Friday, February 13, 2009

Fuses maintenance

Low Voltage Fuse

Fuse terminals and fuseclips should be examined
for discoloration caused by heat from poor contact or
corrosion. Early detection of overheating is possible through
the use of infrared examination. If evidence of overheating
exists, the cause should be determined.

Cleaning and Servicing.
. All fuse-
holder connections should be tightened. All connections to
specifications should be torqued where available. Fuseclips
should be checked to be sure that they exert sufficient pressure
to maintain good contact. Clips making poor contact
should be replaced or clip clamps used. Contact surfaces of
fuse terminals and clips that have become corroded or oxidized
should be cleaned. Silver-plated surfaces should not be
abraded. Contact surfaces should be wiped with a noncorrosive
cleaning agent. Fuses showing signs of deterioration, such
as discolored or damaged casings or loose terminals, should
be replaced.

Replacement. When replacing fuses, fuse-
holders should never be altered or forced to accept fuses that
do not readily fit. An adequate supply of spare fuses with
proper ratings, especially those that are uncommon, will minimize
replacement problems.

Five characteristics should be considered

when replacing fuses:

Interrupting Rating. Fuses should have an interrupting
rating equal to or greater than the maximum fault
current available at their point of application. Fuses have interrupting
ratings from 10,000 amperes to 300,000 amperes.
Voltage. The voltage rating of the fuse should be at
least equal to or greater than the system voltage.
Current. Fuse ampere ratings should be adequate
for the applications. Ratings are determined by the service,
feeder, and branch-circuit conductors, and the loads served.
Time Delay. Time-delay fuses are especially useful on inductive circuits such as motor and transformer
circuits with inrush currents. Time-delay fuses are the
most commonly used fuses on power distribution and motor

Current Limitation. Fuses are designated as either
current-limiting or non-current-limiting based on their speed
of response during short-circuit conditions. Non-currentlimiting
fuses can be replaced with current-limiting fuses, but
current-limiting fuses should not be replaced with noncurrent-
limiting fuses unless a review of the specific application
is undertaken.

Special Purpose. Special-purpose fuses are used for
supplementary protection of power systems and for utilization
equipment such as power rectifiers, variable speed drives, and
solid-state controllers. High-speed or semiconductor-type
fuses are most commonly used in these applications. These
fuses have unique performance characteristics and physical
size. They should be matched to the utilization equipment.

Fuses Rated Over 1000 Volts.

Introduction. Fuses rated over 1000 volts consist of
many parts, some current carrying and some non-current carrying,
all subject to atmospheric conditions. These fuses can
be current limiting or non-current limiting, sand or liquid
filled, or vented expulsion type.

Inspection and Cleaning.

The fuse should be disconnected and the mounting
de-energized from all power sources before servicing. Insulators
should be inspected for breaks, cracks, and burns. The
insulators should be cleaned, particularly where abnormal
conditions such as salt deposits, cement dust, or acid fumes
prevail, to avoid flashover as a result of the accumulation of
foreign substances on their surfaces.

Contact surfaces should be inspected for pitting,
burning, alignment, and pressure. Badly pitted or burned
contacts should be replaced.

The fuse unit or fuse tube and renewable element
should be examined for corrosion of the fuse element or connecting
conductors, excessive erosion of the inside of the fuse
tube, discharge (tracking) and dirt on the outside of the fuse
tube, and improper assembly that might prevent proper operation.
Fuse tubes or units showing signs of deterioration
should be replaced.

Bolts, nuts, washers, pins, and terminal connectors
should be in place and in good condition. Lock or latch
should be checked.



How to Test Contact-Resistance This test is used to test the
quality of the contacts on switches and circuit breakers. A test
set designed for this purpose is available with direct-scale calibration
in microhms, capable of reading contact resistances of
10 microhms or less. An alternate method is to pass a known
level of direct current through the contact structure and to
measure the dc millivolt drop across the contacts. The data
obtained can then be converted to resistance by applying
Ohm’s law. When millivolt drop data is used directly to describe
contact resistance, it is normally stated in terms of the
continuous current rating of the device. Millivolt drop data
obtained at currents lower than the rated continuous current
rating can be converted to the continuous current rating basis
by multiplying the actual millivolt readings by the ratio of the
continuous rated current to the actual test current. The alternate
method requires a source of at least 100 amperes with a
millivolt meter of approximately 0 mV to 20 mV range. The
contact resistance should be kept as low as possible to reduce
power losses at the contacts with the resultant localized heating,
which will shorten the life of both the contacts and near by insulation.

Maintenance of different parts


General. The major function of the air circuit breaker
depends among other things on correct operation of its contacts.
These circuit breakers normally have at least two distinct
sets of contacts on each pole, main and arcing. Some have an
intermediate pair of contacts that open after the main current-
carrying contacts and before the arcing contacts. When
closed, practically the entire load current passes through the
main contacts. Also, high-overload or short-circuit current
pass through them during opening or closing faulted lines. If
the resistance of these contacts becomes high, they will overheat.
Increased contact resistance can be caused by pitted contact
surfaces, foreign material embedded on contact surfaces,
or weakened contact spring pressure. This resistance will
cause excessive current to be diverted through the arcing contacts,
with consequent overheating and burning. The pressure
should be kept normal, which is usually described in the
manufacturer’s instructions.

Arcing Arching contacts are the last to open; any arcing normally
originates on them. In circuit interruption they carry
current only momentarily, but that current might be equal to
the interrupting rating of the breaker. In closing against a
short circuit, they can momentarily carry considerably more
than the short-circuit interrupting rating. Therefore, there
must be positive contact when they are touching. If not, the
main contacts can be badly burned, interrupting heavy faults.
Failure to interrupt might also result.

On magnetic blow-out air breakers, the arc is
quickly removed from the arcing contacts by a magnetic blowout
field and travels to arcing horns, or runners, in the arc
interrupter. The arcing contacts are expendable and will eventually
burn enough to require replacement.

Rules: The general rules for maintaining contacts on all
types of breakers are as follows:

(1) They should be kept clean, smooth, and in good alignment.
(2) The pressure should be kept normal, as prescribed in the
manufacturers’ literature.

