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