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TRANSCRIPT
PRODUCT STANDARDS
The objective of product safety standards is to
prescribe a set of construction criteria that will ensure
the safe operation of the electrical device, as well as
to protect the user. A standard is a written document
issued by a countries’ national safety agency or
standards committee. Product standards clearly
specify how an end user’s device is to be built, tested,
and perform safely. Examples of common U.S. and
European product standards are:
UL 1950 “Safety of Information Technology
Equipment”
UL 544 “Medical and Dental Equipment”
EN 60950 “Safety of Information Technology
Equipment”
EN 60601 “Safety of Medical Equipment”
EN 60065 “Safety Requirements for Mains
Operated Electronic & Related Apparatus for
Household and Similar General Use”
Both UL (Underwriters Laboratories) and CSA
(Canadian Standards Association) issue standards as
well as provide product testing services for obtaining
approvals. In European nations, the IEC (International
Electrotechnical Commission) also issues standards
created via a network of international technical
committees. The European Committee for Electric
Standardization (CENELEC) generates documents
called “European Norms” (EN). These documents are
based on existing IEC product standards with any
national deviations noted. It is the responsibility of
each national safety agency to provide testing for
approvals as IEC does not perform any test functions.
Examples of other national safety agencies are:
BSI British Standards Institution (United Kingdom)
IMQ Instituto ltaliano del Marchio di Qualita (Italy)
MITI Ministry of International Trade & Industry (Japan)
SEMKO Svenska Elektriska Materielkontrollanstalten
(Sweden)
VDE Verband Deutscher Elektrotechniker (Germany)
All European member nations of CENELEC are
mandated to accept the use of EN’s. Members include
Austria, Belgium, Czech Republic, Denmark, Finland,
France, Germany, Greece, Iceland, Ireland, Italy,
Luxembourg, the Netherlands, Norway, Portugal,
Sweden, Spain, Switzerland, and the United Kingdom.
COMPONENT STANDARDS
Most product standards usually mention that
individual components such as fuses, be tested and
approved to their own appropriate construction
standard. Component standards are focused on
defining categories, dimensions, electrical ratings, test
set-ups, and reliability criteria. While there are several
standards for fuses from small board-mounted types
to high voltage power line types, the most common
standards for electronic fuses are:
UL 248-14 Low Voltage Fuses-Supplemental
Fuses
CSA-C22.2 No.248.14 L.V. Fuses-Supplemental
Fuses
IEC 60127-2 Miniature (Cartridge) Fuses
IEC 60127-3 Subminiature Fuses
IEC 60127-4 Universal Modular Fuses (UMF)
Please note that the UL and CSA standards are
identical harmonized documents which took effect
October 1st , 1994. The UMF standards (IEC 60127-4)
was published in 1996. Construction standards for
fuse holders are:
UL 512 Fuse holders
CSA C22.2 No.39-M1987 Fuse holder Assemblies
IEC 60127-6 Fuse holders for Miniature Fuses
APPROVALS
An approval, as determined by an independent testing
agency, certifies conformance to an appropriate
standard. Meeting the requirements of a component
standard, however, does not imply automatic agency
approval. Such approvals must be applied for, and are
only granted after satisfactory testing has been
performed. To carry an Approval mark such as that
issued by UL, CSA, VDE or SEMKO, a fuse or holder
must be manufactured to one of the aforementioned
standards. Approvals play a large part in determining
a fuse or holder’s suitability for a given application. A
summary of approvals follows:
UL Listing: This symbol, granted by the U.S.
agency, guarantees that a fuse has been
manufactured in full compliance with the UL 248-14
standard. A fuse carrying a UL Listing cannot qualify
for either a VDE or SEMKO approval for reasons
explained in the next section.
CSA Certification: This symbol, granted by
the Canadian agency, guarantees that a fuse or
holder has been manufactured in full compliance with
the CSA C22.2 No. 248.14 or CSA C22.2 No.39
standard, respectively. A fuse carrying a CSA
Certification cannot qualify for either a VDE or
SEMKO approval for reasons explained in the next
section.
SEMKO Approval: This symbol, granted by
the Swedish agency, guarantees that a fuse or holder
has been manufactured in full compliance with the
appropriate section of the IEC 60127 standard. A fuse
carrying a SEMKO approval cannot qualify for either a
UL Listing or CSA Certification for reasons explained
in the next section.
VDE Approval: This symbol, granted by the
German agency, guarantees that a fuse or holder has
been manufactured in full compliance with the
appropriate section of the IEC 60127 standard. Until
recently, a fuse carrying a VDE approval could not
qualify for either a UL Listing or CSA Certification for
reasons explained in the next section.
BSI Kitemark License: This symbol, granted by the British agency, guarantees that a fuse
has been manufactured in full compliance with the
appropriate section of the IEC 60127 (BS 4265)
standard. A fuse carrying a BSI approval cannot
qualify for either a UL Listing or CSA Certification for
reasons explained in the next section.
Dentori Approval: This symbol,
granted by the Japan Electrical Testing Laboratory,
guarantees that a fuse has been manufactured in full
compliance with the Japanese MITI standard. This
document is similar to UL 248-14 with subtle
differences in voltage ratings and breaking capacity
criteria.
UL Recognition: UL’s Component Recognition Program allows the testing of
components (including fuses and holders) for which
no UL standard exists or where only certain sections
of a particular UL standard are referenced. A fuse or
holder may be submitted to UL for testing according to
criteria defined by the manufacturer. If basic safety
requirements are met during testing and the
component performs as predicted, it can be UL
Recognized. Fuses built to the European IEC 60127
fuse standard (with SEMKO, VDE, and/or BSI
approvals) are technically qualified to apply for UL
Recognition.
c-UL Listing: This approval is similar to CSA Certification. Since UL is accredited by the Standards
Council of Canada as a Certified Organization and
Testing Organization, it can perform component
testing to applicable Canadian standards. This
approval guarantees full compliance with the selected
Canadian component standard and is technically
equivalent to CSA Certification.
c-UL Recognition: This approval is similar
to that offered by the CSA Component Acceptance
Program. If basic Canadian safety requirements are
met during testing and the component performs as the
manufacturer predicts, it can be c-UL Recognized. As
with UL Recognition, “Conditions of Acceptability” are
listed in the test report.
c-UL-us Listing: This approval combines
the UL Listing and c-UL Listing into a single approval
valid for the U.S. and Canada. This approval
guarantees full compliance with the selected
component standards.
c-UL-us Recognition: This approval
combines the UL Recognition and c-UL Recognition
into a single approval valid for the U.S. and Canada.
As with UL Recognition, “Conditions of Acceptability”
are listed in the test report.
UMF Tulip Mark: This symbol is applied to
Universal Modular Fuses (UMF) that have been found
to be compliant with IEC 60127-4. It is now possible to
obtain a true UL Listing and European approvals for a
UMF fuse.
For each fuse and holder approval、we have listed the
applicable our File Number for each safety agency.
These agencies will require this file number during the
product evaluation process. Contact us if approval
certificates are needed or file numbers are not listed.
UL/CSA vs. IEC
There are significant differences between the
requirements listed in the UL 248-14/CSA 248.14
standard and the IEC 60127 fuse standards. Whereas,
both documents have much in common when
describing physical dimensions and materials used in
construction, they completely contradict each other
when defining pre-arcing time vs. current
characteristics of fuses.
This incompatibility alone makes it impossible to build
an electronic fuse that fully complies with all of these
standards. Knowledge of these fundamental
differences will enable the design to properly select an
approved fuse type for any given application. Since
the fuse is one of the last items selected in the design,
this knowledge can be a time-saver during product
approval.