The main contact surfaces should be clean and
bright. Discoloration of the silvered surfaces, however, is not
usually harmful unless caused by insulating deposits. Insulating
deposits should be removed with alcohol or a silver
cleaner. Slight impressions on the stationary contacts will be
caused by the pressure and wiping action of the movable contacts.
Minor burrs or pitting can be allowed, and projecting
burrs can be removed by dressing. Nothing more abrasive
than crocus cloth should be used on the silvered contact surfaces.
Where serious overheating is indicated by the discoloration
of metal and surrounding insulation, the contacts and
spring assemblies should be replaced in line with the manufacturers’

Contact Pressure The circuit breaker should be closed manually to
check for proper wipe, pressure, and contact alignment,
and to ensure that all contacts make at approximately the
same time. The spacing between stationary and movable
contacts should be checked in the fully open position. Adjustments
should be made in accordance with the manufacturers’

Old contacts: Laminated copper or brush-style contacts found on
older circuit breakers should be replaced when badly burned.
Repairs are impractical because the laminations tend to weld
together when burning occurs, and contact pressure and wipe
are greatly reduced. They can be dressed with a file to remove
burrs or to restore their original shape. They should be replaced
when they are burned sufficiently to prevent adequate
circuit-breaker operation or when half of the contact surface is
burned away. Carbon contacts, used on older circuit breakers,
require very little maintenance. However, inadequate contact
pressure caused by erosion or repeated dressing might cause
overheating or interfere with their function as arcing contacts.

The drawout contacts on the circuit breaker and the
stationary contacts in the cubicle should be cleaned and inspected
for overheating, proper alignment, and broken or weak
springs. The contact surfaces should be lightly coated with a contact
lubricant to facilitate ease of the mating operation.

Arc Interrupters.
General. Modern arc interrupters of medium voltage
magnetic blow-out air circuit breakers are built with only inorganic
materials exposed to the arc. Such materials line the
throats of the interrupter and constitute the interrupter plates
or fins, which act to cool and disperse the arc. The insulation
parts of the interrupter remain in the circuit across contacts at
all times. During the time that the contacts are open, these
insulating parts are subject to full potential across the breaker.
The ability to withstand this potential depends on the care
given the insulation.

Particular care should be taken at all times to keep the
interrupter assembly dry. The materials are not affected much
by humidity, but the ceramic material especially will absorb

The interrupters should be inspected each time the
contacts are inspected. Any residue, dirt, or arc products
should be removed with a cloth or by a light sanding. A wire
brush or emery cloth should not be used for this purpose because
of the possibility of embedding conducting particles in
the ceramic material.

Inspection checks An interrupter should be inspected for the following:

(A) Broken or Cracked Ceramic Parts. Small pieces broken
from the ceramics or small cracks are not important. Large
breaks or expansive cracks, however, can interfere with top
performance of the interrupter. Hence, if more than one or
two broken or badly cracked plates are apparent, renewal of
the ceramic stack is indicated.

(B) Erosion of Ceramics. When an arc strikes a ceramic part
in the interrupter, the surface of the ceramic will be melted
slightly. When solidified again, the surface will have a glazed,
whitish appearance. At low and medium currents, this effect is
very slight. However, large-current arcs repeated many times
can boil away appreciable amounts of the ceramic. When this
happens, the ceramic stack assembly should be replaced.

(C) Dirt in Interrupter. While in service, the arc chute assembly
can become dirty. Dust or loose soot deposited on the inside
surface of the arc chute can be removed by vacuuming or
by wiping with cloths that are free of grease or metallic particles.
Deposits can accumulate on ceramic arc shields from
the arcing process. These deposits, from the metal vapors
boiled out of the contacts and arc horns, can accumulate to a
harmful amount in breakers that receive many operations at
low-or medium-interrupting currents. Particular attention
should be paid to any dirt on the plastic surfaces below the
ceramic arc shield. These surfaces should be wiped clean, if
possible, especially if the dirt contains carbon or metallic deposits.
On breakers that operate thousands of times at low and
medium currents, sufficient tightly adhering dirt can accumulate
on the ceramic arc shields to impair proper interrupting
performance. These arc chutes are of a very hard material,
and a hard nonconducting abrasive is necessary for cleaning.
A flexible, abrasive aluminum oxide disc on an electric drill
can be useful in cleaning arc chutes. The ceramic arc shields
might appear dirty and yet have sufficient dielectric strength.
The following insulation test can be used as a guide in determining
when this complete or major cleaning operation is
required. The arc chutes of medium-voltage circuit breakers
should withstand the 60-Hz-rated maximum voltage for one
minute between the front and rear arc horns. In some applications,
circuit breakers can be exposed to overvoltages, in
which case such circuit breakers should have an appropriate
overpotential test applied across the open contacts. Some
manufacturers also recommend a surface dielectric test of the
ceramic surfaces near the contacts to verify adequate dielectric
strength of these surfaces.

Air-puffer devices used to blow the arc up into the
interrupter should be checked for proper operation. One accepted
method is as follows. With the interrupter mounted on
the breaker in its normal position, a piece of tissue paper is
placed over the discharge area of the interrupter and observed
for movement when the breaker is opened. Any perceptible
movement of the paper indicates that the puffer is functioning

Low-voltage air circuit-breaker arc chutes are of relatively
simple construction, consisting primarily of a wedge-
shaped vertical stack of splitter plates enclosed in an insulating
jacket. An arc chute is mounted on each pole unit directly above
the main contacts. Arc interruptions produce erosion of the splitter
plates. The lower inside surfaces of the insulating jackets will
also experience some erosion and sooty discoloration.

Observation: The arc chutes should be removed and examined as
part of routine maintenance. If the splitter plates are seriously
eroded, they should be replaced. If the interior surfaces of the
enclosing jackets are discolored or contaminated with arc
products, they should be sanded with sandpaper or replaced.
Occasionally, the whole arc chute might need replacing, depending
on the severity of the duty.

Operating Mechanism.
General. The purpose of the operating mechanism
is to open and close the contacts. This usually is done by
linkages connected, for most power breakers, to a power-
operating device such as a solenoid or closing spring for
closing, and that contains one or more small solenoids or
other types of electromagnets for tripping. Tripping is accomplished
mechanically, independently from the closing
device, so that the breaker contacts will open even though
the closing device still might be in the closed position. This
combination is called a mechanically trip-free mechanism.
After closing, the primary function of the operating mechanism
is to open the breaker when it is desired, which is
whenever the tripping coil is energized at above its rated
minimum operating voltage.

