August 1965 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Arthur Steele is probably enjoying retirement from
(still in business today) by now.
In 1965 he had an article published offering guidelines on how
to select the proper type fuse for protecting the circuit at
hand. The correct choice is seldom a simple matter of adding
a margin of some amount onto the known maximum current draw,
especially if you are designing for a commercial or defense
electronics project. Applied voltage, expected current surges,
operational temperature and mechanical stress
(vibration & shock, etc.),
applicable design regulations (UL, Mil-Spec,
etc.), serviceability, and available space
among the factors that need consideration. Do you need a fast-blow,
medium-blow, or slow-blow fuse for that circuit? You'll have
a good understanding after reading Mr. Steele's piece.
The automotive blade fuse type (thumbnail
to left) so common today was not around at the time;
they first came on the scene in the 1980s.
Selecting the Proper Fuse
By Arthur J. Steele, Staff Engineer, Littelfuse, Inc.
Important factors to consider include physical size, amperage,
voltage, type, ambient temperature, and special characteristics.
A number of important rules for proper fusing are included.
A fuse is an intentionally weakened part of an electrical
or electronic circuit. Its purpose-is to protect the other elements
of the circuit from a possibly damaging overload. If it is to
perform its function of protection properly, it must be thoughtfully,
selected and installed or mounted. Fuses are used to protect
many different types of circuits. In some instances we are trying
to protect only the wiring and prevent a fire. In other instances,
we may be protecting a very sensitive meter or an electronic
component in a television set.
Fuses are generally a glass-bodied cartridge with nickel-plated
brass caps. High-amperage fuses are made of a punched zinc alloy
element. Low-amperage fuses are made with a filament wire of
many materials from copper to platinum and many alloys to accomplish
the desired result.
Glass and ceramic cartridge fuses. From left:
3AG medium-lag type; slow-blow fuse; 3AB ceramic anti-vibration
fuse with arc-quenching filler; 8AG low-current instrument fuse;
and 4AG low-voltage medium-lag fuse developed for aircraft use.
Rules of Fusing
The first question that must be answered is, "What am I trying
to protect?" Is it the wiring of a house? the wiring of an electronic
device in industry? the wiring of a very sensitive mechanism
to be used in our space program? In each case, am I trying to
protect against a fire hazard? or am I trying to protect a piece
of equipment? a very sensitive metering device? an expensive
component? or eliminate shock hazard to personnel? There is
a definite series of questions that you should ask yourself
when deciding on a fuse. Ask yourself the following questions:
1. What is the normal operating current?
2. What is the abnormal current at which
the fuse is required to open the circuit?
3. What are the minimum and maximum times
during which abnormal current is permitted?
4. What voltage is applied to the fuse?
5. What is the normal ambient temperature
at the fuse location?
6. Are there pulse characteristics involved
in the circuit?
7. What are the applicable commercial specifications?
8. Are there any applicable government
9. Are there other requirements special
to this application (mechanical or electrical characteristics)
beyond the normal requirements of a commercial fuse?
10. What is the physical size desired?
Fig. 1. Characteristics of fast-, medium-,
and slow-blow fuses.
Types of Fuses Used
The number and types of fuses and fuse mountings required by
the electronics industry have developed over the past 25 years
in about the same proportion as those of resistors and capacitors.
In the same way, too, the pressure from engineers has been to
make fuses smaller and better. By far the greatest number of
fuses used in electronic components and circuit protection are
glass-enclosed, although ceramic is used for the higher current,
250-volt Underwriters' Laboratory listed fuses.
Table 1. Listing of fuses commonly used in
The common fuse families are listed in Table 1. The most
popular fuses, their sizes, and maximum ratings are given, along
with their standard industry type designation. Readers will
be interested to note that there is no logical or rational relationship
between the recognized "AG" designation and the physical size
of the fuse. It has grown rather haphazardly over the past 40
years, starting with the automotive industry. 1AG was the first
"automotive glass" fuse; 2AG was next (no longer used, dropped
in an effort to standardize); 3AG was next, etc.