Time vs. Current Characteristics
The rated current (IRated) of an electronic fuse must be
equal to or greater than the continuous operating
current in the circuit where the fuse is used. The
maximum allowable continuous current through a
given fuse type varies as follows:
Standard Voltage Rating Allowable Continuous
Operating Current(23o)
UL/CSA 250V < 75% x IRated
UL/CSA 125V < 70% x IRated
IEC 250V < 100% x IRated
IEC 125V < 70% x IRated
IEC(UMF) 32V-250V < 80% x IRated
Ul & IEC Time-current Curves Figure 1
This means that UL/CSA fuses must be “oversized” by
a minimum of 35%-40% while IEC fuses (rated at
250V) can be used continuously at their full load rating.
The oversizing factor is required to simulate “real
world” conditions as opposed to tightly controlled lab
test conditions. This factor compensates for line
fluctuations, enclosed fuseholders, air movement,
variances in wire and solders track dimensions, and
differences in contact resistances. This difference in
the definition of rated current means that a 1.0 Amp
IEC fuse is approximately equivalent to a 1.4 Amp
UL/CSA fuse. Sizing information is especially critical
during fuse replacement since an incorrect selection
could lead to nuisance trips (undersizing) or potential
fire hazards (oversizing).
Each fuse standard also dictates non-fusing and
fusing timecurrent limits. Non-fusing time defines the
minimum time which a fuse should carry a specified
current without blowing. Fusing time is defined as the
allowable time range (min-max) which a fuse should
carry a specified current before blowing. A summary
of these characteristics is shown on page 6.
Non-fusing times denoted by a “*” is the time at which
temperature stabilization of the melting element
occurs.
Dimensions
The UL/CSA standard does not specify any
dimensions for miniature cartridge fuses, however,
subminiature “microfuses” are required to have no
principal dimension exceeding 10mm. IEC 60127-2
categorizes requirements for miniature fuses with
dimensions of 5x20mm and 6.3x32mm only. IEC
127-3 covers three configurations of subminiature
fuses-similar to the picofuse, MET and MEF types
shown herein. IEC 60127-4 only covers radial-leaded
subminiature fuses and surface mount fuses. Terminal
spacing vary by voltage rating. All dimensions
including body shape and lead spacing are explicitly
listed.
Current Ratings
UL/CSA only suggests typical current ratings up to a
maximum of 60 Amps in the supplemental fuse
category. It is left up to the manufacturer which
increments to build. UL and CSA approvals cover a
range (min-max) of current ratings (ie. 1 A, 2 A, 3.15 A)
that can be built and tested for approvals. The
maximum current rating mentioned in IEC 60127 is
6.3 Amps. This means that fuses with ratings above
6.3 Amps cannot carry any European approvals. They
are referred to as “non-standard ratings.” If a fuse with
a higher current rating (>6.3 Amps) is used in a device,
it may only be eligible for an application-specific
approval. Additional testing may be performed and the
approval itself may have stipulations.
Miniature Fuse Time-Current Characteristics Table 1
UL/CSA 248-14 IEC 60127-2
%Rated
Current
Current
Range
Fast
Acting
Normal
Blow
Time
Delay
Quick
Acting
I
Quick
Acting
II
Time
Lag
III
Time
Lag
V
Time
Lag
VI
100% 0-10A * *
135% 0-10A <1hr. <1hr.
50mA-6.3A >1hr.
32mA-6.3A >1hr. >1hr. >1hr. 150%
1A-6.3A >1hr.
0-10A <2min. 200%
0-3A >5s
50mA-6.3A <30min.
32mA-6.3A <30min. <2min. <2min. 210%
1A-6.3A <30min.
50mA-3.15A 10ms-2s
4A-6.3A 10ms-3s
32mA-100mA 10ms-500ms 200ms-10s 200ms-10s
125mA-6.3A 50ms-2s 600ms-10s 600ms-10s
275%
1A-6.3A 1s-80s
50mA-6.3A 3ms-300ms
32mA-100mA 3ms-100ms 40ms-3s 40ms-3s
125mA-6.3A 10ms-300ms 150ms-3s 150ms-3s
1A-3.15A 95ms-5s
400%
4A-6.3A 150ms-5s
50mA-6.3A <20ms
32mA-6.3A <20ms
32mA-100mA 10ms-300ms 10ms-300ms
125mA-6.3A 20ms-300ms 20ms-300ms
1A-3.15A 10ms-100ms
1000%
4A-6.3A 20ms-100ms
* Fuse shall not open upon reaching stable operating temperature
Subminiature Fuse Time-Current Characteristics Table 2
UL/CSA 248-14 IEC 60127-4(UMF) IEC 60127-3
%Rated
Current
Current
Range
Fast
Acting
Time-Lag
Time-Lag
II
Quick-Acting
III
Time-Lag
IV
100% 0-10A *
125% 32mA-6.3A >1hr.
50mA-5A >1hr. 150%
40mA-4A >1hr.
0-10A <1min.
32mA-6.3A <2min. 200%
50mA-5A <5s
50mA-5A <30min. 210%
40mA-4A <2min.
50mA-5A 10ms-3s 275%
40mA-4A 400ms-10s
50mA-5A <30ms 3ms-300ms 400%
40mA-4A 150ms-3s
32mA-6.3A 10ms-100ms
50mA-5A <20ms 1000%
40mA-4A 20ms-150ms
* Fuse shall not open upon reaching stable operating temperature
Breaking Capacity
Breaking capacity requirements listed in each standard are shown below:
Standard Voltage
(VAC)
Test
Current(A) Fuse Type
UL 32 1000 Blade
UL/CSA 125 50 Micro
UL/CSA 125 10,000 Miniature
UL/CSA 250 35 Mini(0-1A)
UL/CSA 250 100 Mini(1.1-3.5A)
UL/CSA 250 200 Mini(3.6-10A)
UL/CSA 250 750 Mini(10.1-15A)
UL/CSA 250 1500 Mini(15.1-30A)
IEC
60127-2 250 35or10xIRated Mini(Low)
IEC
60127-2 250 150 Mini(Enhanced)
IEC
60127-2 250 1500 Mini(High)
IEC
60127-3 250 35or10xIRated Subminiature
IEC 125 50 Subminiature
60127-3
IEC
60127-4 32,63 35or10xIRated Submini, SM
IEC
60127-4 125 50or10xIRated Submini, SM
IEC
60127-4 250 100 Submini, SM
IEC
60127-4 250 500 Submini, SM
IEC
60127-4 250 1500 Submini, SM
Breaking Capacity Criteria Table 3
Typically, only ceramic body fuses with a 1500 Amp
rating (HBC) are recommended for use in products
directly connected to the AC mains. It is possible,
however, that EBC (enhanced breaking capacity) fuse
types may be substituted in circuits where maximum
short circuit currents are known not to exceed 150
Amps.
Power Factor
Microfuses built to the UL/CSA standard and
subminiature fuses built to the IEC 60127-3 standard
have their breaking capacity tests conducted at a
power factor of 0.95 to 1.0. Test set-ups on UL/CSA
and IEC 60127-2 miniature fuses use a power factor
of 0.7-0.8 with an exception for IEC 60127-2 glass
fuses. Tests on these low breaking capacity types use
a power factor of 1.0. Required power factors in IEC
60127-4 vary by breaking capacity category.
Stress Tests
Endurance testing is required for all IEC 60127
approved fuses. Miniature fuses are subjected to
120% of rated current (100% for MEF fuses) for 100
cycles followed by one hour at 150% of rated current.
Tests on picofuses and MET fuses are conducted at
80% of rated current for 100 hours. After the test, the
fuse must still conduct and have a maximum voltage
drop increase of 10%. All time lag fuses under IEC
60127 are subjected to high ambient operating tests
at 70℃ for one hour. The fuses cannot open during
the test. None of these parameters are tested in the
UL/CSA standard.
Miscellaneous
Maximum voltage drops and power dissipation values
are specified for all current ratings in the IEC 60127
standard. Neither of these parameters are required in
the UL/CSA standard.