Check points The operating mechanism should be inspected for
loose or broken parts, missing cotter pins or retaining keepers,
missing nuts and bolts, and binding or excessive wear. All
moving parts are subject to wear. Long-wearing and corrosion-
resistant materials are used by manufacturers, and some wear
can be tolerated before improper operation occurs.

Excessive wear usually results in the loss of travel of
the breaker contacts. It can affect operation of latches; they
could stick or slip off and prematurely trip the breaker. Adjustments
for wear are provided in certain parts. In others, replacement
is necessary.

The closing and tripping action should be quick
and positive. Any binding, slow action, delay in operation, or
failure to trip or latch must be corrected prior to returning to

The two essentials to apply in maintenance of the

operating mechanism are KEEP IT SNUG and KEEP IT FRICTION


Breaker Auxiliary Devices.

The closing motor or solenoid, shunt trip, auxiliary
switches, and bell alarm switch should be inspected for correct
operation, insulation condition, and tightness of connections.

On-off indicators, spring-charge indicators, mechanical
and electrical interlocks, key interlocks, and padlocking
fixtures should be checked for proper operation,
and should be lubricated where required. In particular, the
positive interlock feature that prevents the insertion and
withdrawal of the circuit breaker should be tested while it is
in the closed position.

The protective relay circuits should be checked by
closing the breaker in the test position and manually closing
the contacts of each protective relay to trip the circuit breaker.
Trip devices on low-voltage breakers should be tested periodically
for proper calibration and operation with low-voltage/
high-current test devices. Calibration tests should be made to
verify that the performance of the breaker is within the manufacturer’s
published curves. It is very important that manufacturers’
calibration curves for each specific breaker rating be
used. The fact that current-time curves are plotted as a band of
values rather than a single line curve should be taken into

If the breakers are equipped with static-tripping devices,
they should be checked for proper operation and timing in line
with the manufacturer’s recommendations. Some manufacturers
recommend replacement of electromagnetic devices with
static devices in the interest of realizing more precision and a
higher degree of reliability with the latter devices.

Molded case circuit breaker

A molded-case circuit breaker consists of two basic
parts. One part consists of the current-carrying conductors,
contacts, and appropriate operating mechanism necessary to
perform the circuit-switching functions. The second part consists
of the protective element, including the tripping mechanism
associated therewith.
Application Considerations. Molded-case circuit breakers
will trip from exposure to continuous currents beyond
their ratings, and many trip from unduly high ambient temperatures,
from poor or improper connections, from damaged
plug-in members, and from other conditions that transfer
undue heat to the breaker mechanism. Some of these
conditions violate application specifications. A molded-case
circuit breaker applied in a panelboard should not be loaded
in excess of 80 percent of its continuous current rating, where
in normal operation the load will continue for three hours or

Phase-Fault Current Conditions. A typical molded-case
circuit breaker is equipped with both time-delay and instantaneous
tripping devices. Time-delay tripping has inverse time
characteristics that provide a shorter tripping time for higher
overloads. Under moderate, short-duration overloads, the circuit
breaker allows sufficient time for applications such as motor
starting. Under severe overloads, the circuit breaker will
trip quickly, providing adequate protection for conductors
and insulation. For high-fault currents, the magnetic tripping
device responds to open the circuit breaker immediately.

Ground-Fault Tripping.
It should be recognized that
standard molded-case circuit breakers are not generally
equipped with ground-fault sensing and protection devices
and, therefore, will not normally trip and clear low-level
ground faults that can do immense damage.

Types of Molded-Case Circuit Breakers.

Molded-case circuit breakers can generally be divided
into three major categories depending on the type of trip unit

(1) Factory sealed, noninterchangeable trip
(2) Interchangeable trip
(3) Solid state
Types of Maintenance.
Maintenance of molded-case circuit
breakers can generally be divided into two categories: mechanical
and electrical. Mechanical maintenance consists of inspection
involving good housekeeping, maintenance of proper
mechanical mounting and electrical connections, and manual
operation as outlined in the following paragraphs.

Inspection and Cleaning. Molded-case circuit breakers
should be kept clean of external contamination so that internal
heat can be dissipated normally. Further, a clean case will
reduce potential arcing conditions between live conductors,
and between live conductors and ground. The structural
strength of the case is important in withstanding the stresses
imposed during fault-current interruptions. Therefore, an inspection
should be made for cracks in the case, and replacements
should be made if necessary.

Loose Connections. Excessive heat in a circuit breaker
can cause a malfunction in the form of nuisance tripping and
possibly an eventual failure. Loose connections are the most
common cause of excessive heat. Periodic maintenance
checks should involve checking for loose connections or evidence
of overheating. Loose connections should be tightened
as required, using manufacturers’ recommended torque values.
Molded-case circuit breakers having noninterchangeable
trip units are properly adjusted, tightened, and sealed at the
factory. Those having interchangeable trip units installed away
from the factory could overheat if not tightened properly during
installation. All connections should be maintained in accordance
with manufacturers’ recommendations.

Mechanical Mechanism Exercise. Devices with moving
parts require periodic checkups. A molded-case circuit
breaker is no exception. It is not unusual for a molded-case
circuit breaker to be in service for extended periods and never
be called on to perform its overload-or short-circuit-tripping
functions. Manual operation of the circuit breaker will help
keep the contacts clean, but does not exercise the tripping
mechanism. Although manual operations will exercise the
breaker mechanism, none of the mechanical linkages in the
tripping mechanisms will be moved with this exercise. Some
circuit breakers have push-to-trip buttons that should be
manually operated in order to exercise the tripping mechanism

Friday, February 6, 2009

Frequently asked question on Siemens breakes

Q1) Must the neutral conductor always be connected to the left pole of the ACB?
ANS: On 4-pole circuit-breakers, the neutral conductor must always be connected all on the left.
Otherwise this can cause malfunction of electronic over current release.

Also for non-automatic breakers the neutral always has to be connected to the left phase (due to the design layout of the breaker).

Q2) Are the circuit breakers 3WL able to detect and evaluate:
• Power flow direction?
• Energy flow direction?
• Reverse current flow?
ANS: For the circuit breaker 3WL with electronic tripping unit ETU45B or ETU76B, equipped with the metering function (and voltage transformer) there are extended protective functions available as described in the operating instructions (revision 11.2006) on page 9-100.
These functions can be used for tripping the breaker..
These signals can be displayed on the ETU or made available as floating contact using a digital output module.
Using COM15, these data are accessible via Profibus-DP.