As the industry grew, additional sizes were necessary and
they did not necessarily fall within the physical size order.
In addition, the SAE (Society of Automotive Engineers) established
a specification in an effort to try to prevent over-fusing by
making the fuses non-interchangeable. As the amperage rating
was increased, the physical length of the fuses was increased,
thus preventing over-fusing. This approach, however, has not
been too practical due to the limited number of amperage ratings
In general, then, the "AG" designations only classify the
fuse as to the specific physical size of the envelope in which
the fuse is placed.
The need for smaller and better fuses has led to the development
of such fuses as the "Microfuse" and "Picofuse." The "Microfuse"
is being used in the Gemini space program. It is small, accurate,
and reliable, and is produced in either plug-in or pigtail versions.
The "Picofuse" is a pigtail-type fuse with either radial or
axial leads. This fuse is the smallest of protectors - accurate
and fast-acting. It may be used where space and weight are at
a premium and where mechanical and electrical requirements are
Fig. 2. Typical family of characteristic
curves for 8AG high-speed instrument fuses of various current
ratings. Most of these fuses are low-current devices with bead
or filament construction.
Fuses are the safety valves of electrical circuits, therefore
it is important that they operate or "blow" before damage occurs
in either the equipment or the wiring being protected. Conversely,
we do not want to have nuisance blowing. Thus, fuses must not
blow too easily and cause open circuits when the equipment is
It becomes obvious that time and current are the controlling
factors in the function of a fuse. There is a time and current
relationship at which the circuit will operate satisfactorily
and cause no damage to equipment or wiring. There is also a
time and current relationship at which the equipment will be
damaged. In other words, the time-current characteristics of
the fuse must conform to the time-damage characteristics of
There are three basic types of fuse characteristics for different
types of applications. Fast-blowing fuses are used for instrument
protection where fast action is necessary in order to protect
the equipment. Medium-blowing fuses are used for general applications.
Slow-blowing fuses are used for applications where a time lag
is desirable (see Fig. 1).
Fast-acting fuses are designed to carry 100% of current rating
and blow rapidly at very slight percentages of overload. For
example, they will open the circuit at a 200% overload within
a maximum of 5 seconds. The greater the overload, the more rapidly
the circuit opens. There is very little mass in the filament
used, therefore the reaction can take place very quickly under
short-circuit conditions. This filament is generally made of
silver, platinum, or other precious metal alloys.
These fuses are manufactured with wire diameters as low as
0.000020". Obviously this filament will open the circuit under
adverse conditions before any damage can be done in other parts
of the circuit. Typical examples of the fast-acting fuses are
the 8AG instrument fuses (see Fig. 2), "Microfuses," and "Picofuses."
This super-small, fast-acting ceramic cartridge
fuse will almost fit through the eye of a needle. Fuse is only
7/32" long and 0.078" in diameter and is available at up to
There are three basic constructions of these fast-acting
fuses. One, the bead-type construction, uses a small onyx bead
holding two heavy wires, across which is placed a fine filament.
The second is a filament-type construction employing a diagonal
lineup to insure the constant blowing characteristics of the
fuse; and third, the element construction for heavy amperage
fuses in this design. By observing the characteristic curves,
it will be noted that as the amperage rating increases there
is an inherent lag; automatically built-in, due to the increase
in mass of the fusible link.
A group of medium-lag indicating fuses is
shown. When this fuse blows, the silver-plated indicating pin
extends from the end of the fuse. This serves as a visual indicator.
The pin can also be used to actuate an audible alarm circuit
Medium-Lag & Slow-Blow Fuses
Medium-lag fuses are by far the most widely used. They are
used in automotive, commercial, and industrial electronic equipment,
as well as in appliances. These fuses are designed to protect
a non-critical device, wiring, or equipment at minimum cost.
