| |
LW1DSE > TECH 06.01.18 20:43l 449 Lines 22743 Bytes #999 (0) @ WW
BID : 1129-LW1DSE
Read: GUEST
Subj: Fuses
Path: IW8PGT<IZ3LSV<ED1ZAC<LU4ECL<LW2DQC<LW3DBH<LU1DBQ<LU7DQP
Sent: 180106/1756Z @:LU7DQP.#LAN.BA.ARG.SOAM #:26640 [Lanus Oeste] FBB7.00i
From: LW1DSE@LU7DQP.#LAN.BA.ARG.SOAM
To : TECH@WW
[――― TST HOST 1.43c, UTC diff:5, Local time: Mon Nov 20 18:54:51 2017 ®®®]
* FUSES *
***********
In electronics and electrical engineering a fuse, short for 'fusible
link', is a type of overcurrent protection device. Its essential component is
a metal wire or strip that melts when too much current flows. When the metal
strip melts, it opens the circuit of which it's a part, and so protects the
circuit from excessive current.
A practical fuse was one of the essential features of Edison's
electrical power distribution system. An early fuse was said to have
successfully protected an Edison installation from tampering by a rival
gas-lighting concern.
Fuses (and other overcurrent devices) are an essential part of a power
distribution system to prevent fire or damage. When too much current flows
through a wire, it may overheat and be damaged, or even start a fire. Wiring
regulations give the maximum rating of a fuse for protection of a particular
circuit. Local authorities will incorporate national wiring regulations as
part of law. Fuses are selected to allow passage of normal currents, but to
quickly interrupt a short circuit or overload condition.
Contents:
1) Fuse characteristics
1.1) Interrupting rating
1.2) Voltage rating
2) Fuse Markings
2.1) Fuse approvals
3) Fuse terminologies
4) Fuse Packages
4.1) Glass vs. Ceramic Construction
4.2) Measurements
4.3) Special Features
5) Automotive fuses
5.1) Blade type
5.2) Bosch type
6) High voltage fuses
7) Fuses compared with circuit breakers
8) Fuse boxes
9) British plug fuse
10) Other fuse types
1) Fuse characteristics
The speed at which a fuse operates depends on how much current flows
through it. Manufacturers of fuses plot a time-current characteristic curve,
which shows the time required to melt the fuse and the time required to clear
the circuit for any given level of overload current.
Where several fuses are connected in series at the various levels of a
power distribution system, it is very desirable to clear only the fuse (or
other overcurrent devices) electrically closest to the fault. This process is
called "coordination" and may require the time-current characteristics of two
fuses to be plotted on a common current basis. Fuses are then selected so that
the minor, branch, fuse clears its circuit well before the supplying, major,
fuse starts to melt. In this way only the faulty circuits are interrupted and
minimal disturbance occurs to other circuits fed by the supplying fuse.
Where the fuses in a system are of similar types, simple rule-of-thumb
ratios between ratings of the fuse closest to the load and the next fuse
towards the source can be used.
Fuses are often characterized as "fast-blow", "slow-blow" or
"time-delay", according to the time they take to respond to an overcurrent
condition. The selection of the characteristic depends on what equipment is
being protected. Semiconductor devices may need a fast or ultrafast fuse for
protection since semiconductors may have little capacity to withstand even a
momentary overload. Fuses applied on motor circuits may have a time-delay
characteristic, since the surge of current required at motor start soon
decreases and is harmless to wiring and the motor.
1.1) Interrupting rating
A fuse also has a rated interrupting capacity, also called breaking
capacity, which is the maximum current the fuse can safely interrupt.
Generally this should be higher than the maximum prospective short circuit
current. Miniature fuses may have an interrupting rating only 10 times their
rated current. Fuses for small low-voltage wiring systems are commonly rated
to interrupt 10,000 amperes. Fuses for larger power systems must have higher
interrupting ratings, with some low-voltage current-limiting "high rupturing
capacity" (HRC) fuses rated for 300,000 amperes. Fuses for high-voltage
equipment, up to 115,000 volts, are rated by the total apparent power
(megavolt-amperes, MVA) of the fault level on the circuit.