Please be aware that add-up values are resettled when the ETU is disconnected from the auxiliary supply.
Either, these functions - in addition to the delay time as given in the operating instructions - require a response time of about 1 second.

Q3) Is ground-fault detection with 3WL ACB in an IT-System feasible?
ANS: No, it is not possible to achieve ground-fault detection with 3WL ACB in an IT-System.
One has to use an insulation monitoring device for ungrounded networks.
These devices permanently monitor the insulation resistance between the ungrounded single or three-phase AC-supply and a protective conductor and instantaneously signal occurring faults.

We recommend the use of a 3UG insulation monitoring relay for ungrounded networks

Q4) Which maintenance and inspection work can be carried out on a circuit breaker?
ANS) Information about maintenance of the circuit breaker is included in the operating instructions, part 24; also it is given in separate link on this page. Furthermore maintenance and inspection have to be done according the requirements of the switchboard manufacturer or the end user.

Apart from an inspection of the contact system and the arc cutes the circuit breaker basically is maintenance-free.
Within the routine maintenance of the breaker some operations should be done (if there were no switching operations during service) to move the mechanical system.

Independent from this an inspection has to be done after any short-circuit tripping.

Q5) How is ground-fault protection achieved with 3WL circuit-breakers?
ANS: Vectorial summation current formation (method 1 - ETU27B or higher):
The three phase currents and the N-conductor current are measured directly.
The electronic trip unit determines the ground-fault current by means of Vectorial summation current formation for the three phase currents and the N-conductor current.
For 4-pole breakers all CTs are integrated, for 3-pole breakers an additional external CT for the neutral protection is required.
This Rogowski-coil 3WL9111-0AA2.-0AA0 or 3WL9111-0AA3.-0AA0, named -T5 in our documents, has to be wired to terminals X8.9 and 10 as shown in the operating instructions on page 9-124 (edition 11.2006).

Direct measurement of the ground-fault current (method 2 - ETU45B or higher):
A standard transformer with the following data is used for measurement of the ground-fault current:
1200 A/1 A, Class 1, 15 VA (5 A secondary cannot be used).
The transformer can be installed directly in the grounded neutral point of a transformer.
This CT (T6 in our documents) has to be wired to terminals X8.11 and 12; the polarity of the CT does not effect the detection.

Q6) What is the selection criteria for 3WL circuit breaker used for protection of capacitors?
ANS)According to IEC, capacitor units have to be appropriate for operation with a current, whose r.m.s value does not exceed 1.3 x the current flowing at sinusoidal rated voltage and rated frequency.
Furthermore, there is an excess of 15 % over the real value of the energy tolerated.
The maximal current for switching and permanent load for the circuit-breaker shall be 1.5 x nominal rated capacitor current.

Q7)For which applications of ACBs 3WL and MCCBs 3VL are the very low settings of the short time delayed short circuit protection Isd = 1.5 x Ir useful?
ANS)With the aid of the short time delayed short circuit protection Isd it is possible to inhibit trappings as consequences of rejection inrush currents or current spikes.
The specific setting value for Isd depends mainly on the existing application.
There exist generators with very low short circuit currents.
That’s why the short time delayed short circuit protection is provided with setting options down to Isd = 1.5 x Ir.
Furthermore the different Isd-setting options are useful for application specific discrimination considerations.

Q8) How can the ETU (electronic tripping unit) be activated to make the parameterization?
The correct state of the over current release is displayed by the green flashing "Active" LED in heart-beat rhythm.
The "Active" LED starts flashing at:

- current of 80A for frame size I and II,
- current of 150A for frame size III or
- supply of 24V DC auxiliary voltage (from ETU 45B)

When the current exceeds set operating current of over current release (I >= Ir), then yellow "Alarm" LED on the ETU turns on.

The "floating state" between activated and non-activated over current release, at load-currents close to activation-limit of circuit-breaker with ETU45B is displayed with the message "Waiting for Trip Unit".

Due to this, the protection function is ensured even without additional supply voltage. The power supply is provided by the internal current transformer.

Tripping characteristic shows the behavior of the over current release when it is activated via a current before the release. If release (instantaneous short-circuit release) happens immediately after switching on the breaker and due to this, the over current release has not activated yet, the opening time takes about 3 to 10ms longer.

Q9) May the 3WL circuit breaker be used in systems of e. g. 150 HZ or 400 HZ?


The electronic trip unit is designed for 50/60 HZ +-10 % - there were no tests for other frequencies.
At higher frequencies the energy transformers would overload the power supply unit of the ETU.

For 400 HZ the 3WN6 e. g. with trip unit type "B" (but not "N" or "P") can be used.
At this frequency the breaker has to be oversized per 30-40% due to higher eddy-current losses.
May be that for 800 A a breaker 1250 A has to be used, but the setting of the trip unit may remain at 800 A.

Q10) Can a 3WL be used with a voltage above 1000V?
Breakers 3WL12/3WL13 with option A05 (Ue = 1000 V) were tested with a voltage of 1000V +5% =>1050V.
They were not tested at any higher rated voltages Ue.
Therefore it is not allowed to use 3WL breakers for Ue > 1050 V maximum value.

Q11) Are there negative effects to be expected in the operation of 3WL with frequency converters?
Frequency converters generate harmonics and often cable-born noise voltages, mostly common-mode interferences, in the line supply.
Reasons are:
- Low-frequency harmonics caused by 6-pulse supplies.

- Switching reactors on the line side, causing high voltage peaks.
- Switching the IGBT of the converter with very fast switching times.
- Resonant circuits producing high voltage peaks.

Most important is to have EMC-correct design of system configurations:

- Metal cabinet parts must be connected together and must be grounded.
- Signal cables and power cables must be routed separately.
- Always use shielded signal cables.

Low-frequency harmonics are reduced using line reactors.
The electronic system of the ETU calculates the rms-value and is therefore suitable for systems being afflicted with harmonics. The ETU is tested according EN 60947-2, Annex F, including electromagnetic compatibility (EMC) tests, immunity tests and harmonics.

Attention should be paid to the use of frequency converters in IT-systems. In non-grounded line supplies these peaks cannot discharge to earth via PE/PEN conductor.

For ACBs switching frequency converters, especially in IT-systems, we suppose the use of an additional EMC-filter for the ETU. This filter is connected between CT and ETU and reduces noise voltages.

Ordering: Order code Z=F31 or separately by using order-no. 3WL9111-0AK32-0AA0.