In general, they are designed to carry 110% of rated current
for a minimum of four hours. Circuits will be protected if an
overload of 135% of rating is placed on the fuse; it will open
in less than one hour. If an overload of 200% of rating is placed
on the circuit, the fuse will open the circuit in a maximum
of 30 seconds. Notice that the action of these fuses is not
as rapid as the fast-acting fuse. However, they are perfectly
satisfactory in the majority of general applications for fuse
protection. All SFE fuses fall into this category, as do many
of the 3AG and 3AB fuses.
The slow-blow fuse is one that has a time-lag characteristic.
Many circuits or pieces of equipment have a built-in pulse characteristic
which is normal for the operation of this equipment. The starting
of a motor, for example, produces the pulse characteristic referred
to. Until the motor is up to speed, current far higher than
the normal running current flows in the circuit. This is normal
operation for this circuit, so we do not want the fuse or protector
to open the circuit during these conditions. Therefore, a fuse
must be designed with sufficient mass or time lag to prevent
nuisance blowing. In some highly inductive or capacitive electronic
circuits, this same characteristic is required.
Subminiature fuses may be assembled into
cylindrical pin contacts which are then inserted into military
and industrial connectors.
In the design of fast-acting fuses, we want to have a minimum
amount of mass in order to cause the fuse to open the circuit
as quickly as possible. For the slow-blow characteristic, the
reverse is true. We want a period of delay, therefore additional
mass of one type or another is added to the fusible link, thus
causing a delay. This can be done by using a relatively large
mass of low-melting point alloy placed near the heat-generating
source. The fuse, being a thermal device, requires a certain
amount of time for the current passing through a certain resistance
to cause a reaction. It requires a certain amount of time for
the current passing through the heat-generating source, or the
resistance of the fuse, to bring the temperature of the low-melting
point alloy to the point where it becomes liquid, triggering
the mechanical action of a tensioning spring, and opening the
circuit. The time that is required for this reaction is called
the slow-blow or lag characteristic of the fuse.
The low-melting alloys are made up of tin, lead, bismuth,
and small quantities of other materials to control the desired
melting point. The heat-generating source takes many forms,
depending on the amperage rating of the fuse. In some instances
a small carbon or wirewound resistor is added, or the resistance
of a coil spring becomes the heat-generating source.
Fig. 3. Characteristics of slow-blow fuses
for motor protection.
In most applications, the time-lag characteristic is desirable
only in a limited range of operating currents. The current values
in excess of this limited range require the fast action. therefore,
slow-blow fuses are designed to carry 110% of rating for a minimum
of four hours and open the circuit at 135% of rating within
one hour, have a minimum of 5-second delay at 200% of rating,
and open the circuit within 2 minutes.
For overloads in excess of 700%, a fusible link is placed
in series with the slow-blow fuse so that fast action is obtained
in opening the circuit when an overload of this nature is encountered.
Three types of characteristics are actually available in the
slow-blow type fuse. First, a long life at the normal operating
characteristic; second, a time delay to prevent nuisance blowing
under slight overloads; and third, rapid action under short-circuit
or high overload conditions. This type of fuse is used in TV
horizontal sweep circuits, power supplies, motor circuits, etc.
See Fig. 3.
There are many different ways of mounting fuses. The least
expensive is the addition of a pigtail-or wire-to the ends of
the fuse, which can then be soldered directly into the circuit.
The second method, and one which is very commonly used, is the
open-clip-type mounting where the fuse is supported by metal
fuse clips and the circuitry is either wired or attached by
screw terminals or solder lugs to the fuse clip. The third refinement
is the fuse-extractor post, which is generally panel-mounted,
giving access to the fuse from either the front or back panel
without danger of shock hazard to the operator. In automotive
or low-voltage applications, another method of mounting is offered
in the fuse retainer, or in-line type mounting.