Universal Modular Fuse-links (IEC 60127-4)
In an effort to recognize the efforts of manufacturers to
build products for a global marketplace, a single fuse
standard acceptable to both North American and
European safety agencies was published in 1996.
This document enables a single fuse type to obtain
worldwide approvals such as UL and VDE. Specific
dimensions, printed circuit board layouts and electrical
rating are specified. Voltage rating increments are 32V,
63V, 125V, and 250V. Solderability testing of
terminations (leads and SM pads) is also required.
Note that time vs. current characteristics are tested at
125%, 200% and 1000% of rated current adding more
confusion to proper selection. Presently, OUR offers
one surface mount Chip Fuse (No.446) that complies
with IEC 60127-4.
The CE symbol is only an identification of
the conformity of guidelines in connection with the
technical harmonization within the European
Community. This means that the CE symbol is not a
quality or standard conformity approval, but an
administrative symbol. OUR products comply with the
Low Voltage Directives 73/23/EWG. This is valid for
components with an operating voltage or rated voltage
between 50VAC and 1000VAC or 75VDC and
1500VDC. The CE symbol, according to the Directive,
will be shown on the fuse or holder shipping box only.
QUALITY ASSURANCE
All of OUR manufacturing plants worldwide have
applied for and received the Certificate of Registration
designating OUR as an ISO 9001 manufacturer. The
manufacture of high quality circuit protection
components necessitates extensive knowledge and
experience over many yeas. The ISO 9000 Series is
an internationally recognized family of standards
considered to be the premier benchmark of quality
assurance systems. “ISO Certified” means that the
processes used by a company to accept a purchase
order, review it’s requirements, process internal
paperwork and to design, produce, test, and deliver
the product, meet or exceed the requirements of the
DIN ISO 9001 standard as determined by an
independent auditor. For a copy of our current ISO
Certificate, please contact us.
Vendor materials are sample tested in accordance
with IEC 410 (MIL STD 105D, ISO 2859). During the
production process, statistical process controls (SPC)
and in-line process controls are utilized. Upon
completion of the manufacturing process, all fuses
undergo a 100% cold resistance test. The final test of
manufactured parts is Test Level II with an Acceptable
Quality Level (AQL) value of 0.25 or 0.4 in accordance
with IEC 2859 T1, whereby we distinguish between:
Critical Faults – present danger to body or life which
exclude use for safety reasons (i.e. not blowing within
the prescribed time-current limits).
Main Faults – that which limits application of fuse (i.e.
improperly marked or dimensions exceeding
tolerance levels).
Secondary Faults – that which impairs appearance
but does not affect normal operation.
The time vs. current characteristic test is carried out
according to Test Level S4 with a total AQL value of
0.65. Because this is a destructive test, it is based on
an extreme value observation. Non-fusing tests are
performed on fuses with high cold resistance values
while fusing tests are conducted on fuses with low
cold resistance values. Test certificates by
commission and lot number are available for fuses
that ship in industrial size containers.
MANUFACTURER RESPONSIBILITY
With regard to product safety of the device and
reliability of the fuse-link, a correct selection is
important. Only when choosing correctly and when
using as agreed under consideration of the safety
principle (ie. human beings, animals, and intrinsic
values must be protected against danger) can a
definite function of the fuse as a protective component
be possible. Please refer to VDE 0022 Section 2.3.5 –
Self responsibility of the manufacturer of electrical
devices which states: “Any person involved in the
production of electrical plant systems or the
manufacture of electrical equipment including those
dealing with the operation of such systems or
equipment, is in accordance with present
interpretation of the law, individually responsible for
every aspect of compliance to the recognized rules
and procedures of electrical engineering.
Standards & Testing Agencies
Agency Address Contact
BELLCORE Telcordia Technologies 8 Corporate Place
Piscataway, NJ 08854 USA
TEL: +1/ (732) 699-5800 FAX: +1/ (732) 336-2559
WEB: http://www.telcordia.com
BSI
British Standards Institution Quality Assurance
Services PO Box 375 Milton Keynes MK 14 6ll
England
TEL: +44/ 1 908 22 09 08 FAX: +44/ 1 908 22 06 71
WEB: http://www.bsi.org.uk
CCC CEPREI Box 1501 510610 Guangzhou China TEL: +86/ 20 8723 7006 FAX: +86/ 20 8723 6171
CSA Canadian Standards Association 178 Rexdale
Boulevard Toronto, Ontario M9W 1R3 Canada
TEL: +1/ (416) 747-4000 FAX: +1/ (416) 747-4149 WEB: http://www.csa.ca
DIN Deutsches Institute fur Normung Burggrafenstrasse 6
Postfach 11 Q7 D-10787 Berlin Germany
TEL: +49/ 30 26 011 FAX: +49/ 30 26 011263 WEB: http://www.din.de
FCC Federal Communication Commission 1919 M Street
NW Washington, DC 20554 USA
TEL: +1/ (202) 418-0200 FAX: +1/ (202) 418-2825 WEB: http://www.fcc.gov
IEC International Electrotechnical Commission 3,Rue de
Varembe CH-1211 Geneva 20 Switzerland
TEL: +41/ 22 919 0211 FAX: +41/ 22 919 0300 WEB: http://www.iec.ch
ITU International Telecommunications Union Place des
Nations CH-1211 Geneva 20 Switzerland TEL: +41/ 22 730 5111 WEB: http://www.itu.ch
ISO
International Standards Organization 1, Rue de
Varembe Case Postale 56 CH-1211 Geneva 20
Switzerland
TEL: +41/ 22 749 0111 FAX: +41/ 22 733 3430 WEB: http://www.iso.ch
MITI
Japanese Industrial Standards Office Agency of
Industrial Science & Technology 1-3-1 Kasumigaseki
1-Chome Chiyoda-ku, Tokyo 100
TEL: +81/ 3 501 9296 FAX: +81/ 3 580 1418
WEB: http://www.miti.go.jp/index-e.html
SEMKO Svenska Elektriska Materielkontrollanstalten Box
1103 16422 Kista Sweden
TEL: +46/ 8 750 00 00 FAX: +46/ 8 750 60 30
WEB: http://www.semko.se
TÜV RW- TÜV Anlagenteknik GmbH Abt. 7. 1 Steubenstr.
53 D-45138 Essen Germany
TEL: +49/ 201 825 3413 FAX: +49/ 201 825 3209
WEB: http://www.tuev.rwtuv.de
UL Underwriters Laboratories, Inc. 333 Pfingsten road
Northbrook, IL 60062 USA
TEL: +1/ (708) 272-8800 FAX: +1/ (708) 272-8129 WEB: http://www.ul.com
VDE Verband Deutscher Electrotechnicker Merianstrasse
28 D-63069 Offenbach am Main Germany
TEL: +49/ 69 8306 0 FAX: +49/ 69 8306 555 WEB: http://www.vde.de
FUSE PURPOSE
Fuse links protect electrical devices and components
from overcurrents and short circuits. This is achieved
automatically by the melting of a fuse wire through
which a fault current flows. An irreversible, physical
separation is created thereby cutting off current flow
through that conductor. Fuse links are rated so as to
reliably interrupt current flow when it reaches a
predictable magnitude for a fixed duration. For all
practical purposes, a fuse is invisible to the circuit.
An optimum matching of fuse characteristics to the
protective requirements of any device is important to
provide both adequate protection of the end-user and
maximum utilization of other circuit components. In
order to minimize consequential damage and to
comply with the requirements and standards of
modern electrotechnology, we have compiled in the
following paragraphs essential selection and
application criteria for electronic fuses.
VOLTAGE RATING
Standards establish voltage ratings for various types
of fuses. A fuse may be operated at any voltage level
below its voltage rating. These ratings indicate that a
fuse will reliably interrupt fault currents in a circuit
where the operating voltage is equal to, or less than
the rated voltage of the fuse. Since fuses are only
sensitive to changes in circuit current, it is not until the
fuse wire actually melts, that the rated voltage and
available power becomes an issue. It is important that
fuse construction prohibits an open circuit voltage
from causing an arc “restrike” across open fuse
terminals. Standards either require that a fault voltage
remain applied for 30 seconds subsequent to
interruption of current flow without an arc restrike
occurring, or that the insulation resistance across the
blown fuse measures at least 0.1 MOhm. For this
reason, the rated voltage of the fuse must be at least
equal to or greater than the operating voltage of the
device.