Q13) How about the function of the thermal memory?
Thermal memory of 3WL operates in following way:
Shortly before tripping energy value data responding to the actual current flow are read into a clock unit and the time counter of the clock module is resetted.
The buffered clock unit automatically continues operating.
When re-energizing the energy value data as well as the time counter are read out from clock device and converted in values which are interpretable for protection functions.

Does the display of the ETU45B of the circuit breaker 3WL an auxiliary supply to show the measured currents or is the minimum current of 80 A resp. 120 A sufficient ?
If ETU45B is not supplied with auxiliary power while minimum current is present, then background light is deactivated.
The display, however, is active and shows the measured currents.
1 Test device

The handheld test device is used to verify the proper operation of the trip unit, the energy transformers and current transformers as well as the tripping solenoid F5 and the data display

1.1 View

(1) LED for operating voltage indication

(2) Control buttons

(3) 6 LED's to show test results

1.2 Preparations

- Switch off and isolate the circuit-breaker

- Note the setting values of the overload release

- Switch off the earth-fault protection at the over current release, if available (Ig = OFF)

- Setting value IR = 1.0 In

- Interrupt external voltage supply for the electronic system, if available

- Remove the cap from the test connector X25 of the trip

1.3 Connection
: If this sequence is not followed then there may be false tripping& false result

(1) Test socket at the over current release

(2) SUB-D, 40-pole (test device) to socket connector, 40-pole or, with ETU release 2 and higher, SUB-D, 40-pole (test device) to plug connector, 40-pole

(3) Voltage supply

(4) Test device

1.4 Voltage supply

The test device is supplied by an AC power system 230 V or 115 V, 50/60 Hz. The factory setting is 230 V.The changeover switch is located on the printed circuit board inside the test device.

1.5 Operation

The status test starts immediately after connecting the voltage supply and queries the various components and parameters of the trip unit. If the status test is successful, the "ETU STATUS" LED will light up. Otherwise, the "ETU STATUS" LED will flash. It is possible to draw conclusions about the fault cause from the way in which it flashes

The status test can be repeated any time by pressing the "START" button for at least three seconds

Note: The status test is not supported by the over current releases type
ETU25B / ETU27B / ETU15B with identification number less than 253030xxxxxx / 273030xxxxxx / 150704xxxxxx .By pressing the "L" button for app. 3 seconds while switching on the test device power supply, the status test can be skipped for these types

Testing the current transformers

To test the current sensors and energy transformers, press the "START" button briefly (less than 2 sec).

A lit-up LED confirms the proper operation of the corresponding transformer. If an LED flashes, the corresponding transformer is not available, not properly connected or defective. Energy-transformers within CT’s will be tested "OK", if within the limits of 3.5 - 12 ohms and with an inductance above 300 mH.
External earth-fault-CT within the limits of 2.5 - 11 ohms and inductance above 500 mH will be tested similarly.
The length of the testing-period necessary may reach 65 sec.

Result of neutral CT check (for ETU release 2)

A flashing light (1 second on, 1 second off) indicates an error in the area of the transducer of the neutral conductor. The cause of this will either be a missing transducer (e.g. no external neutral CT connected), a missing connection to the transducer or a faulty transducer.
A rapid flashing light (0.5 seconds on, 0.5 seconds off) indicates an error in the area of the energy transformer of the neutral conductor.
The cause of this will either be a missing energy transformer (e.g. if an external neutral CT is connected), a missing connection to the energy transformer or a faulty energy transformer.

Testing the tripping function

Note Over current releases type ETU25B resp. ETU27B with an identification number smaller than 250205 xxxxxx resp. 270206xxxxxx do only react to a test of the L-tripping function.

- Charge the storage spring by hand

- Close

To test the tripping function, press one of the buttons "L", "S", "I", "N" or "G".

The test of tripping function will fail, if the corresponding protective a function of the ETU is not activated or available.

The circuit breaker trips after the time delay plus 2 seconds. The tripping reason can be inquired through the "QUERY" button at the trip unit. The trip cause storage function is available only, if the trip unit had been activated for least 10 min before tripping. Otherwise, the trip unit doesn't have the corresponding protective function or is defective.

Testing the display of the measured values

Once a tripping test has been carried out, if the ETU is not activated, the storage capability of the tripping reasons must be checked using the Query button. To check the correctness of the measured values in the display or via remote transmission press the "I" and "N" keys simultaneously.

A current is successively simulated via the measuring transformers in L1, L2, L3, N and G for 30 sec. The LED of the respective transformer will flash. The test can be considered successful if current is indicated in the corresponding position.

Activation of trip unit

To activate the trip unit press the “N“and “G“keys simultaneously

The trip unit is activated up to pressing another key.
With this function the “T.U.-Error“-LED can be checked, if the status test had finished with the error “Trip unit defective“.

1.6 Finishing

- Restore the noticed settings
- Mount the Cover on X25

Check points during maintenance

Following points need to check while carrying out the maintenance activity of 3WL & 3WT air circuit breaker. This check points are applicable to the ACB of other make also. For better life of your breaker make the annual maintenance contract with us for detail contact

• Closing coil operation

• Shunt release operation

• U/V release operation

• Earth fault release operation

• O/L & S/C release operation

• Cradle mechanism checking

• Cradle plug contact

• Test/service limit switch operation

• Breaker operating mechanism

• Checking of main Fixed & moving contacts

• Auxiliary switch operation

• Switching lock mechanism

• Lamination block checking

• Arc chamber

• Spring charging motor

• Spring charging operation

• Gear box operation

• Insulation test (if required)

• Earth terminal

• Door interlock
Most of the customer doesn’t know which information has to be given while ordering the ACB spares parts & hence they face the difficulty. Firstly it is very important to identify the correct spare part & its exact name which is given in Function of different parts with image link on the page .Generally breaker MLFB is sufficient to order required spares.
But if breaker MLFB is not available then following information is required to order required spares.

Shunt coil: Supply voltage

Closing coil: Supply voltage

Under voltage coil: Supply voltage, whether it is with time delay or without time delay

Motorized operating mechanism: supply voltage

Safety shutter: Number of poles in breaker, Size of breaker i.e. SIZE I SIZE II or SIZE III

Current Transformer: Size of breaker i.e. SIZE I SIZE II or SIZE III

Electronic Trip Unit (ETU): Only type number of ETU

Draw out Mechanism: Number of poles in breaker, Size of breaker i.e. SIZE I SIZE II or SIZE III

Arc chute cover: Number of poles in breaker, Size of breaker i.e. SIZE I SIZE II or SIZE III

Pole Set (Main contact set= 1 Fixed + 1 Moving): Number of poles in breaker, Size of breaker i.e. SIZE I ,SIZE II or SIZE III, Normal Current carrying capacity, Breaking capacity of contacts in kilo Ampere like 65 KA,80 KA,100 KA.