1.2) Voltage rating
As well as a current rating, fuses also carry a voltage rating
indicating the maximum circuit voltage in which the fuse can be used. For
example, glass tube fuses rated 32 volts should never be used in line-operated
(mains-operated) equipment even if the fuse physically can fit the fuseholder.
Fuses with ceramic cases have higher voltage ratings. Fuses carrying a 250 V
rating may be safely used in a 125 V circuit, but the reverse isn't true as
the fuse may not be capable of safely interrupting the arc in a circuit of a
higher voltage. Medium-voltage fuses rated for a few thousand volts are never
used on low voltage circuits, due to their expense and because they can't
properly clear the circuit when operating at very low voltages.
2) Fuse Markings
A sample of the many markings that can be found on a fuse.
Surface Mount Fuses on 8 mm tape: Each fuse measures 1.6 mm x 0.79 mm
and hasn't markings.Most fuses are marked on the body, or end caps to markings
show their ratings. Surface mount technology "chip type" fuses feature little
or no markings making identification very difficult.
When replacing a fuse, it is important to interpret these markings
correctly as fuses that may look the same, could be designed for very
different applications. Fuse markings will generally convey the following
information:
* Ampere rating of the fuse
* Voltage rating of the fuse
* Time-current characteristic ie. element speed
* Approvals
* Manufacturer / Part Number / Series
* Breaking capacity
2.1) Fuse approvals
The majority of fuse manufacturers build products that comply with a
set of guidelines and standards, based upon the application of the fuse. These requirements are devised by many different Government agencies and certification authorities[3]. Once a fuse has been tested and proven to meet the required standard, it may then carry the approval marking of the certifying agency.
3) Fuse terminologies
Rated Current IN This is the maximum current that the fuse can continuously
pass without interruption to the circuit, or harmful effects on its
surroundings.
#) The Iύt: This value is a direct measure of the energy required to blow the
fuse element and is an important characteristic of the fuse. It determines
the energy to which the object to be protected by the fuse will be subjected,
before the fuse blows.
#) Voltage Drop: the values of the voltage drop across a fuse are usually
given by the manufacturer. A fuse may become hot due to the energy
dissipation in the fuse wire at rated current conditions. The voltage drop
should be taken into account particularly when using a fuse in low-voltage
applications.
#) Breaking Capacity The breaking capacity is the maximum current that can
safely be interrupted by the fuse. The maximum short-circuit current that
could occur under fault conditions should not exceed the rated breaking
capacity of the fuse. Some fuses are designated High Rupture Capacity (HRC)
and are usually filled with sand or a similar material.
#) Voltage Rating The voltage rating of a fuse should always be greater than
or equal to the circuit voltage. Fuses can be used without detriment at
voltages less than their rating, though the fuses' own resistance should be
taken into account at very low voltages.
4) Fuse Packages
Fuses come in a vast array of sizes & styles[4] to cater for the
immense number of applications in which they are used. While many are
manufactured in standardised package layouts to make them easily
interchangeable, a large number of new styles are released into the
marketplace every year. In terms of fuse body construction, ceramic is the
most commonly used material. Glass & plastic are also used in lower voltage
applications.
* Cartridge (ferrule) fuses have a cylindrical body terminated with metal end
caps. While many cartridge fuses are symmetrical, some cartridge fuses are
manufactured with differing body proportions to reduce the possibility of
inserting an incorrect fuse into the holder (circuit). An example of such a
fuse range is the 'bottle fuse', which in appearance resembles the shape of
a bottle.
* Fuses designed for soldering to a printed circuit board have radial or axial
wire leads, however surface mount fuses have a smaller fuse body size, and
have solder pads instead of leads.
* Fuses used in higher voltage/ampere circuits as required by industrial
applications, commonly feature metal tags or blades located on each end of
the fuse. Tags allow the fuse to be bolted into the fuse holder while
blades slot into metal pressure clamps located on the fuse holder. Blade
type fuses often require the use of a special purpose extractor tool to
remove them from the fuse holder.