VOLTAGE DROP
The maximum voltage drop across a fuse is based on
the specifications listed in the standards. It is
dependant upon the construction of the fuse and is
measured at 100% of rated current. A fuse element is
generally a thin wire or etched metal film exhibiting
some resistance to current flow. Because of this
resistance, a small voltage drop occurs across the
fuse terminals. In most cases, this voltage drop can
be ignored, as it is insignificant when compared to the
operating voltage of the circuit. However, attention
must be paid to the internal voltage drop of fuses with
low current ratings (50mA, 63mA, etc.) especially in
electrical circuits with low operating voltage drops due
to the extremely small cross sectional areas of their
melting elements. Voltage drops at 100% of rated
current are listed for most fuse types.
CURRENT RATING
The current rating of a fuse identifies its
current-carrying capacity based on a controlled set of
test conditions. When subjected to a current flow in
excess of its rated current for a predictable amount of
time, a fuse should reliably create an open circuit
condition. In order to avoid nuisance blowing and to
adequately protect the circuit, the rated current of a
fuse must be equal to or greater than the full load
operating current of the circuit and lower than the
lowest overload current.
The nominal operating current of a device is the
magnitude of current drawn (in RMS Amperes) after it
has been energized, warmed up, and is operating
normally. To determine the full load continuous
operating current of a circuit, the possible occurrence
of supply voltage fluctuations must be considered. In
cases of turn-on inrush currents or periodically
repeating pulses, please refer to the section on
melting integrals to derive the required fuse current
rating.
If a fuse manufactured according to the UL Standard
is replaced by a similarly rated fuse manufactured
according to the IEC 60127 Standard, safe blowing of
the fuse is no longer guaranteed. This is due to the
difference in each standard’s definition of rated
current. When loaded with its rated current, a UL fuse
is intended to eventually blow, whereas an IEC fuse is
designed to operate continuously under similar
conditions. Knowledge of how these definitions differ
is vital to selection of the proper fuse and its
appropriate current rating.
Ambient operating temperature and fuse contact
resistance must also be considered when determining
the fuse rating.
TEMPERATURE
Ambient temperature refers to the air temperature
immediately surrounding the fuse (1 cm) and is not to
be confused with room temperature. The fuse ambient
temperature is higher in many cases, because it is
enclosed or mounted near other heat-producing
components such as resistors, transformers, etc. To
prevent premature blowing of the fuse, ambient
temperatures should not exceed 70℃.
Contact resistance between the fuse and its
connections to a circuit can cause additional heat
buildup in the vicinity of the fuse terminals. Fuse clips,
fuseholders, hookup wire and solder track dimensions
must be sized adequately to minimize the effect of
contact resistance. Please refer to Table 4 for details.
Derating of the Fuse Rated Current Figure2
Higher ambient temperatures mean additional loading
on a fuse element. Since technical data is presented
based on a standard 23℃ ambient temperature,
either forced air cooling or de-rating of a fuse may be
necessary to ensure reliable operation in a higher
ambient environment. Derating is the process of
selecting a fuse with a higher rated current to allow for
its operation in an ambient environment above 23℃.
See Figure 2 above for typical derating criteria. For
example, a 1 Amp time-lag fuse shall be derated to
780mA when operating at 70℃ ambient conditions.
This is calculated by multiplying 1.0A times 0.78
(100%-22%) from the y-axis.
RESISTANCE
The resistance of a fuse is the measured voltage drop
divided by the applied test current. It is usually an
insignificant part of the total circuit resistance,
however, when using low amperage fuses in low
voltage circuits, the fuse resistance should be taken
into account.
It is common to refer to both the cold resistance and
hot resistance of a fuse. The actual operating point of
a fuse is typically somewhere in between these cold
and hot resistance points.
Cold resistance is measured using a currents of no
more than 10% of the fuse’s nominal rated current.
Cold resistance remains nearly constant at and below
10% of rated current. For this reason, this parameter
is an excellent predictor of a fuse quality level when
measured on a production line. Hot resistance is
calculated using the stabilized voltage drop across the
fuse at 100% of nominal rated current. Cold
resistance data on individual fuse types is available
upon request.
TIME VS. CURRENT CURVES
The lowest suitable fuse rated current is obtained by
rounding up the calculated value to the standardized
or typical current rating values shown on the fuse data
pages. The time vs. current characteristic curves
shown with each fuse series are a graphical
representation of the “pre-arcing” (fusing) time as a
function of fault current. Pre-arcing time is defined as
that period of time from the moment a current
sufficient to rupture the fuse element begins to flow
until arcing occurs. Pre-arcing time includes the actual
melting time of the fuse, as well as the heating-up
period.
The time vs. current characteristic curve
demonstrates the relationship between pre-arcing
time and fault current magnitude. Pre-arcing time is
expressed in seconds on the vertical (y) axis. The
ratio, IFault / IRated (fault current magnitude divided by
the rated current of the fuse), is expressed as a pure
number (no units) on the horizontal (x) axis. The
blowing curves are shown as an envelope for all listed
rated currents. The region between the two curves
identifies the range of blowing times for all ratings of a
particular fuse series. For standardized fuses, these
curves are defined by their respective fuse standard.
Individual curves exist for each rated current and may
be obtained upon request.
These curves enable the most precise selection of
fuse type and current rating for any application. They
should be used to verify that the fuse performance will
match all known circuit start-up and operating
conditions.
BREAKING CAPACITY
The breaking capacity of a fuse is a measure of the
maximum fault current which the fuse will safely
interrupt without exploding, rupturing, or causing a fire.
During a fault or short circuit condition, a fuse may
receive an instantaneous overload current many times
greater than its rated current. The fuse immediately
senses this increase in current flow and the element
begins to overheat and melt. The fuse must safely
interrupt this current flow and subsequently prevent
any potential arc restrike before a catastrophic fault or
short circuit conditions causes physical damage to the
downstream equipment.
The breaking capacity of a fuse is dependent on a
number of factors including fuse construction, circuit
operating voltage, current type (AC or DC) and circuit
power factor. The use of sand or ceramic disks in
miniature and subminiature fuse construction can
increase the maximum breaking capacity. These
materials tend to distribute the heat generated from a
fault condition over the entire fuse element. The
breaking capacity of a fuse is much lower in DC
applications than in AC applications. In an AC circuit,
the quenching of the arc is assisted by the periodic
passage of both voltage and current sine waves
through zero. This allows momentary clearing (cooling)
to occur within the confines of the fuse casing. As this
waveform effect is not present in DC circuits.
Interruption of similar fault currents can prove more
difficult. Greater reliance is made on the special
design features incorporated into the fuse casing for
arc-quenching purposes. In AC circuits, the power
factor (cos φ) can also have a considerable influence
on arc quenching behavior. If the circuit power factor
is reduced below the rated voltage of the fuse, the
breaking capacity of the fuse increases nonlinearly.
In all cases, the breaking capacity for fuses listed
herein are tested at a specific voltage level, current
level, and power factor.
MELTING INTEGRAL
The thermal energy required to melt a specific fuse
element is termed the melting integral and is
represented as a product of I2t (Amps2 x sec.). This
value is a parameter of the fuse itself and is
determined by the construction materials used.
