SIST-TP IEC TR 60269-5:2022

IEC TR 60269-5 ®
Edition 2.1 2020-12
CONSOLIDATED VERSION
TECHNICAL
REPORT
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Low-voltage fuses –
Part 5: Guidance for the application of low-voltage fuses

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IEC TR 60269-5 ®
Edition 2.1 2020-12
CONSOLIDATED VERSION
TECHNICAL
REPORT
colour
inside
Low-voltage fuses –
Part 5: Guidance for the application of low-voltage fuses

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.120.50 ISBN 978-2-8322-9218-1

IEC TR 60269-5 ®
Edition 2.1 2020-12
CONSOLIDATED VERSION
REDLINE VERSION
colour
inside
Low-voltage fuses –
Part 5: Guidance for the application of low-voltage fuses

– 2 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Fuse benefits . 12
5 Fuse construction and operation . 13
5.1 Components . 13
5.2 Fuse-construction . 13
5.2.1 Fuse link . 13
5.2.2 Fuse-link contacts . 14
5.2.3 Indicating device and striker . 15
5.2.4 Fuse-base . 15
5.2.5 Replacement handles and fuse-holders . 15
5.3 Fuse operation . 15
5.3.1 General . 15
5.3.2 Fuse operation in case of short-circuit . 15
5.3.3 Fuse operation in case of overload . 16
5.3.4 Fuse link pre-arcing time current characteristic: . 17
5.3.5 Fuse operation in altitudes exceeding 2 000 m . 18
6 Fuse-combination units . 18
7 Fuse selection and markings . 20
8 Conductor protection . 23
8.1 General . 23
8.2 Utilization category gG . 23
8.3 Utilization category gN and gD . 25
8.4 Utilization category gR and gS . 25
8.5 Utilization category gU . 26
8.6 Utilization category gK . 26
8.7 Utilization category gPV . 26
8.8 Utilization category gBat . 26
8.9 Protection against short-circuit current only . 26
9 Selectivity of protective devices . 26
9.1 General . 26
9.2 Selectivity between fuses . 27
9.2.1 General . 27
9.2.2 Verification of selectivity for operating time ≥ 0,1 s . 27
9.2.3 Verification of selectivity for operating time < 0,1 s . 28
9.2.4 Verification of total selectivity . 28
9.3 Selectivity of between circuit-breakers upstream of and fuses . 28
9.3.1 General . 28
9.3.2 Verification of selectivity for operating time ≥ 0,1 s . 29
9.3.3 Verification of selectivity for operating time < 0,1 s . 29
9.3.4 Verification of total selectivity . 29
9.4 Selectivity of between fuses upstream of and circuit-breakers . 30

© IEC 2020
9.4.1 General . 30
9.4.2 Verification of selectivity for operating time ≥ 0,1 s . 30
9.4.3 Verification of selectivity for operating time < 0,1 s . 30
9.4.4 Verification of total selectivity . 30
10 Short-circuit damage protection . 32
10.1 General . 32
10.2 Short-circuit current paths . 32
10.3 Current limitation . 33
10.4 Rated conditional short-circuit current, rated breaking capacity . 33
11 Protection of power factor correction capacitors . 33
12 Transformer protection . 34
12.1 Distribution transformers with a high-voltage primary . 34
12.2 Distribution transformers with a low-voltage primary . 35
12.3 Control circuit transformers . 35
13 Motor circuit protection . 35
13.1 General . 35
13.2 Fuse and motor-starter coordination . 36
13.3 Criteria for coordination at the rated conditional short-circuit current I . 36
q
13.4 Criteria for coordination at the crossover current I . 37
co
13.5 Criteria for coordination at test current “r” . 37
14 Circuit-breaker protection in a.c. and d.c rated voltage circuits . 38
15 Protection of semiconductor devices in a.c. and d.c. rated voltage circuits . 38
15.1 General recommendations . 38
15.2 Fuse application with inverters . 40
15.2.1 Inverters . 40
15.2.2 Purpose of the fuse . 41
15.2.3 Current carrying capacity . 45
15.2.4 Voltage considerations . 45
15.2.5 I t characteristics . 46
15.2.6 Breaking range . 46
16 Fuses in enclosures . 46
16.1 General . 46
16.2 Limiting temperature of utilization category gG fuse-links according to
IEC 60269-2 – System A . 47
16.3 Other fuse-links . 47
17 DC applications . 47
17.1 General . 47
17.2 Short-circuit protection . 47
17.3 Overload protection . 48
17.4 Time-current characteristics . 49
18 Automatic disconnection for protection against electric shock for installations in
buildings . 49
18.1 General . 49
18.2 Principle of the protection . 50
18.3 Examples . 51
19 Photovoltaic (PV) system protection . 52
19.1 General . 52
19.2 Selection of PV fuse-links . 53