When many fuses are used, it is desirable to have some indication
of which fuse has blown; therefore, indicating fuse-extractor
posts are available. In this type, a lamp in the transparent
knob of the fuse post lights up at the time of fuse failure,
indicating which circuit has the blown fuse. When the fuse is
replaced with a good fuse, the light is no longer energized.
There are also available special types of fuses with built-in
indicators to show when a fuse has blown.
There are some specialized holders for specific applications,
for example, the limited-current fuses and fuse holders used
for TV sets. These limited-current fuses are made with a pair
of tabs on one cap, which are indexed with a pair of slots in
the head of the fuse holder. As the amperage rating is increased,
the tab width is increased, thus making it impractical to use
a higher rating of fuse.
Common Fusing Errors
There are some common fusing errors in the use of fuses which
create problems for the user and, in some instances, result
in damaged equipment. One of the most common errors is over-fusing.
A piece of equipment may be designed to use a certain amperage
fuse - which gives adequate protection. Because of overloading,
the fuse blows occasionally and when the fuse is replaced, the
equipment seems to work all right. Because of this, the obvious
conclusion to the layman is, "Put in a larger fuse and everything
will be OK." A larger fuse is installed and suddenly there is
a cloud of smoke due to the continuing overload and the use
of a fuse that is not capable of opening the circuit because
it is overrated for that particular application.
Always replace a blown fuse with the same rating fuse that
was originally designed into the equipment. There have been
many instances where 1/4-amp. fuses have been replaced by 20-amp.
units in order to prevent nuisance blowing. Obviously, over
a period of time this can only lead to ultimate failure of the
Another common fusing error is using the wrong type of fuse
in a piece of equipment. For example, a piece of equipment that
has a high inrush surge should not use a medium- or fast-blowing
fuse. In this type of application, a slow-blow fuse should be
used, thus preventing nuisance blowing and the possibility of
up rating or over-fusing. On the other hand, for the protection
of a delicate instrument or an instrument circuit, we want a
fuse that will open the circuit as quickly as possible in order
to prevent damage. In this case, the use of a medium- or slow-blow
fuse would not be correct. An instrument fuse, or fast-blowing
fuse would be the correct choice. Remember, use the right type
of fuse for your particular application.
The use of a fuse with the wrong voltage rating is also a
common fusing error. The voltage rating of a fuse is the maximum
voltage it will break without arcing or bursting. (This is assumed
to be in a direct-current system with a 10,000 ampere capacity
power supply, unless otherwise indicated.) In other words, 32-volt
fuses should not be used in 250-volt circuits. This can be dangerous
to the user as well as to the equipment in which it is installed.
All voltage ratings on fuses are maximum voltages; fuses may
be used up to and including their voltage rating.
These slow-blow fuses have different lengths
and different widths of bayonet locking tabs on the fuse caps.
The fuse post accepts only one type so that over-fusing is prevented.
The ambient temperature is a contributing factor in the opening
of a fuse. One of the common errors in fusing is a complete
disregard of the temperature in which the fuse is required to
operate. We have stated that the fuse is actually a thermal
device. It operates by virtue of the heat dissipated in the
fuse element. Therefore, an increase in ambient temperature
can also cause de-rating of the fuse. Conversely, a decrease
in ambient temperature can also cause an increase in rating
in the fuse. Thus, when temperatures other than normal room
temperature are encountered, they should be taken into account
in selecting appropriate fuses.
Other minor considerations often overlooked yet responsible
for fusing errors include pulse fatigue, vibration, shock, and
exceptional atmospheric conditions. All of these conditions
should be considered at the time the equipment is designed and
the fuse application determined. This is another good reason
for replacing fuses that have blown in service with the same
type of fuse that was in the original installation.
In summary, when choosing a fuse for your particular application,
select the proper unit by carefully considering the following:
1. size, 2. amperage, 3. voltage, 4. type, 5. ambient temperature,
and 6. special characteristics.
The selection of proper fuses can prevent nuisance blowing,
fire hazard, and danger to personnel.
Posted June 13, 2015