Laboratory tests are conducted on individual fuse
ratings to determine a nominal melting I2t. During
these tests, a fault current (typically 10 times the rated
current of the fuse) is applied long enough to melt the
element. Studies have shown that the melting integral
of the fuse element remains constant when; (1) there
is no heat dissipation from the surface of the element
to the surrounding materials, and (2) no heat within
the fuse casing is conducted away by the fuse
contacts or terminals. Adiabatic heating occurs when
all of the heat energy causes the element to
completely melt. Upon test completion, the fuse
melting integral is easily calculated by squaring the
applied fault current and multiplying by the actual
melting time. This value is independent of
temperature and voltage fluctuations.
For fast-acting fuses, this Guide lists maximum
melting integrals. Fast-acting fuses are used to
protect devices from short duration, high amplitude
fault conditions. Turn-on transients or pulsed currents
are usually not present. For this reason, the circuit
designer requires a particular fuse rating to open prior
to a catastrophic fault. An example would be the
protection of semiconductor components which are
sensitive to any overcurrent condition. Selecting a
fuse rating with a maximum I2t value less than the
semiconductor’s own I2t rating guarantees that the
fuse will open before the semiconductor component is
damaged.
Time lag fuses are listed with their minimum melting
integral values. For capacitive or inductive circuits,
where normally anticipated turn-on transients, inrush
currents, surge or pulse currents are present, a
time-lag fuse is required. Selecting a fuse rating with a
minimum I2t value sufficiently greater than the
calculated I2t limit of the transient or pulse waveform
guarantees that the fuse will not cause nuisance
interruptions due to transient or pulse conditions.
When a fuse is pulsed with periodic inrush or
repetitive transient currents sufficiently powerful to
warm the fuse element but not strong enough to
cause the fuse to blow, the thermal stress caused by
the cyclical expansions and contractions of the fuse
element can lead to mechanical fatigue and
premature failure. Selecting the appropriate current
rating involves choosing a fuse with an I2t value
greater than or equal to the I2t value of the pulse
multiplied by a Pulse Factor (Fp). The formula for this
relationship is as follows:
I2tFuse ≥ I2tPulse X Fp
From Figure 3, determine which waveform
approximates the circuit turn-on condition. Calculate
the waveform’s approximate melting integral (I2tPulse)
using the corresponding formula from Figure 3.
Accuracy of this calculation can be improved by
obtaining from 3 to 6 inrush current oscillograms and
using the maximum recorded peak inrush current in
the calculation. This peak inrush current will vary
according to the level of the supply voltage at the
instant the device is turned on.
Pulse Waveform
Current-Time Integral Waveforms Figure 3
Ideally, an inrush current oscillogram should be
recorded with circuit energization coinciding with the
peak of the supply voltage waveform. Multiply the
resulting I2tPulse by the Pulse Factor (Fp). To determine
Fp, estimate the amplitude of the inrush current and
quantity of transient pulses the fuse will be subjected
to over the lifetime of the device and the approximate
time interval between pulses. Refer to the Pulse
Factor Derating Curve in Figure 4 when selecting Fp
for either a fast acting or time lag fuse.
The product of I2tPulse and Fp determines the prorated
I2t value of the inrush current waveform over the life
expectancy of the device. Lastly, select a fuse rating
with a minimum melting integral (I2tFuse) that exceeds
this prorated value.
For more complex waveforms or rapidly repeating
pulses (< 100ms interval), contact OUR for design
assistance. We strongly recommend that the selected
fuse be tested under normal, fault, and simulated life
0
0
0
0
I
I
I
I
0.386I
t 5t
tp
tp
I2tp
(1/2)I2tp
(1/3)I2tp
(1/2)I2t
cycle conditions to ensure a proper selection.
FUSE TYPES
Fuses can be classified by their construction as
miniature, subminiature, or blade-type.
Miniature cartridge fuses are commonly available in
dimensions of 5x20mm and 6.3x32mm. Their main
advantage is the relative ease of replacement when
used in conjunction with open fuse blocks or enclosed
fuseholders.
Subminiature fuse include the MEF, MET, MSF,
picofuse, and SM Fuses for surface mounting. These
have very small dimensions and are ideal for compact
circuit board layouts. Axial or radial leaded fuses, as
well as newer surface mounted types, permit cost
effective installation methods via automatic placement.
The MEF and MSF fuse series are electrically
equivalent to miniature glass cartridge, but offer much
lower voltage drop values and a higher resistance to
impact and vibration.
Blade-type fuses are typically used in low voltage,
high current applications. The automotive industry has
standardized on three sizes – small, standard, and
large. Blade-type fuses have become popular in many
non-automotive applications due to their ease of
replacement and low cost.
Protectors are recommended for protection of
secondary circuits or low voltage IC’s. Similar to high
grade fusible resistors, protectors are not bound by
competing component standards, and thus are
applicable worldwide.
Further subdivision of the above referenced fuse
types can be made under the topics of Manufacturing
Standard, Time vs. Current characteristic and Fault
Current Interruption Capability; each of which has
already been discussed and is summarized below.
The North American component standard for
miniature and subminiature fuses is UL 248-14. The
component standard adopted in Europe is IEC 60127.
Because of the fundamental differences in how rated
current is defined, it is not always possible to
interchange fuses built to these different standards.
Fuses are also subdivided according to their relative
prearcing Time vs. Current characteristics. This
characteristic determines how fuses react to varying
fault current conditions. While the UL Standard
identifies fuses with either Normal Blow or Time Delay
characteristics, the IEC standard defines several
characteristic types and lists their symbols as follows:
Pulse Factor Derating Curve Figure 4
Symbol Characteristic IEC 60127 (Sheet No.)
FF Very Quick-acting -4
F Quick-acting -2 -3 -4
M Medium Time-lag
T Time-lag -2 -3 -4
TT Long Time-lag -4
The UL Normal Blow characteristic roughly
corresponds to the IEC Quick-acting characteristic.
Since fuses exhibiting these characteristics have low
melting integrals, they are typically used to protect
devices which do not normally generate inrush or
surge currents. The standard application for Normal
Blow or Quick-acting fuses is to protect devices where
overcurrents or short circuit currents must be quickly
interrupted.
The higher melting integral (I2t) values associated with
UL Time Delay and IEC Time-lag fuses make them
ideal for protection of applications exhibiting turn-on
inrush, transient, or repetitive pulse currents. Such
applications include capacitive loads (i.e. battery
charging circuits) or inductive loads (i.e. power supply
and motor load circuits).
Fuses are also classified according to their fault
current interruption capability, commonly referred to
as the “Breaking Capacity” rating. Please refer to
Table 3 for breaking capacity values associated with
UL and IEC standards.
MARKINGS
Both UL/CSA and IEC fuse standards state that fuse
casings shall be marked with:
Manufacturers Name or Trademark
Rated Current (A or mA)
Rated Voltage (Volts)
Symbol for Time-Current Characteristic
Additionally, IEC 60127-2 requires a symbol for rated
breaking capacity (“L” for Low, “E” for Enhanced, “H”
for High) on miniature cartridge fuses. With the
exception of current rating, “microfuses” built to
UL/CSA 248-14 may have the above information
printed on the smallest shipping package only. At the
option of the manufacturer, fuses may also display
approval marks, part numbers, and color coding.
Picofuses and surface mount fuses, because of their
size, only have the manufacturer’s logo and current
rating printed on the body. Examples of various
markings are shown below:
Fuse Markings Figure 5
FUSEHOLDERS
Fuseholders can be either an open type or totally
enclosed type. Depending on the type of fuseholder
selected for an application, consideration must be
given to ambient temperature, electrical conductor
cross-sectional area and the maximum power
dissipation (self-heating) of the fuse chosen for use in
conjunction with the fuseholder. Maximum power
dissipation values are especially critical when
selecting enclosed fuseholders, since excessive heat
built up inside the fuseholder may cause material
damage and will certainly result in premature fuse
failure. The maximum power dissipation capability (ie.
rated power acceptance) of a fuseholder is listed on
the individual data pages. These values are valid for
an ambient temperature of 23℃ . When ambient
temperatures above 23℃ are encountered, power
acceptance derating curves have to be considered.
