.and.determine.important.differences.
between.products..You.should.complete.
Basics of
Electricity
before.attempting.
Basics of Circuit Breakers
..An.
understanding.of.many.of.the.concepts.covered.in.
Basics of
Electricity
.is.required.for.
Basics of Circuit Breakers
..
If.you.are.an.employee.of.a.Siemens.Energy.&.Automation.
authorized.distributor,.fill.out.the.final.exam.tear-out.card.and.
mail.in.the.card..We.will.mail.you.a.certificate.of.completion.if.
you.score.a.passing.grade..Good.luck.with.your.efforts.
Siemens.is.a.trademark.of.Siemens.AG..Product.names.
mentioned.may.be.trademarks.or.registered.trademarks.of.their.
respective.companies..Specifications.subject.to.change.without.
notice.
National Electrical Code
®.and.
NEC
®.are.registered.trademarks.
of.the.National.Fire.Protection.Association,.Quincy,.MA.0269-
747..
Portions.of.the.
National Electrical Code®
.are.reprinted.with.
permission.from.NFPA.70®,.
National Electrical Code®
.
Copyright.©.2008,.National.Fire.Protection.Association,.Quincy,.
MA.0269..This.reprinted.material.is.not.the.complete.and.
official.position.of.the.National.Fire.Protection.Association.
on.the.referenced.subject,.which.is.represented.only.by.the.
standard.in.its.entirety.
NEMA®.is.a.registered.trademark.and.service.mark.of.the.
National.Electrical.Manufacturers.Association,.Rosslyn,.VA.
22209.
Underwriters.Laboratories.Inc.®.and.UL®.are.registered.
trademarks.of.Underwriters.Laboratories.Inc.,.Northbrook,.IL.
60062-2096.
Other.trademarks.are.the.property.of.their.respective.owners.
3
Need for Circuit Protection
Current and Temperature
.
Current.flow.in.a.conductor.always.generates.heat..The.greater.
the.current.flow,.the.hotter.the.conductor..Excess.heat.is.
damaging.to.electrical.components.and.conductor.insulation..
For.that.reason,.conductors.have.a.rated.continuous.current.
carrying.capacity.or.
ampacity
..Overcurrent.protection.devices,.
such.as.
circuit breakers
,.are.used.to.protect.conductors.from.
excessive.current.flow..These.protective.devices.are.designed.
to.keep.the.flow.of.current.in.a.circuit.at.a.safe.level.to.prevent.
the.circuit.conductors.from.overheating..
Normal Current Flow
Excessive Current Flow
Excessive.current.is.referred.to.as.
overcurrent
..The.
National
Electrical Code® (NEC®)
.defines.overcurrent.as.
any current in
excess of the rated current of equipment or the ampacity of a
conductor
..
It may result from overload, short circuit, or ground
fault
(Article.00-Definitions).
Reprinted with permission from NFPA 70®, National Electrical Code®,
Copyright © 2008, National Fire Protection Association, Quincy, MA 02169.
This reprinted material is not the complete and official position of the
National Fire Protection Association on the referenced subject, which is
represented only by the standard in its entirety.
4
Overloads
.
An.
overload
.occurs.when.too.many.devices.are.operated.on.
a.single.circuit,.or.a.piece.of.electrical.equipment.is.made.to.
work.harder.than.it.is.designed.for..For.example,.a.motor.rated.
for.0.amps.may.draw.20,.30,.or.more.amps.in.an.overload.
condition..In.the.following.illustration,.a.package.has.become.
jammed.on.a.conveyor,.causing.the.motor.to.work.harder.and.
draw.more.current..Because.the.motor.is.drawing.more.current,.
it.heats.up..Damage.will.occur.to.the.motor.in.a.short.time.if.
the.problem.is.not.corrected.or.the.circuit.is.shut.down.by.the.
overcurrent.protector.
Conductor Insulation
.
Motors,.of.course,.are.not.the.only.devices.that.require.circuit.
protection.for.an.overload.condition..Every.circuit.requires.some.
form.of.protection.against.overcurrent..Heat.is.one.of.the.major.
causes.of.insulation.failure.of.any.electrical.component..High.
levels.of.heat.can.cause.the.insulation.to.breakdown.and.flake.
off,.exposing.conductors..
Good Insulation
Insulation Affected by Heat
5
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