Lamination Block: Size of breaker i.e. SIZE I, SIZE II or SIZE III, Normal Current carrying capacity, Breaking capacity of contacts in kilo Ampere like 65 KA,80 KA,100 KA.

Connecting Terminals on chassis: Size of breaker i.e. SIZE I, SIZE II or SIZE III, Normal Current carrying capacity, Its arrangement i.e. Horizontal, Vertical Etc.

Rating Plug: Current Rating

Recommended spares parts of breakers in stock

Following are the Spares of 3WL & 3WT breaker can be maintained in the stock to minimize the breakdown time.

1) Shunt coil

2) Under voltage coil (With or without delay)

3) Motorized operating mechanism

4) Current transformer

5) E.T.U. Release

6) Draw out mechanism

7) Wiring Harness

8) Arc chute

9) Main contact set (1Fixed+1moving)

10) Lamination blocks or Jaw contacts

11) Connecting terminals

12) Rating plug

Frequency of inspection of breaker

Contact assemblies need to be changed according their condition but at least after

10,000 operation for SIZE 1 & SIZE 2

5000 operation for SIZE 3

1000 operation for SIZE 2 & SIZE3 when used in1000V appliances

(Note: To calculate number of operation of circuit breaker you can use mechanical counter available as a option with circuit breaker)

Switchgear operator has to determine the inspection intervals depending on the breaker application condition but at least

• One time per annum

• After interruption of heavy loads

• After trips by over current release(Electronic trip unit)

Down stream connected non automatic circuit breaker have to be inspected also after 1000 switching operation, an inspection has to be comprise

• Arc chutes & contact system

• Mechanical functionality

• Main & auxiliary circuit, function & connecting quality

• Plausibility control of trip unit setting& correction, if necessary

Draw out guide frame with arc chute covers installed; have to be replaced after no more than three short circuit interruption of circuit breaker

Arc chutes& contact system must be replaced depending upon their condition, but latest after 10000 switching operation

Depending upon the circuit breaker stress it may also be necessary to replace the operating system after 10000switching operation

Sunday, January 25, 2009

Selective Coordination

Selective coordination is the application of circuit protective devices in series such that under overload or fault conditions, only the upstream device nearest the fault will open. The rest of the devices remain closed, leaving other circuits unaffected. In the following example a short circuit has occurred in the circuit fed by branch circuit breaker “C”. Power is interrupted to equipment supplied by circuit breaker “C” only. All other circuits remain unaffected.

Circuit Breaker Coordination
Time current curves are useful for coordinating circuit breakers. If the trip curves of main breaker “A”, feeder breaker “B”, and branch breaker “C” are placed on the same graph, there should be no overlapping, indicating the breakers are coordinated. The three circuit breakers in the following example have been coordinated so that for any given fault value, the tripping time of each breaker is greater than tripping time for the downstream breaker(s). In the following illustration, circuit breaker “C” is set to trip if a 400 amp fault current remains for .04 seconds. Circuit breaker “B” will trip if the fault remains for .15 seconds, and circuit breaker “A” if the fault remains for .8 seconds. If a 400 amp fault occurs downstream from circuit breaker “C”, it will trip first and clear the fault. Circuit breakers “A” and “B” will not trip.

Series-Connected Systems

When selecting circuit breakers, it is extremely important to know both the maximum continuous amperes and the available fault current. NEC® article 110.9 states:

Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

There are two ways to meet this requirement. The first method is to select circuit breakers with individual ratings equal to or greater than the available fault current. This means that, in the case of a building with 65,000 amperes of fault current available at the service entrance, every circuit breaker must have an interrupting rating of at least 65,000 amperes.

The second method is to select circuit breakers with a series combination rating equal to or greater than the available fault current at the service entrance. The series-rated concept requires the main upstream breaker to have an interrupting rating equal to or greater than the available fault current of the system, but subsequent downstream breakers connected in series can be rated at lower values. For example, a building with 65,000 amperes of available fault current might only need the breaker at the service entrance to have an interrupting rating of 65,000 amperes. Additional downstream breakers can be rated at lower values. The series combination must be tested and listed by UL.

Time-Current Curves

Time-current curves:
similar to the one shown on the following page, are used to show how fast a breaker will trip at any magnitude of current. The following illustration shows how to read a time-current curve. The figures along the bottom (horizontal axis) represent multiples of the continuous current rating (In) for the breaker. The figures along the left side (vertical axis) represent time in seconds.
Time in Seconds Multiple of In

To determine how long a breaker will take to trip at a given multiple of In, find the multiple on the bottom of the graph and draw a vertical line to the point where it intersects the curve. Then draw a horizontal line to the left side of the graph and find the time to trip. For example, in this illustration a circuit breaker will trip when current remains at six times In for .6 seconds. Note that the higher the current, the shorter the time the circuit breaker will remain closed. Time-current curves are usually drawn on log-log paper. Many time-current curves also show the bandwidth, tolerance limits, of the curve.
From the information box in the upper right hand corner, note that the time-current curve illustrated on the next page defines the operation of a Siemens MG frame circuit breaker. For this example, operation with an 800 ampere trip unit is shown, but, depending upon the specific breaker chosen, this circuit breaker may be purchased with a 600, 700, or 800 amp continuous current rating.

Overload Protection:
The top part of the time-current curve shows the continuous
Current performance of the circuit breaker. The black line shows the nominal performance of the circuit breaker and the gray band represents possible variation from this nominal performance that can occur even under specified conditions.

Using the example of an MG breaker with an 800 amp continuous current rating (In), note that the circuit breaker can be operated at 800 amps (1.0 times In) indefinitely without tripping. However, the top of the trip curve shows that an overload trip will occur in 10,000 seconds at 1000 amps (1.25 times In). Additionally, the gray area on either side of the trip curve shows the range of possible variation from this response.

Keep in mind that this trip curve was developed based upon predefined specifications, such as operation at a 40°C ambient temperature. Variations in actual operating conditions will result in variations in circuit breaker performance.