Power Acceptance Derating Curve Figure 6
The derated power acceptance value of the
fuseholder shall be greater than the maximum power
dissipation of the chosen fuse to insure a proper
selection. Particular care must be taken in application
where miniature ceramic cartridge fuses are used,
especially operation of these series in the “overload”
region cannot be excluded. Ceramic fuses can
generate excessive heat when operated at
150%-200% of rated current for several minutes.
Power dissipation values may become higher than
those listed in the respective data pages. The
recommended minimum cross sectional area of the
conductor connected to any fuseholder is determined
based on the anticipated maximum load condition,
normal circuit operating current and the power
dissipation value of the intended fuse. Note that the
circuit operating current is not equal to the rated
current of the intended fuse.
Table 4 lists minimum cross-sectional areas for
conducting paths (solder tracks) to PC board-mounted
fuseholders and conductor (wire) sizes required for
panel mount fuseholders.
Unlike fuses, individual fuseholders can be tested and
approved by both North American and European
safety agencies. However, the maximum permitted
fuseholder operating current varies according to each
agency’s individual approval. Operating currents in
accordance with UL/CSA approvals merely indicate
the maximum current-carrying capacity of the
fuseholder terminals. By comparison, operating
currents in accordance with European safety agency
approvals (SEMKO/VDE) include consideration of the
maximum power dissipation of the intended fuse at a
minimum conductor cross-section and maximum
ambient temperature. Thus, the approved operating
current limits in accordance with UL/CSA are
considerably higher than the approved limits in
accordance with SEMKO/VDE. In order to achieve the
highest degree of safety and to prevent possible
damage, it is recommended that fuse and holder
combinations be tested under actual load conditions.
Most circuit protection applications require that
enclosed fuseholders limit user contact with live parts
during operation or servicing. Fuseholders
manufactured with integral protection against electric
shock are termed “Shocksafe”. These holders are
designed such that live parts are not accessible when
the fuseholder is correctly assembled and installed on
panels or printed circuit boards. In addition, live parts
are not accessible during insertion or removal of the
fuse carrier. Compliance is checked by a “test finger”
as described in the standard IEC 60529. Shocksafe
fuseholders are commonly used in instrumentation,
medical and consumer products. All other
non-shocksafe fuseholders specified the in following
data pages fall under the PC1 category which
describes fuseholders without integral protection
against shock (ie. fuse blocks).
INSTALLATION
Miniature fuses may be installed in clips, open blocks,
enclosed fuseholders, or axial leads for direct
soldering. When bending wires on all axial leaded
fuses, a mechanical support is recommended. The
wires should be bent at a minimum of two (2) lead
diameters from the fuse body to prevent damage.
All subminiature fuses and protectors with wire leads
are suitable for wave and iron (hand) soldering
procedures. For wave soldering, we recommend a
soldering time of three (3) seconds at 260 ℃
maximum. For iron soldering, a duration of
approximately one (1) second at 350℃ is sufficient.
Heat sinking should be considered.
Surface mount fuses are constructed to handle all
typical SM soldering processes including wave, IR
reflow, and vapor phase. For inquiries about special
soldering methods, please contact us.
All protectors, subminiature and surface mount fuses
can be exposed to aqueous-based board wash
solutions. Since miniature cartridge fuses are not
sealed, provisions should be made if these types are
to be exposed to any board washing.
Maximum Load Values
Operating Current Fuse Power Dissipation
Minimum Cross Sections(Copper)
Conducting Path Conductor Size AWG
6.3A 1.6W 0.1mm2 0.75mm2 18-17
10.0A 2.5W 0.2mm2 1.50mm2 15-13
16.0A 4.0W 0.2mm2 2.50mm2 13-11
Conductor Requirements Table 4
Company: Fax to: OUR
Address: Applications Engineering
Fax: +86/ (0755)8286 5705(Shenzhen)
Fax: +86/ (0510)8561 5399(Wuxi)
Tel: Fax: +86/ (852)2541 9339(Hong Kong)
Fax:
E-mail:
1. Where do you intend to approve your device?
□ North America
□ Europe
□ Japan
□ Worldwide
□ No approvals required
2. How will the fuse or protector be mounted?
□ Replaceable, withholder or clips
□ Solderable, with leads
□ Solderable, surface mount
□ Specify
3. What is the maximum effective operating voltage?
V □ AC □ DC
4. What is the maximum continuous operating current?
A
5. What is the maximum ambient temperature in close proximity to the fuse or protector?
℃
6. What fault currents can occur?
What melting times do you require?
IFault1: A tMelt1,max: s
IFault2: A tMelt2,max: s
7. What is the maximum possible fault current?
IFault,max: A
8. Are there current pulses when turning on the device?
□ Yes, with the following waveform (Circle one)
You may also attach an oscillogram
□ No, there are no inrush current pulses
9. Insert the waveform melting integral, if known.
A2s
10. Quantity of pulses projected over the product life.
□
□ 1000
□ 10,000
□ 100,000
□ 1,000,000
11. Description of the device in which the fuse or protector will be installed (ie. primary circuit of TV set).
12. Estimated annual usage: pcs.
□ Please provide samples.
□ Please provide a quotation.
Company: Fax to: OUR
Address: Applications Engineering
Fax: +86/ (0755)8286 5705(Shenzhen)
Fax: +86/ (0510)8561 5399(Wuxi)
Tel: Fax: +86/ (852)2541 9339(Hong Kong)
Fax:
E-mail:
1. Where do you intend to approve your device?
□ North America
□ Europe
□ Japan
Worldwide
□ No approvals required
2. How will the fuse or protector be mounted?
Replaceable, withholder or clips
□ Solderable, with leads
□ Solderable, surface mount
□ Specify
3. What is the maximum effective operating voltage?
240 V AC □ DC
4. What is the maximum continuous operating current?
1.0 A
5. What is the maximum ambient temperature in close proximity to the fuse or protector?
40 ℃
6. What fault currents can occur?
What melting times do you require?
IFault1: 50 A tMelt1,max: 0.01 s
IFault2: A tMelt2,max: s
7. What is the maximum possible fault current?
IFault,max: 150 A
8. Are there current pulses when turning on the device?
Yes, with the following waveform (Circle one)
You may also attach an oscillogram
□ No, there are no inrush current pulses
9. Insert the waveform melting integral, if known.
? A2s
10. Quantity of pulses projected over the product life.
□
□ 1000
□ 10,000
100,000
□ 1,000,000
11. Description of the device in which the fuse or protector will be installed (ie. primary circuit of TV set).
Primary circuit of switching power supply
12. Estimated annual usage: 50K pcs.
Please provide 10 samples.
Please provide a quotation.
Background
Transient overvoltages occurring in telephone lines
can usually be traced to two sources: (1) atmospheric
interference such as lightning, and (2) AC power lines.
Solid state circuitry in telephone subscriber equipment
and central office equipment is extremely sensitive to
these overvoltages and excessive currents. Various
types of current and voltage sensitive components
can be selected to protect telecommunications
equipment from lightning-induced and power
system-induced overvoltages. Listed herein is
background information and circuit design
considerations.
Equipment
Analog Telephones
ISDN Telephones
Modems
Facsimile Machines
Answering Machines
Switching Line Cards
Router Adapter Cards
Videoconferencing Equipment
xDSL Equipment
Line Test Equipment
PBX Equipment
Lightning
Nearby lightning strikes are the most common source
of atmospheric interference on telephone lines. Most
telephone lines are jacketed in shielded cables.
Nearby lightning can cause excessive currents to flow
on the shield surface, thereby inducing transient
overvoltages between the internal conductor and
shield itself. The levels of these overvoltages depend
on the construction of the cable and it’s internal
impedance. Indirect lightning can enter buried
telephone lines via ground currents. These currents
will exit the shield at each grounded pole along it’s
path until fully dissipated. Since the cable has a fixed
impedance, a potential gradient will be created along
the cable’s entire length. The gradient will induce a
potential difference between the cable and soil. At
several points along the cable, it’s dielectric value will
be exceeded, causing a puncture. At each puncture
point, a portion of the lightning currents will eventually
enter sensitive telecommunication equipment.