4.1) Glass vs. Ceramic Construction
While glass fuses have the advantage of a visible fuse element for
inspection purposes, they have a low breaking capacity which generally
restricts them to applications of 15 A or less at 250 VAC. Ceramic fuses have
the advantage of a higher breaking capacity facilitating their use in higher
voltage/ampere circuits. Filling a fuse body with sand provides additional
protection against arcing in an overcurrent situation.
4.2) Measurements
Cartridge fuses are generally measured as the overall length and
diameter of the fuse. Due to the large variety of cartridge fuses available,
fuse identification relies on accurate measurements as fuses can differ by
only a few millimeters between types. 'Bottle style' cartridge fuses also
require the measurement of the cap diameter as this varies between ampere
ratings.
Other fuse packages can require a variety of measurements such as;
body (width x height x depth)
blade or tag (width x height x depth)
overall length of the fuse (when the fuse features blades or tags)
overall width of the fuse (when the fuse features 2 bodies)
width of the mounting holes (when the fuse features tags)
distance between blades (when radially configured)
fixing centre (when the fuse features tags - see below)
Fuses fitted with tags require the fixing centre measurement. This
measurement is the distance between the tag mounting holes on either end of
the fuse as measured from the centre of each mounting hole.
4.3) Special Features
Glass cartridge and plug fuses allow direct inspection of the fusible element.
Other fuses have other indication methods including:
*) Indicating pin or striker pin: extends out of the fuse cap when the
element is blown.
*) Indicating disc: a coloured disc (flush mounted in the end cap of the fuse)
falls out when the element is blown.
*) Element window: a small window built into the fuse body to provide visual
indication of a blown element.
*) Flag: an external sprung arm that is released to an extended position once
the element is blown.
*) External trip indicator: similar function to striker pin, but can be
externally attached (using clips) to a compatible fuse.
*) Some fuses allow a special purpose microswitch[7] or relay unit to be fixed
to the fuse body. When the fuse element blows, the indicating pin extends
to activate the micro switch or relay which in turn triggers an event.
5) Automotive fuses
Plug-in type fuses come in three physical sizes: mini, ATO and maxi.
Automotive fuses protect the wiring and electrical equipment for
vehicles. They are generally rated for circuits no higher than 24 volts
direct current.
5.1) Blade type
Plug-in fuses (also called blade or spade fuses), with a plastic body
and two prongs that fit into sockets, are used in automobiles. These types of
fuses come in three different physical dimensions: mini (or minifuse), ATO®
(or ATC) and maxi (or maxifuse).
The physical dimensions, including the connector, of the fuses are as
follows (LxWxH) (ampere ratings in the parenthesis):
mini: 10.9x3.6x16.3 mm (2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A)
ATO: 19.1x5.1x18.5 mm (1A, 2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A, 40A)
maxi: 29.2x8.5x34.3 mm (20A, 30A, 40A, 50A, 60A, 70A, 80A)
It is possible to replace an ATO-type plug-in fuse with a circuit
breaker that has been designed to fit in the socket of a ATO-sized fuse
holder. These circuit protectors are more expensive than a regular fuse.
5.2) Bosch type
Bosch type fuses are used in old (often European) automobiles. The
physical dimension of this type of fuse is 6x25 mm with conical ends. Bosch
type fuses usually use the same color coding for the rated current. The DIN
standard is 72581/1
Color ³ Ampere
ΔΔΔΔΔΔΔΔΔΔΔΔΕΔΔΔΔΔΔΔΔΔΔΔΔ
yellow ³ 5A
white ³ 8A
red ³ 16A
blue ³ 25A
6) High voltage fuses
Fuses are used on power systems up to 115,000 volts AC. High-voltage
fuses are used to protect instrument transformers used for electricity
metering, or for small power transformers where the expense of a circuit
breaker is not warranted. For example, in distribution systems, a power fuse
may be used to protect a transformer serving 1-3 houses. A circuit breaker at
115 kV may cost up to five times as much as a set of power fuses, so the
resulting saving can be tens of thousands of dollars.