– 4 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
19.2.1 Fuse utilization category . 53
19.2.2 PV string fuses . 53
19.2.3 Fuse replacement . 53
19.2.4 Unearthed or Ungrounded PV Systems . 53
19.2.5 Functional earthing fuses . 53
19.2.6 PV array and PV sub-array fuses . 53
19.2.7 Fuse monitoring . 54
19.2.8 Breaking capacity . 54
19.2.9 Voltage of gPV fuses . 54
19.2.10 Rated current of gPV fuses . 54
20 Protection of wind mills . 54
21 Guidance for the selection of a fuse for the protection of Battery systems . 55
21.1 General . 55
21.2 Voltage characteristics . 55
21.2.1 Rated voltage . 55
21.3 Current carrying capability . 55
21.3.1 Rated current . 55
21.4 Breaking capacity . 55
Annex A (informative) Coordination between fuses and contactors/motor-starters . 56
A.1 General . 56
A.2 Examples of suitable fuse-links used for motor protection . 56
A.3 Values of I t and cut-off current observed in successful tests of fuse-
link/motor-starter combinations worldwide . 57
A.4 Criteria for coordination at the rated conditional short-circuit current I . 60
q
A.4.1 General . 60
A.4.2 Maximum operating I t and cut-off current . 60
A.4.3 Guidance for choosing the maximum rated current of an alternative fuse
type . 61
A.4.4 Further guidance . 61
A.5 Criteria for coordination at test current "r" . 62
A.6 Types of coordination . 63
Bibliography . 66

Figure 1 – Typical fuse-link according to IEC 60269-2. 14
Figure 2 – Typical fuse-link according to IEC 60269-2. 14
Figure 3 – Current-limiting fuse operation . 16
Figure 4 – Fuse operation on overload . 17
Figure 5 – Time current characteristic for fuse-links . 17
Figure 6 – Currents for fuse-link selection . 25
Figure 7 – Selectivity – General network diagram . 27
Figure 8 – Verification of selectivity between fuses F and F for operating
2 4
time t ≥ 0,1 s . 28
Figure 9 – Verification of selectivity between circuit-breaker C and fuses F and F . 29
2 5 6
Figure 10 – Verification of selectivity between fuse F and circuit-breaker C for
2 3
operating time t ≥ 0,1 s . 31
Figure 11 – Verification of selectivity between fuse F and circuit-breaker C for
2 3
operating time t < 0,1 s . 32
Figure 12 – Fuse and motor-starter coordination . 37

© IEC 2020
Figure 13 – DC circuit . 48
Figure 14 – DC breaking operation . 48
Figure 15 – Fuse operating time at various d.c. circuit time constants . 49
Figure 16 – Time-current characteristic . 51
Figure 17 – Inverter double-way connection with arm fuses for regenerative or non-
regenerative load . 40
Figure 18 – Inverter double-way connection with d.c. loop fuses for regenerative or
non-regenerative load . 40
Figure 19 – Multi inverters systems double-way connection with d.c. loop fuses for
regenerative or non-regenerative load . 41
Figure 20 – Capacitor discharge . 42
Figure 21 – Voltage across the capacitor . 43
Figure 22 – Inductance of the circuit . 44
Figure A.1 – Collation of cut-off currents observed in successful coordination at I . 58
q
Figure A.2 – Pre-arcing and operating I t values of fuses used in successful
coordination tests as a function of contactor rated current AC3 . 59
Figure A.3 – Pre-arcing and operating I t values of fuses used in successful
coordination tests as a function of fuse rated current I . 60
n
Figure A.4 – Illustration of the method of selection of the maximum rated current of a
fuse for back-up protection of a contactor of rating I = X amperes . 63
e
Figure A.5 – Withstand capabilities of a range of contactors and associated overload
relays at test current "r" . 64
Figure A.6 – Illustration of a method of deriving curves of maximum peak current at
test current "r" as a function of fuse rated current . 65