Lightning Interference Figure7
Power System
The power source for telecommunications equipment
is taken from commercial AC power lines. Typically,
telephone and power lines are run side-by-side
suspended on poles or buried in the earth.
A telephone line consists of many pairs of conductors
arranged in twisted pairs called tip and ring lines.
Regardless of location, twisted pairs are capable of
picking up transient overvoltages and transmitting
them to central office or subscriber equipment. Two
sources of power system interference can be
attributed to power induction and power cross. Power
induction is the result of circulating currents in the
earth or high currents in adjacent lines induced on
telephone lines due to their close proximity to power
lines. The more damaging source of transient
overvoltages is from direct physical contact (ie.
short-circuit) between telephone and power lines.
Power cross can last several minutes and cause
excessive voltage development and current flow into
telephone equipment. Power cross has been traced
as the cause of many building and equipment fires.
Power System Interference Figure 8
Effects
During recent years, increased usage of telephone
lines for voice, data, and video transmission has
caused an increased sensitivity to overvoltages. Solid
state IC’s are much more susceptible to damage or
failure from transients and line noise conditions. Since
these surges are typically unidirectional impulses,
they can cause permanent damage to components,
equipment fires and shock hazards. Temporary faults,
such as tip and ring imbalance, are also common
occurrences. The predominant industry standards are
below.
UL 1950 – 3rd Edition
When introduced in April 2000, one objective of UL
1950, “Safety of Information Technology Equipment,”
was to protect users from fire hazard and electrical
shock. Functionality of the equipment was not
covered under this standard. Test procedures in
Section 6.6 required that the telecommunication
equipment be wrapped in cheesecloth and connected
to a power source through a 2 Amp time delay fuse or
No. 32 AWG wire. A series of overvoltages and
overcurrents simulating long and short term power
induction and power cross conditions are applied
across the equipment’s tip and ring line input. The
purpose of the 2 Amp test fuse or No. 32AWG wire is
to simulate building wiring. Equipment is to be tested
in both line-to-ground and line-to-line modes as
shown:
Test Voltage(VRMS) Current(A) Duration
L1/M1 600 40 1.5sec
L2/M2 600 7 5sec.
L3/M3 600 2.2 30min.
UL 1950 Test conditions Table5
The varying currents simulate short and long term
induction and power cross. In all tests, the purpose of
the equipment’s overcurrent protector (fuse) is to
safely interrupt the fault current without ignition or
charring of the cheesecloth and before the wiring
simulator opens. The equipment fuse must limit
current to within the acceptable range as shown in
Figure 9. The fuse’s melting integral (I2t) must be less
than that of the wiring simulator at all test points to
guarantee a successful circuit interruption.
Additionally, tests L4/M4 prescribe criteria where the
equipment fuse is jumped red and 600 Volts is applied
at 135% of the fuse current rating (2.2 Amps max) for
up to 30 minutes. Power dissipation (selfheanting) is
monitored in nearby circuit components.
UL 1950 Wiring Simulator Characteristics Figure9
All telecommunication lines entering a building pass
through a primary protector shunting voltages in
excess of 600 Volts. Unfortunately, these coarse
protectors allow potentially damaging voltages to
remain on the lines. The purpose of the secondary
equipment overvoltage device is to clamp
overvoltages during power cross conditions and
protect the downstream load. When solid state
overvoltage devices fail, it is usually in the shorted
mode, producing a limited impedance path of current
flow. Depending on the impedance value, this can
potentially cause a fire hazard in the building wiring or
the equipment. The equipment fuse is used to
eliminate these hazardous conditions. The designer
should exercise caution during fuse selection, since
too low of a current rating may pass UL 1950
satisfactorily, but not withstand the simulated lightning
surge waveforms describe in the next sections. Too
high of a fuse rating can potentially allow damaging
currents to destroy downstream components. At
present, UL 1950 – 3rd Edition must be referenced to
test all new Information Technology equipment. In
2005, all new equipment must be Listed to UL 60950
– 3rd Edition.
FCC Part 68
Most products or devices which interface to the U.S.
or Canadian telecommunications network must be
registered with the Federal Communications
Commissions under Part 68. “Connection of Terminal
Equipment to the Telephone Network” is a
governmental regulation which prescribes a series of
tests and procedures that are designed to determine if
equipment has any potential to harm the network.
This standard does not address subscriber equipment
requirements – only harm to network criteria.
Specifically, Section 302 lists a series of simulated
lightning waveforms that potentially can cause fire and
shock hazards to humans and equipment. The basic
tests are as follows:
Surge Type Voltage
(V)
Current
(A) Waveform
#
Pulses
Metallic 800 100 10x560μs 2
Longitudinal 1500 200 10x160μs 2
Longitudinal 2500 1000 2x10μs 6
FCC Test Conditions Table 6
Metallic surges are applied tip to ring while
longitudinal surges are applied between tip to ground
and ring to ground. The primary objective of these
tests is to allow for telecommunications equipment
interface circuitry to safely divert lightning-like
transients to ground. If the circuitry maintains the path
to ground after the transient subsides, the potential for
network interference exists due to line imbalance. The
equipment should remain fully operational after these
surge voltages are applied. This means that the
equipment fuse does not open and the overvoltage
device returns to it’s high impedance state.
FCC Metallic Surge Waveform Figure 10
Bellcore 1089
Many manufacturers are now required to not only
pass UL 1950 and FCC Part 68, but also comply with
Bellcore GR-1089-CORE: “Electromagnetic
Compatibility and Electrical Safety – Generic Criteria
for Network Telecommunications Equipment.” Already
a leading provider of communications software and
consulting services based on extensive research,
Telcordia Technologies (formerly bellcore) creates the
business solutions that make information technology
work for telecommunications carriers, businesses and
governments worldwide. It is quite common for
equipment supplied to the Bell operating company
clients to be subjected to GR-1089-CORE. This
standard essentially combines lighting immunity, AC
power cross, and power induction criteria into one
document. Because of it’s higher energy surge
waveforms documents. Because of it’s higher energy
surge waveforms and higher pulse repetition, the
designer must select more surge tolerant circuitry
than required by UL and FCC alone. Unlike UL
1950-3rd Edition, Bellcore does not impose a limit on
the fuse rating.
These simulated lightning and AC power fault tests
are applied to the telecommunications (tip & ring)
ports of the equipment and assume it is externally
protected by 3-milcarbon blocks. First – level lightning
surge criteria, shown in Table 7, states that the
equipment shall be undamaged and continue to
operate properly after the waveform is removed.
Second-level lightning surge criteria, shown in Table 8,
allows the equipment to sustain damage, but shall not
present a fire, fragmentation, or electrical safety
hazard.
Surge No. Voltage
(v)
Current
(A) Waveform #Pulses
1 600 100 10x1000μs 25
2 1000 100 10x360μs 25
3 1000 100 10x1000μs 25
4 2500 500 2x10μs 10
5 1000 25 10x360μs 5
Bellcore 1st Level Lightning Table 7
Surge No. Voltage(V) Current(A) Waveform #Pulses
1 5000 500 2x10μs 1
Bellcore 2nd Level Lightning Table 8
Note that in Table 7, Surge No. 3 may be performed in
lieu of Surge Nos. 1 and 2 to expedite testing. The
equipment must withstand a total of 50 pulses (25 per
polarity) for Surge Nos.1 thru 3 and remain
operational. Various longitudinal test connections of
the tip and ring ports to the surge generator and
ground are specified in the standard.