Instantaneous Trip:
The middle and bottom parts of this time-current curve show
Instantaneous trip (short circuit) performance of the circuit breaker. Note that the maximum clearing time (time it takes for the breaker to completely open) decreases as current increases. This is because of high-speed contact designs which utilize the magnetic field built up around the contacts. As current increases, the magnetic field strength increases, which speeds the opening of the contacts?

This circuit breaker has an adjustable instantaneous trip point from 3250 to 6500 amps, which is approximately four to eight times the 800 amp continuous current unit rating. This adjustment affects the middle portion of the trip curve, but not the top and bottom parts of the curve. The breaker shown in this example has a thermal-magnetic trip unit. Circuit breakers with solid-state trip units typically have additional adjustments

True RMS Sensing

Some solid state circuit breakers sense the peak values of the current sine wave. This method accurately measures the heating effect of the current when the current sine waves are perfectly sinusoidal. Frequently, however, the sine waves are harmonically distorted by non-linear loads. When this happens, peak current measurement does not adequately evaluate the true heating effect of the current.

Siemens solid state trip unit circuit breakers incorporate true root-mean-square (RMS) sensing to accurately sense the effective value of circuit current. True RMS sensing is accomplished by taking multiple, instantaneous “samples” of the actual current wave shape for a more accurate picture of its true heating value.

The microcomputer in Siemens solid state trip unit breakers samples the AC current waveform many times a second, converting each value into a digital representation. The microcomputer then uses the samples to calculate the true RMS value of the load current. This capability allows these circuit breakers to perform faster, more efficiently and with repeatable accuracy.

Being able to monitor true RMS current precisely is becoming more important in today’s electrical distribution systems because of the increasing number of power electronic devices being used that can distort the current waveform.

Adjustable Trip

Curves One of the key features of solid state trip unit circuit breakers is the ability to make selective adjustments to the circuit breaker’s time-current curve. The time-current curve shown here is for a circuit breaker in the SJD6-SLD6 family.

Solid State Circuit Breaker

Adjustments The type of trip unit included in a circuit breaker determines the specific time-current curve adjustments available. . The following illustration and associated table describes the adjustments available.

Circuit Breaker Ratings

Voltage Rating:
Circuit breakers are rated according to the maximum voltage they can handle. The voltage rating of the circuit breaker must be at least equal to the circuit voltage. The voltage rating of a circuit breaker can be higher than the circuit voltage, but never lower. For example, a 480 VAC circuit breaker could be used on a 240 VAC circuit. A 240 VAC circuit breaker could not be used on a 480 VAC circuit. The voltage rating is a function of the circuit breaker’s ability to suppress the internal arc that occurs when the circuit breaker’s contacts open.
Some circuit breakers have what is referred to as a “slash” voltage rating, such as 120/240 volts. In such cases, the breaker may be applied in a circuit where the nominal voltage between any conductor and ground does not exceed the lower rating and the nominal voltage between conductors does not exceed the higher rating.

Continuous Current Rating
Every circuit breaker has a continuous current rating which is the maximum continuous current a circuit breaker is designed to carry without tripping. The current rating is sometimes referred to as the ampere rating because the unit of measure is amperes, or, more simply, amps.

The rated current for a circuit breaker is often represented as In. This should not be confused with the current setting (Ir) which applies to those circuit breakers that have a continuous current adjustment. Ir is the maximum continuous current that circuit breaker can carry without tripping for the given continuous current setting. Ir may be specified in amps or as a percentage of In.

As mentioned previously, conductors are rated for how much current they can carry continuously. This is commonly referred to as the conductor’s ampacity. In general, the ampere rating of a circuit breaker and the ampacity of the associated conductors must be at least equal to the sum of any non-continuous load current plus 125% of the continuous load current.

Circuit breakers are rated on the basis of using 60° C or 75° C conductors. This means that even if a conductor with a higher temperature rating were used, the ampacity of the conductor must be figured on its 60° C or 75° C rating.

Frame Size:

The circuit breaker frame includes all the various components that make up a circuit breaker except for the trip unit. For any given frame, circuit breakers with a range of current ratings can be manufactured by installing a different trip unit for each rating. The breaker frame size is the highest continuous current rating offered for a breaker with a given frame.

Interrupting Rating:
Circuit breakers are also rated according to the maximum level of current they can interrupt. This is the interrupting rating or ampere interrupting rating (AIR). Because UL and IEC testing specifications are different, separate UL and IEC interrupting ratings are usually provided.
When designing an electrical power distribution system, a main circuit breaker must be selected that can interrupt the largest potential fault current that can occur in the selected application. The interrupting ratings for branch circuit breakers must also be taken into consideration, but these interrupting ratings will depend upon whether series ratings can be applied. Series-connected systems are discussed later in this course.

The interrupting ratings for a circuit breaker are typically specified in symmetrical RMS amperes for specific rated voltages. As discussed in Basics of Electricity, RMS stands for root-mean-square and refers to the effective value of an alternating current or voltage. The term symmetrical indicates that the alternating current value specified is centered around zero and has equal positive and negative half cycles. Siemens circuit breakers have interrupting ratings from 10,000 to 200,000 amps.

Circuit breaker Contact design:

Straight-through contact
The current flowing in a circuit controlled by a circuit breaker flows through the circuit breaker’s contacts. When a circuit breaker is turned off or is tripped by a fault current, the circuit breaker interrupts the flow of current by separating its contacts.
Many circuit breakers use a straight-through contact arrangement, so called because the current flowing in one contact arm continues in a straight line through the other contact arm.

Blow-Apart Contacts:
As an improvement over the straight-through contact design, Siemens developed the blow-apart contact design now commonly used by circuit breakers with higher interrupting ratings. With this design, the two contact arms are positioned parallel to each other as shown in the following illustration. As current flows through the contact arms, magnetic fields are set up around each arm. Because the current flow in one arm is opposite in direction to the current flow in the other arm, the two magnetic fields oppose each other. Under normal conditions, the magnetic fields are not strong enough to force the contacts apart.

When a fault develops, current increases rapidly causing the strength of the magnetic fields surrounding the contacts to increase as well. The increased strength of the opposing magnetic fields helps to open the contacts faster by forcing them apart.

By reducing the time required to open circuit breaker contacts in the event of a fault condition, the blow-apart contact design exposes the electrical equipment protected by the circuit breaker to less damaging heat.