Table 1 – Derating factors for different altitudes . 18
Table 2 – Definitions and symbols of switches and fuse-combination units . 19
Table 3 – Fuse application . 21
Table 4 – Maximum operational voltage of a.c. fuse-links . 22
Table 5 – Typical operational voltage ratings of d.c. fuse-links . 22
Table 6 – Fuse selection for power factor correction capacitors (fuses according to
IEC 60269-2, system A) . 34
Table 7 – Conventional non fusing current . 39
Table 8 – Time constants of typical d.c. circuits . 49
Table A.1 – Examples of typical fuse-link ratings used for motor-starter protection
illustrating how the category of fuse-link can influence the optimum current rating . 57
Table A.2 (Table 12 of IEC 60947-4-1:2009) – Value of the prospective test current
according to the rated operational current . 62
Table A.3 – Types of coordination . 63

– 6 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW-VOLTAGE FUSES –
Part 5: Guidance for the application of low-voltage fuses

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
This consolidated version of the official IEC Standard and its amendment has been
prepared for user convenience.
IEC 60269-5 edition 2.1 contains the second edition (2014-03) [documents
32B/621A/DTR and 32B/624/RVC] and its amendment 1 (2020-12) [documents
32B/694/DTR and 32B/697A/RVDTR].
In this Redline version, a vertical line in the margin shows where the technical content
is modified by amendment 1. Additions are in green text, deletions are in strikethrough
red text. A separate Final version with all changes accepted is available in this
publication.
© IEC 2020
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60269-5, which is a technical report, has been prepared by subcommittee 32B: Low-
voltage fuses, of IEC technical committee 32: Fuses.
This second edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) recommendations for fuse operations in high altitudes added
b) more details for operational voltages added
c) recommendations for photovoltaic system protection added
d) numerous details improved
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60269 series, under the general title: Low-voltage fuses, can be
found on the IEC website.
The committee has decided that the contents of the base publication and its amendment will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 8 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
INTRODUCTION
Fuses protect many types of equipment and switchgear against the effects of over-current
which can be dramatic:
– thermal damage of conductors or bus-bars;
– vaporisation of metal;
– ionisation of gases;
– arcing, fire, explosion,
– insulation damage.
Apart from being hazardous to personnel, significant economic losses can result from
downtime and the repairs required to restore damaged equipment.
Modern fuses are common overcurrent protective devices in use today, and as such provide
an excellent cost effective solution to eliminate or minimize the effects of overcurrent.

© IEC 2020
LOW-VOLTAGE FUSES –
Part 5: Guidance for the application of low-voltage fuses

1 Scope
This technical report, which serves as an application guide for low-voltage fuses, shows how
current-limiting fuses are easy to apply to protect today's complex and sensitive electrical and
electronic equipment. This guidance specifically covers low-voltage fuses up to 1 000 V a.c.
and 1 500 V d.c. designed and manufactured in accordance with IEC 60269 series. This
guidance provides important facts about as well as information on the application of fuses.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary. Available from
http://www.electropedia.org/
IEC/TR 60146-6, Semiconductor convertors – Part 6: Application guide for the protection of
semiconductor convertors against overcurrent by fuses
IEC 60269 (all parts), Low-voltage fuses
IEC 60269-1:2006, Low-voltage fuses - Part 1: General requirements
IEC 60269-1:2006/AMD1:2009
IEC 60269-1:2006/AMD2:2014
IEC 60269-2, Low-voltage fuses – Part 2: Supplementary requirements for fuses for use by
authorized persons (fuses mainly for industrial application) – Examples of standardized
systems of fuses A to K
IEC 60269-3, Low-voltage fuses – Part 3: Supplementary requirements for fuses for use by
unskilled persons (fuses mainly for household or similar applications) – Examples of
standardized systems of fuses A to F
IEC 60269-4:2009, Low-voltage fuses – Part 4: Supplementary requirements for fuse-links for
the protection of semiconductor devices
IEC 60269-6, Low-voltage fuses – Part 6: Supplementary requirements for fuse-links for the
protection of solar photovoltaic energy systems
IEC 60364-4-41:2005, Low-voltage electrical installations – Part 4-41: Protection for safety –
Protection against electric shock
IEC 60364-4-43:2008, Low-voltage electrical installations – Part 4-43: Protection for safety –
Protection against overcurrent
IEC 60364-5-52, Low-voltage electrical installations – Part 5-52: Selection and erection of
electrical equipment – Wiring systems