Bellcore Surge No.3 Waveform Figure11
First-level AP Power Fault criteria, shown in Table 9,
states that the equipment shall be undamaged and
continue to operate properly after the waveform is
removed. Second-level AC Power Fault criteria,
shown in Table 10, allows the equipment to sustain
damage, but shall not present a fire, fragmentation, or
electrical safety hazard.
Test
No.
Voltage
(VRMS) Current(A) Duration
#
Pulses
1 50 0.33 15 min. 1
2 100 0.17 15 min. 1
3 200,400,600 1 1 sec. 60
4 1000 1 1 sec. 60
5 600 See Std. 5 sec. 60
Bellcore 1st Level AC Power Fault Table9
Test No. Voltage(VRMS) Current(A) Duration
1 120,277 25 15 min.
2 600 60 5 sec.
3 600 7 5 sec.
4 100-600 2.2 15 min.
5 600 See Std. 15 min.
Bellcor 2nd Level AC Power Fault Table10
In Table 10, tests Nos. 1 and 2 simulate secondary
and primary power cross conditions, respectively. The
remaining tests prescribe criteria for short and long
term power induction. The designer, at their discretion,
may elect to substitute prior UL 1950 test results for
this section. Only Test No. 1, conducted at 277 Volts,
must be performed, since it is not addressed in UL
1950.
Table 10 requirements are intended to cover
non-customer premises equipment (non-CPE).
Criteria for terminal equipment (telephone sets, PBX,
etc.) located on the customer premises is subjected to
two AC power cross conditions. First, 300VAC is
applied between exposed conductive surfaces and
ground and then 600 VAC is applied in both metallic
and longitudinal modes across the tip & ring ports and
ground. These test voltages are initially applied at 30
VAC and raised slowly such that current is limited to
100mA and the voltage does not rise greater than 20
percent over any 15 minute interval. Voltage is
increased until one of the following conditions occurs:
(1) the maximum voltage is reached, (2)the current
reaches 20 Amps or the wiring simulator (1.6A glass
fuse) opens or, (3) the equipment fails open-circuited.
If the wiring simulator opens or the equipment fails,
the equipment has failed the power cross test.
ITU-T Recommendation K.20
The International Telecommunications Union (ITU) is
an intergovernmental organization, consisting of 185
member states within which the public and private
sectors cooperate for the development of
telecommunications. The ITU adopts international
regulations and treaties governing all terrestrial and
space uses of the frequency spectrum, within which
countries adopt their own national legislation. ITU also
develops standards to facilitate the interconnection of
telecommunication systems worldwide regardless of
the type of technology used. Recommendation K.20
named, “Resistibility of Telecommunications Switching
Equipment to Overvoltages and Overcurrents” was
issued in October of 1996. This standard relates to
PBX exchanges and similar switching centers and is
concerned mainly with stress conditions to be applied
to points of connections to subscriber lines. Tests
include surges due to lightning on or near telephone
central offices, short term induction of voltages from
adjacent power lines or railways, and direct contact
between telephone lines, power lines and electrostatic
discharges. A summary of tests in Table 11 follows:
Test No.Voltage
(V)
Current
(A) Duration #Pulses
1a 1000 67 10x700μs 30
1b 4000 See Std. 10x700μs 30
2a 600 1 0.2 sec. 5
2b 600 1 1 sec. 5
3 230 60,3,1 15 min. 1/5
ITU – T K.20 Test Conditions Table 11
Tests 1a and 1b describe metallic and longitudinal
lightning surges conducted at 1000 volts and 4000
Volts peak. Test 1b only applies to equipment exposed
to harsh environments where a specific type of
overvoltage device is agreed upon. The time interval
between consecutive surges is one minute and the
polarity is reversed between each surge.
In Tests 2a and 2b, power induction is simulated with
five surges applied at 600Volts thru a 600Ω resistor
for 0.2 seconds and one second, respectively. In the
aforementioned tests, the equipment shall withstand
the surges without damage and shall continue to
operate within the specified limits afterwards. A power
cross voltage of 230 Volts in Test 3 is applied for 15
minutes each through varying resistor values. These
values are to simulate power cross incidents at
varying distances from the telephone exchange. The
power cross tests may damage the equipment
irreparably, but the fuse shall safely interrupt the fault
and no fire may occur.
Telecom Circuit Protector
While many types of OUR time-lag or time-delay fuses
may be considered for telecom line protection, we
recommend the surface mountable Telecom Circuit
Protector (TCP). The TCP has been tested under the
telecommunications standards described herein.
Product features include:
* Low Profile Chip
* Small Footprint
* Non-flammable Housing
* Auto-Insertable
* Surge Tolerant
* Low Voltage Drop
* Zero Leakage Current
Protector Selection
Circuit analysis must be examined to determine the
proper TCP and resistor ratings to successfully pass
FCC lightning waveforms. Two typical tip & ring
interface circuits are shown below:
Fused Tip Line Interface Figure 12
Fused Tip & Ring Line Interface Figure 13
In these sample interface circuits, the series resistor(s)
are employed to limit the peak currents seen during
the various lightning immunity tests. The TCP is
required to safely interrupt current flow to the
equipment during a sustained power cross condition.
The overvoltage device, placed in parallel, should limit
and divert transient overvoltages seen during a power
cross or power induction condition. Upon sensing a
transient, the overvoltaged device will switch to a low
“on state” voltage. Current conduction will continue
through the over-voltage device until the fuse
interrupts the circuit or the fault is removed.
In order for the OUR TCP-1A to pass FCC Part 68
metallic surges operationally, the following series
resistor example calculations from Figure 13 can be
made.
FCC Part 68 Lightning Surges (10X560μs)
1) Total Loop Impedance:
RTotal = RSource + RTip +RRing
(where RTip = RRing):
RTotal = RSource + 2RTip
2) Generator Source Impedance:
RSource = I(Source)
V(Source)
RSource = 100A
800V = 8Ω
3) Fuse Peak Surge Current:
IPeak (10x560μs) = Value from Data sheet
IPeak (10x560μs) = 40 Amps
4) Series Resistor Value (s):
RTotal = (10x560us)I(Peak
V(Source)
RTotal = 40A
800V = 20Ω
RTip = RRing = 2
R(Source))-(R(Total)
RTip = RRing = 2
8ohm)-(20ohm = 6Ω
For circuits corresponding to Figure 12 where only the
tip line is fused, the resistor value should be doubled
to 12Ω or greater. Since internal impedances can
vary in different applications, the designer is urged to
perform these transient overvoltage tests on the
equipment prior to submission for agency approval.
Please note that the TCP-1.5 A with a FCC Part 68
surge current rating of 108 Amps will not require any
additional series resistance.
Recall that the resistor’s purpose is to limit peak
currents prescribed in the lightning simulation tests.
Please note that surge current capability of the
overvoltage device must also be checked if series
resistors are eliminated from the circuit. Overvoltage
device manufacturers typically provide surge
waveform test data on their components as required
by FCC, ITU, and Bellcore. Contact OUR for other
circuit design examples when compliance with
Bellcore or ITU standards is required.
For power supply or battery charger applications in
telecommunications equipment, please refer to the
following fuse types: 5x20mm, MEF, MAT, MST, Chip
Fuses, and Blade-type fuses.
Standards Agencies
Underwriters Laboratories, Inc.
333 Pfingsten Road
Northbrook, IL 60062
USA
TEL: +1/ (708) 272-8800
FAX: +1/ (708) 272-8129
WEB: www.ul.com
Federal Communication Commission
1919 M Street NW
Washington, DC 20554
USA
TEL: +1/ (202) 418-0200
FAX: +1/ (202) 418-2825
WEB: www.fcc.gov
Telcordia Technologies (Bellcore)
8 Corporate Place
Piscataway, NJ 08854
USA
TEL: +1/ (732) 699-5800
FAX: +1/ (732) 336-2559
WEB: www.telcordia.com
International Telecommunications Union
Place des Nations
CH-1211 Geneva 20
Switzerland
TEL: +41/ 22 730 5111
WEB: www.itu.ch