Large power fuses use fusible elements made of silver, copper or tin to
provide stable and predictable performance. High voltage expulsion fuses
surround the fusible link with gas-evolving substances, such as boric acid.
When the fuse blows, heat from the arc causes the boric acid to evolve large
volumes of gases. The associated high pressure (often greater than 100
atmospheres) and cooling gases rapidly extinguish (quench) the resulting arc.
The hot gases are then explosively expelled out of the end(s) of the fuse.
Other special High Rupturing Capacity (HRC) fuses surround one or more
parallel connected fusible links with an energy absorbing material, typically
silicon dioxide sand. When the fusible link blows, the sand absorbs energy
from the arc, rapidly quenching it, creating an artificial fulgurite in the
process.
7) Fuses compared with circuit breakers
Fuses have the advantages of often being less costly and simpler than a
circuit breaker for similar ratings. The blown fuse must be replaced with a
new device which is less convenient than simply resetting a breaker and
therefore likely to discourage people from ignoring faults. On the other hand
replacing a fuse without isolating the circuit first (most building wiring
designs do not provide individual isolation switches for each fuse) can be
dangerous in itself, particularly if the fault is a short circuit.
High rupturing capacity fuses can be rated to safely interrupt up to
300,000 amperes at 600 V AC. Special current-limiting fuses are applied ahead
of some molded-case breakers to protect the breakers in low-voltage power
circuits with high short-circuit levels.
"Current-limiting" fuses operate so quickly that they limit the total
"let-through" energy that passes into the circuit, helping to protect
downstream equipment from damage. These fuses clear the fault in less than
one cycle of the AC power frequency. Circuit breakers cannot offer similar
rapid protection.
Circuit breakers which have interrupted a severe fault should be
removed from service and inspected and replaced if damaged.
Circuit Breakers must be maintained on a regular basis to ensure their
mechanical operation during an interruption. This is not the case with fuses,
in which no mechanical operation is required for the fuse to operate under
fault conditions.
In a multi-phase power circuit, if only one fuse opens, the remaining
phases will have higher than normal currents, and unbalanced voltages, with
possible damage to motors. Fuses only sense overcurrent, or to a degree,
over-temperature, and can't usually be used independently with protective
relaying to provide more advanced protective functions, for example, ground
fault detection.
Some manufacturers of medium-voltage distribution fuses combine the
overcurrent protection characteristics of the fusible element with the
flexibility of relay protection by adding a pyrotechnic device to the fuse
operated by external protection relays.
8) Fuse boxes
Fuse boxhold electrical consumer units (also called fuse boxes) were
fitted with fuse wire that could be replaced from a supply of spare wire that
was wound on a piece of cardboard. Modern consumer units contain magnetic
circuit breakers instead of fuses. Cartridge fuses were also used in consumer
units and sometimes still are, as miniature circuit breakers (MCBs) are
rather prone to nuisance tripping.
In North America, fuse wire was never used in this way, although
so-called "renewable" fuses were made that allowed replacement of the fuse
link. It was impossible to prevent putting a higher-rated or double links
into the holder ("overfusing") and so this type must be replaced.
The "Wylex standard" was very popular in the United Kingdom up until
recently when the wiring regulations started demanding Residual-Current
Devices (RCDs) for sockets that could feasibly supply equipment outside the
equipotential zone. The design doesn't allow for fitting of RCDs (there were
a few wylex standard models made with an RCD instead of the main switch but
that isn't generally considered acceptable nowadays either because it means
you lose lighting in the event of almost any fault) or residual-current
circuit breakers with overload (RCBOs) (an RCBO is the combination of an RCD
and an MCB in a single unit). The one pictured is fitted with rewirable fuses
but they can also be fitted with cartridge fuses and MCBs. There are two
styles of fuse base that can be screwed into these units one designed for the
rewirable fusewire carriers and one designed for cartridge fuse carriers.