– 10 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
IEC 60947 (all parts), Low-voltage switchgear and controlgear
IEC 60947-3:20082015, Low-voltage switchgear and controlgear – Part 3: Switches,
disconnectors, switch-disconnectors and fuse-combination units
IEC 60947-4-1:2009, Low-voltage switchgear and controlgear – Part 4-1: Contactors and
motor-starters – Electromechanical contactors and motor-starters
IEC/TR 61912-1:2007, Low-voltage switchgear and controlgear – Overcurrent protective
devices – Part 1: Application of short-circuit ratings
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
switch (mechanical)
mechanical switching device capable of making, carrying and breaking currents under normal
circuit conditions, which may include specified operating overload conditions and also
carrying, for a specified time, currents under specified abnormal conditions such as those of
short-circuits
Note 1 to entry: A switch may be capable of making but not breaking, short-circuit currents.
[SOURCE: IEC 60050-441:1984, 441-14-10]
3.2
disconnector
mechanical switching device that, in the open position, complies with the requirements
specified for isolating function
Note 1 to entry: Some disconnectors may not be capable of switching load.
[SOURCE: IEC 60050-441:1984, 441-14-05, modified (modified definition and Note 1 to entry
added)]
3.3
fuse-combination unit
combination of a mechanical switching device and one or more fuses in a composite unit,
assembled by the manufacturer or in accordance with his instructions
[SOURCE: IEC 60050-441:1984, 441-14-04, modified (Note removed)]
3.4
switch-fuse
switch in which one or more poles have a fuse in series in a composite unit
[SOURCE: IEC 60050-441:1984, 441-14-14]
3.4.1
single-break and double-break
switch-fuse must be single break (it opens the circuit on one side of the fuse link) or double
break (it opens the circuit on both sides of the fuse link)
3.5
fuse-switch
switch in which a fuse-link or a fuse-carrier with fuse-link forms the moving contact

© IEC 2020
[SOURCE: IEC 60050-441:1984, 441-14-17]
3.5.1
single-break and double-break
fuse-switch must be single break (it opens the circuit on one side of the fuse link) or double
break (it opens the circuit on both sides of the fuse link)
3.6
Switching device SD
device designed to make or break the current in one or more electric circuits
Note 1 to entry: A switching device may perform one or both of these operations.
[SOURCE: IEC 60050-441:1984, 441-14-01, modified (Note 1 to entry added)]
3.7
short-circuit protective device SCPD
device intended to protect a circuit or parts of a circuit against short-circuits by interrupting
them
3.8
overload protection
protection intended to operate in the event of overload on the protected section
[SOURCE: IEC 60050-448:1995, 448-14-31]
3.9
overload
operating conditions in an electrically undamaged circuit, which cause an over-current
[SOURCE: IEC 60050-441:1984, 441-11-08]
3.10
overcurrent
current exceeding the rated current
[SOURCE: IEC 60050-442:1998, 442-01-20]
3.11
rated conditional short-circuit current (of a switching device)
I
q
prospective current that a switching device, protected by a short-circuit protective device, can
satisfactorily withstand for the operating time of that device under test conditions specified in
the relevant product standard
3.12
selectivity of protection
ability of a protection to identify the faulty sections and/or phase(s) of a power system
Note 1 to entry: Whereas the terms “selectivity” and “discrimination” have a similar meaning according to the IEV
definitions, this report prefers and uses the term “selectivity” to express the ability of one over-current device to
operate in preference to another over-current device in series, over a given range of over-current. The effect of
standing load current on selectivity in the overload zone is also considered.
[SOURCE: IEC 60050-448:1995, 448-11-06, modified (Note 1 to entry added)]

