uit breaker is not properly
maintained, it will derate and require
re-calibration. The NEMA Low-Volt-
age Standard states after a perfor-
mance at or near its interrupting rat-
ing (IR), it is not to be inferred that
the circuit breaker can again meet its
IR without being inspected and if nec-
essary, repaired. If the breaker is put
back into operation without being re-
paired, a serious safety hazard could
exist. These maintenance issues do
not exist with fuses because once
they operate they are always replaced
with a brand new fuse.
Oversizing
Circuit breakers often need to be
oversized (some as much as 1100%
of full load current while still being
within code) to account for the inrush
currents of industrial load applications
(i.e. motors and transformers). Fuses
can be sized closer to load current,
which in turn provides much better pro-
tection for the equipment from damage
due to current overloads and short cir-
cuits.
Type II No Damage
Protection
Type II is one of two damage levels
defined in IEC standard 947-4-1 and
UL 508E. Based on these standards,
only Type II coordination allows mini-
mal damage to either the contacts or
overload relay of a motor starter, as
long as the calibration is not lost and
the device is reservicable after a fault
occurs. Fuses are the only protective
devices available that are current-lim-
iting enough to limit the available short
The proper selection of overcurrent pro-
tective devices for branch circuits is
an important decision affecting the
safety, reliability and efficiency of an
electrical system. In addition to of-
fering a greater degree of protection,
the performance of a properly sized
fuse provides significant advantages
to an electrical system when com-
pared to the performance of a circuit
breaker in an equivalent system.
Initial and Preventative
Maintenance Costs
The initial cost for an electrical distri-
bution system employing circuit
breakers is up to 300% more than an
equivalent system employing fuses.
Breakers are standardized with initial
interrupting capacities (IC) of
10,000AIC, 22,000AIC, 42,000AIC,
64,000 AIC,...etc. As the interrupting
No Damage protection is
fast becoming the industry
standard for both IEC and
NEMA type devices.
Fuses Vs. Breakers
Electrical
System
Protection
Volume 2 circuit current to a non-destructive
level. Most circuit breakers can not
provide Type II"No Damage" protec-
tion. While no device can prevent an
initial fault from occurring, the protec-
tion provided to components by prop-
erly sized fuses will insure that the
components will remain functional af-
ter the fault. No Damage protection
has already been embraced as the
standard (SAE HS1734) for the pro-
tection of both IEC and NEMA rated
devices by the automotive industry
and is quickly becoming the standard
in other industries.
Component Protection
When a fault occurs, fuses will open
within 1/4 and 1/2 cycles (0.00833
seconds). Depending on the applica-
tion, some breakers can take as long
as 17 cycles (0.28322 seconds) to
open. As an example, an uninsulated
ten gauge wire with 30,000 amperes
of current applied to it will reach well
in excess of 1000
o
F in approximately
3/4 of a cycle. Under this moderate
At the inception of a fault, a branch circuit can reach peak available current (Ip) without a current-limiting
protector. The heat produced reaches temperatures that melt conductors as well as insulation, and the
magnetic forces bend conductors and supports. When protected with a current-limiting fuse, however, the
let-through current is only a fraction if Ip, usually opening the fuse in less than one-half cycle. Type 2
coordination assures that no harm to people or damage to equipment results from short-circuit currents.
do not exist because they do not re-
quire maintenance and must be re-
placed after they operate.
Because most fuses have a standard
200,000 AIR (300,000 AIR in some
cases), fuse changes are not required
during service upgrades. In compari-
son, standard breakers have relatively
low AIC (10,000 to 42,000 AIC) and
thus become obsolete and must be re-
placed when the available fault current
from the utility rises.
The city of Chicago is a prime ex-
ample of the importance of interrupt-
ing ratings. For years, the available
fault current supplied from the utility
in Chicago was approximately 39,000
amperes. Construction of skyscrap-
ers began and the local utility
changed the power supply to accom-
modate these new buildings. This re-
sulted in a new available fault current
of 107,000 amperes. The local utility,
however, is not required to and did
not inform any of its customers (who
were using 42,000 AIC circuit break-
ers) of the change. This dangerous
situation is avoided if a fuse with a
200,000 AIR is used.