Over the years MCBs have been made for both styles of base. With both styles
of base higher rated carriers had wider pins so a carrier couldn't be changed
for a higher rated one without also changing the base. Of course with
rewirable carriers a user could just fit fatter fusewire or even a totally
different type of wire object (hairpins, paper clips, nails etc.) to the
existing carrier.
In North America, fuse boxes were also often used, especially in homes
wired before about 1950. Fuses for these panels were screw-in "plug" type
(not to be confused with what the British call plug fuses), in screw-thread
holders similar to Edison-base incandescent lamps, with ratings of 5, 10, 15,
20, 25, and 30 amperes. To prevent installation of fuses with too high a
current rating for the circuit, later fuse boxes included rejection features
in the fuseholder socket. Some installations have resettable miniature
thermal circuit breakers which screw into the fuse socket. One form of abuse
of the fuse box was to put a penny in the socket, which defeated the
overcurrent protection function and resulted in a dangerous condition. Plug
fuses are no longer used for branch circuit protection in new residential or
industrial construction.
9) British plug fuse
20 mm 200 mA glass cartridge fuse used inside equipment and 1 inch 13
A ceramic British plug fuse.The BS 1363 13 A plug has a BS 1362 cartridge
fuse inside. This allows the use of 30 A/32 A (30 A was the original size; 32
A is the closest European harmonised size) socket circuits safely. In order
to keep cable sizes manageable these are usually wired in ring mains. It also
provides better protection for small appliances with thin flex as a variety
of fuse ratings (1 A, 2 A, 3 A, 5 A, 7 A, 10 A 13 A with 3, 5 and 13 being
the most common) are available and a suitable fuse should be fitted to allow
the normal operating current while protecting the appliance and its cord as
well as possible. With some loads it is normal to use a slightly higher rated
fuse than the normal operating current. For example on 500 W halogen
floodlights it is normal to use a 5 A fuse even though a 3 A would carry the
normal operating current. This is because halogen lights draw a significant
surge of current at switch on as their cold resistance is far lower than
their resistance at operating temperature.
In most other wiring practices the wires in a flexible cord are
considered to be protected by the branch circuit overcurrent device, usually
rated at around 15 amperes, so a plug-mounted fuse is not used. Small
electronic apparatus often includes a fuseholder on or in the equipment, to
protect internal components only.
10) Other fuse types
So-called "self-resetting" fuses use a thermoplastic conductive element
known as a Polymeric Positive Temperature Coefficient (or PPTC) thermistor
that impedes the circuit during an overcurrent condition (through increasing
the device resistance). The PPTC thermistor is self-resetting in that when
the overcurrent condition is removed, the device will revert back to low
resistance, allowing the circuit to operate normally again. These devices are
often used in aerospace/nuclear applications where replacement is difficult.
A "thermal fuse" is often found in consumer equipment such as coffee
makers or hair dryers. They contain a fusible, temperature-sensitive alloy
which holds a spring contact mechanism normally closed. When the surrounding
temperature gets too high, the alloy melts and allows the spring contact
mechanism to break the circuit. The device can be used to prevent a fire in a
hair dryer for example, by cutting off the power supply to the heater
elements when the air flow is interrupted (e.g. the blower motor stops or the
air intake becomes accidentally blocked). Thermal fuses are a 'one shot',
non-resettable device which must be replaced once they have been activated.
Extracted from Wikipedia.com.
Compiled and translated to ASCCI by LW1DSE Osvaldo
Almirante Brown
Buenos Aires
Argentina
10/11/2007
ΙΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝ»
Ί Osvaldo F. Zappacosta. Barrio Garay (GF05tg) Alte. Brown, Bs As, Argentina.Ί
Ί Mother UMC ζPC:AMD486@120MHz 32MbRAM HD SCSI 8.4Gb MSDOS 7.10 TSTHOST1.43C Ί
Ί 6 celdas 2V 150AH. 18 paneles solares 10W. Ί
Ί lw1dse@yahoo.com ; lw1dse@gmail.com Ί
ΘΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΌ
Read previous mail | Read next mail
| |