– 12 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
4 Fuse benefits
The current-limiting fuse provides complete protection against the effects of overcurrents by
protecting both, electric circuits and their components. Fuses offer a combination of
advantageous features, for example:
a) High breaking capacity (high current interrupting rating).
b) No need for complex short-circuit calculations.
c) Easy and inexpensive system expansion in case of increased fault currents.
d) High current limitation (low I t values).
e) Mandatory fault elimination before reenergizing.
Fuses cannot be reset, thus forcing the user to identify and correct the fault condition
before re-energizing the circuit.
f) Reliability.
No moving parts to wear out or become contaminated by dust, oil or corrosion. Fuse
replacement ensures protection is restored to its original level when the fuse is replaced.
g) Cost effective protection.
Compact size offers low cost overcurrent protection at high short-circuit levels.
h) No damage for starters and contactors (type 2 protection according to IEC 60947-4-1).
By limiting short-circuit energy and peak currents to extremely low levels, fuses are
particularly suitable for type 2 protection without damage to components in motor circuits.
Compact size offers economical overcurrent protections at high short-circuit levels
i) Safe, silent operation.
No emission of gas, flames, arcs or other materials when clearing the highest levels of
short-circuit currents. In addition, the speed of operation at high short-circuit currents
significantly limits the arc flash hazard at the fault location.
j) Easy coordination.
Standardized fuse characteristics and a high degree of current limitation ensure effective
coordination between fuses and other devices.
k) Standardized performance.
Fuse-links designed and manufactured in accordance with IEC 60269 series ensure
availability of replacements with standardized characteristics throughout the world.
l) Improved power quality.
Current-limiting fuses interrupt high fault currents in a few milliseconds, minimizing dips or
sags in system supply voltage.
m) Tamperproof.
Once installed, fuses cannot be modified or adjusted thus preserving their level of
performance and avoiding malfunction.
n) No maintenance.
Properly sized fuses require no maintenance, adjustments or recalibrations. They can
remain in service providing originally designed overcurrent protection levels for many
decades.
o) High level of energy efficiency.
The resistance and therefore the power dissipation of the fuse is very low compared with
other protection devices. The magnitude of power loss compared to the power transmitted
by rated current is much less than 0,1%.
p) Excellent personnel and equipment protection in case of arc flash.

© IEC 2020
q) Fuse-links will operate independent of the operation position of the fuse. The operation
position is usually vertical. Other positions of use are permissible. The deratings of the
manufacturers of the fuse must be observed.
Properly sized current limiting fuses operating in their current limiting range interrupt
currents due to arcing fault in a few milliseconds, keeping arc energy well below
hazardous and damaging levels.
5 Fuse construction and operation
5.1 Components
A fuse is a protective device comprising
• the fuse-link,
• the fuse-base,
• the fuse-carrier or replacement handle.
These components may be integrated in a fuse combination unit.
5.2 Fuse-construction
5.2.1 Fuse link
Figures 1 and 2 show the design of typical low-voltage fuse-links for industrial application.
Such fuse-links are commonly called current-limiting or high breaking capacity fuse-links.
Fuse-links according to IEC 60269-2 (fuses for industrial application) are available in current
ratings up to 6 000 A.
Fuse-links according to IEC 60269-3 (fuses for household application) are available in current
ratings up to 100 A.
The fuse-element is usually made of flat silver or copper with multiple restrictions in the cross-
section, called notches. This restriction (or notch) pattern is an important feature of fuse
design, normally achieved by precision stamping.
M-effect (see 5.3.3) material is sometimes added to the fuse-element to achieve controlled
fuse operation in the overload range. The purity of the fuse-element materials and their
precise physical dimensions are of vital importance for reliable fuse operation.