Arc Chute Assembly :
As the contacts open a live circuit, current continues to flow for a short time by jumping the air space between the contacts in the form of an arc. When the contacts open far enough, the arc is extinguished and the current flow stops.

Minimizing the arc is important for two reasons. First, the arc can damage the contacts. In addition, the arc ionizes gases inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases could cause the molded case to rupture.

Circuit breakers commonly use an arc chute assembly to quench the arc. This assembly is made up of several “U” shaped steel plates that surround the contacts. As the arc is developed, it is drawn into the arc chute where it is divided into smaller arcs, which are extinguished faster.

Types of Circuit Breakers

Instantaneous Magnetic Trip-Only Circuit Breakers -
As the name indicates, instantaneous magnetic-trip-only circuit breakers provide short circuit protection but do not provide overload protection. This type of circuit breaker is typically used in motor control applications where overload protection is provided by an overload relay.

For example, in the circuit shown below, a three-pole instantaneous magnetic-trip-only circuit breaker provides short circuit protection while the overload protection for the motor is provided by an overload relay which is part of a motor starter.

Thermal-Magnetic Circuit Breakers,

This type of circuit breaker is called a thermal-magnetic circuit breaker because it has a trip unit that senses heat to detect an overload and senses a magnetic field generated by current to detect a short circuit.

As described in the Circuit Breaker Design portion of this book, this type of circuit breaker trips immediately when a short circuit occurs, but delays an appropriate amount of time before tripping in the event of an overload.

Interchangeable Trip Ckt breaker

The user cannot change the trip unit on many circuit breakers, but some circuit breakers have an interchangeable trip feature. This feature allows the user to change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating
Note: Care must be exercised when considering interchangeable trip circuit breakers. A circuit breaker may be listed by Underwriters Laboratories, Inc.® (UL®) for a specific interchangeable trip unit only. Circuit breaker frames are usually designed to prevent the installation of an improper trip unit size or type.

Current Limiting Circuit Breakers

Many electrical power distribution systems can deliver large Short circuit currents to electrical equipment. This high current can cause extensive damage. Current limiting circuit breakers protect expensive equipment by significantly reducing the current flowing in the faulted circuit.

One way to accomplish current limiting is with an additional set of contacts that feature two moveable arms. These are referred to as dual-pivot contacts, which separate even more quickly than the single-pivot contacts. The dual-pivot contacts are connected in series with the single-pivot contacts. As with the single-pivot design, current flows in opposite directions through the contact arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases.

In an overload condition where current may only be one to six times normal current, the contacts remain closed until the breaker trips. When a short circuit occurs, fault current is extremely high and both sets of contact arms open simultaneously, generating high impedance arcs. The contact gap of the dual-pivot contacts increases more rapidly, therefore generating arc impedance more rapidly. Once the arcs are extinguished, the dual-pivot contacts close on their own due to spring tension. The single-pivot contacts are held open by the breaker mechanism, which will have tripped during the fault and must be manually reset.

The frame on current limiting circuit breakers of this design is extended to allow room for the dual-pivot set of contacts. Siemens current limiting breakers can handle fault currents of up to 200,000 amps.

Solid State Circuit Breakers
Solid state circuit breakers function similarly to thermal-magnetic breakers and have a mechanical breaker mechanism but incorporate a solid state trip unit. The solid state trip unit allows this type of circuit breaker to have programmable features and a greater degree of accuracy and repeatability.

Similar to other types of trip units, the solid state trip unit:

• Senses magnitude of current flow

• Determines when current becomes excessive

• Determines when to send a trip signal to the breaker mechanism

The brains of a solid state trip unit are a microprocessor. Adjustments on the trip unit allow the user to select numerical values the microprocessor will use in performing protective functions. Current sensors mounted in the trip unit monitor the value of load current. The value of current is reduced to a low level and converted to a digital voltage, which is used by the microprocessor. The microprocessor continuously compares the line current with the value set by the user. When current exceeds a preset value for the selected time, the trip unit sends a signal to a magnetic latch. The magnetic latch opens the breaker’s contacts, disconnecting the protected circuit from the power source.

Types of over current Protective Devices

A fuse is a one-shot device. The heat produced by over current causes the current carrying element to melt open, disconnecting the load from the source voltage.

Nontime-Delay Fuse:
Fuses without time delay provide excellent short circuit protection. When an over current situation occurs, heat builds up rapidly in the fuse. Fuses without time delay usually hold 500% of their rating for approximately one-fourth second, after which the current carrying element melts. This means that these fuses cannot be used in motor circuits which often have inrush currents of greater than 500%.

Time-Delay Fuses:
Time-delay fuses provide overload and short circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds to allow motors to start.

Circuit Breaker:

The National Electrical Code® defines a circuit breaker as a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined over current without damage to itself when properly applied within its rating.

Circuit breakers provide a manual means of energizing and de-energizing a circuit. In addition, circuit breakers provide automatic over current protection of a circuit. A circuit breaker allows a circuit to be reactivated quickly after a short circuit or overload is cleared. Unlike fuses which must

be replaced when they open, a simple flip of the breaker’s operating handle restores the circuit.

All circuit breakers perform the following functions:

when an over current occurs.

• MEASURE the amount of over current.

• ACT by tripping the circuit breaker in a time frame necessary to prevent damage to itself and the associated load cables.

Circuit Breaker Operation
In the following illustration, an AC motor is connected through a circuit breaker to a voltage source. When the circuit breaker is closed, a complete path for current exists between the voltage source and the motor allowing the motor to run. Opening the circuit breaker breaks the path of current flow and the motor stops. The circuit breaker will open automatically during a fault, or can be manually opened. After the fault has been cleared, the breaker can be closed allowing the motor to operate.

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. Over current 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.


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 10 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 over current 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 over current. 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.

Short Circuits:

When two bare conductors touch, either phase to phase or phase to ground, a short circuit occurs. When a short circuit occurs, resistance drops to almost zero. Short circuit current can be thousands of times higher than normal operating current.

Ohm’s Law demonstrates the relationship of current, voltage, and resistance. For example, a 240 volt motor with 24 Ω of resistance would normally draw 10 amps of current.

I =E/R = 240/24=10 Amps

When a short circuit develops, resistance drops. If resistance drops to 24 milliohms, current will be 10,000 amps.

I = 240/0.024=10,000 Amps

The heat generated by this current will cause extensive damage to connected equipment and conductors. This dangerous current must be interrupted immediately when a short circuit occurs.