Single-Phasing Protection
While no device can prevent or elimi-
nate single phasing, an overcurrent pro-
tection device must be able to safely
and effectively disable power to the re-
maining active legs of the circuit. Due
to the design of a circuit breaker, when
one phase is opened, all of the phases
are physically opened. This inherently
prevents extended single-phase opera-
tion. A properly sized fused system,
although operating in a different man-
ner, will achieve the same result.
When one of the phases opens, the
remaining two will always experience
overcurrents. This will cause the other
two fuses to open, preventing power
from reaching the device.
Fuse opens within 1/4 to 1/2 cycle
Current
Point
of fault
Time
Normal
load
current
Heat Energy
Breaker operates here
Peak available current (Ip)
Fuses
Breakers
amplitude of short circuit a fuse will
protect the wire, a circuit breaker will
not.
Interrupting Rating
Circuit breakers, like fuses are only
rated to safely interrupt their maximum
current once. Once a breaker has op-
erated at or near its interrupting rating,
the breaker may not adequately pro-
tect the circuit again without the
proper maintenance or repair. If
proper maintenance and repair is ne-
glected, extensive equipment dam-
age and possible injury to personnel
can occur. With fuses, these issues A system comprised of fuses can be
coordinated with relative ease by
making sure the amperage ratings are
within with designated ratios.
A system utilizing circuit breakers may
coordinate easily in the overload re-
gion of a fault, but the difficulty oc-
curs in the instantaneous or short cir-
cuit region. A pair of breakers operat-
ing in the instantaneous region will
both open due to a short circuit. A rela-
tively minor fault on a branch circuit
containing breakers will frequently
cause all circuit breakers in the cur-
rent path to open (including the main
circuit breaker). This can result in an
entire facility experiencing a power
outage. Compared to these inconve-
niences, and considering the relative
ease of keeping within fuse line-to-
load side ratios, the advantages of
using fuses to achieve selective co-
ordination are significant.
Required Maintenance
Circuit breaker manufacturers state
that breakers require annual mainte-
nance in order to assure their rated
performance levels. This time-con-
suming process is often neglected. An
IEEE survey reported that 40% of
tested circuit breakers are faulty. Fur-
thermore, after five years of use many
breakers, if not properly maintained,
become completely inoperative. One
circuit breaker manufacturer stated
that "Nine times out of ten, circuit
breakers fail because of lack of main-
tenance, cleaning, and lubrication".
Selective
Coordination
A complete power failure cripples pro-
duction and creates a tremendous
amount of lost profit. Isolation of a
faulted current from the remainder of
the facility is becoming mandatory in
todays modern electrical systems. It
is not enough to select protective de-
vices based solely on their ability to
carry the system load current and in-
terrupt the maximum fault current at
their respective levels. A properly engi-
neered system will allow only the pro-
tective device closest to an overcurrent
to open, leaving all upstream equip-
ment in service.
Todays molded case circuit breakers
do not give an option for internal lubri-
cation and calibration. In contrast, fuses
do not require maintenance or calibration.
Robust Designs
Circuit breakers are mechanical de-
vices with moving parts that need con-
stant and consistent maintenance to
keep their calibration. Damage of one
component may result in a breakers
inability to function properly.
Because fuses are electrical devices,
they will function in accordance with
electrical conditions present without
regard to mechanical failures.
In conclusion, the proper selection of
overcurrent protective devices for
branch circuits is an important deci-
sion affecting the safety, reliability, ef-
ficiency and cost of an electrical sys-
tem. Although a circuit breaker might
initially appear to be a more conve-
nient device, initial costs, safety, and
long term maintenance need to be
considered. Properly selected fuses
provide a much greater degree of pro-
tection to personnel and equipment
in a smaller and more cost effective
package. When all the factors are
taken into consideration, fuses are
clearly the better choice for electrical
system protection.
A system composed of
fuses can be coordinated