– 14 – IEC TR 60269-5:2014+AMD1:2020 CSV
© IEC 2020
Key
1 Blade contact
2 Fuse-elements
3 Fuse body
4 End cap
5 Filler
Figure 1 – Typical fuse-link according to IEC 60269-2

Key
1 Blade contact
2 Fuse-element
3 Fuse body
4 Endplate (with gripping lug)
5 Indicator wire
6 M-effect material
7 Filler
8 Indicator
Figure 2 – Typical fuse-link according to IEC 60269-2
5.2.2 Fuse-link contacts
Fuse-link contacts provide electrical connection between the fuse-link and fuse-base or fuse
carrier. The contacts are made of copper or copper alloys and are typically protected against
the formation of non-conductive layers by plating.

© IEC 2020
5.2.3 Indicating device and striker
Some fuses are equipped with indicators or strikers for rapid recognition of fuse-link operation.
Fuses equipped with strikers also provide means for mechanical actuation (e.g. for a switch of
remote signalling) as well as a visual indication.
5.2.4 Fuse-base
The fuse-base is equipped with the matching contacts for accepting the fuse-link, connecting
means for cables or busbars and the base insulator.
5.2.5 Replacement handles and fuse-holders
Replacement handles or fuse-carriers, where applicable, enable changing fuse-links in a live
system under specified safety rules. They are made of insulating material and subjected to
tests as required for safety tools. For some systems, fuse-carriers are an integral part of the
fuse-holder, eliminating the need for an external replacement handle.
5.3 Fuse operation
5.3.1 General
Fuses are designed to operate under both short-circuit and overload conditions. Typically
short-circuits are current levels at or above 10 times the fuse’s rating, and overloads are
current levels below 10 times the fuse’s rating.
5.3.2 Fuse operation in case of short-circuit
During a short-circuit, the restrictions (notches) all melt simultaneously forming a series of
arcs equal to the number of r
...

IEC TR 60269-5 ®
Edition 2.0 2014-03
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
colour
inside
Low-voltage fuses –
Part 5: Guidance for the application of low-voltage fuses

Fusibles basse tension –
Partie 5: Lignes directrices pour l’application des fusibles basse tension

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IEC TR 60269-5 ®
Edition 2.0 2014-03
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
colour
inside
Low-voltage fuses –
Part 5: Guidance for the application of low-voltage fuses

Fusibles basse tension –
Partie 5: Lignes directrices pour l’application des fusibles basse tension

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XA
ICS 29.120.50 ISBN 978-2-8322-1448-0

– 2 – IEC TR 60269-5:2014 © IEC 2014
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Fuse benefits . 12
5 Fuse construction and operation . 13
5.1 Components . 13
5.2 Fuse-construction . 13
5.2.1 Fuse link . 13
5.2.2 Fuse-link contacts . 14
5.2.3 Indicating device and striker . 14
5.2.4 Fuse-base . 14
5.2.5 Replacement handles and fuse-holders . 14
5.3 Fuse operation . 15
5.3.1 General . 15
5.3.2 Fuse operation in case of short-circuit . 15
5.3.3 Fuse operation in case of overload . 15
5.3.4 Fuse link pre-arcing time current characteristic: . 16
5.3.5 Fuse operation in altitudes exceeding 2 000 m . 17
6 Fuse-combination units . 18
7 Fuse selection and markings . 19
8 Conductor protection . 21
8.1 General . 21
8.2 Utilization category gG . 22
8.3 Utilization category gN and gD . 23
8.4 Utilization category gR and gS . 23
8.5 Utilization category gU . 24
8.6 Utilization category gK . 24
8.7 Utilization category gPV . 24
8.8 Protection against short-circuit current only . 24
9 Selectivity of protective devices . 24
9.1 General . 24
9.2 Selectivity between fuses . 25
9.2.1 General . 25
9.2.2 Verification of selectivity for operating time ≥ 0,1 s . 25
9.2.3 Verification of selectivity for operating time < 0,1 s . 26
9.2.4 Verification of total selectivity . 26
9.3 Selectivity of circuit-breakers upstream of fuses . 26
9.3.1 General . 26
9.3.2 Verification of selectivity for operating time ≥ 0,1 s . 27
9.3.3 Verification of selectivity for operating time < 0,1 s . 27
9.3.4 Verification of total selectivity . 27
9.4 Selectivity of fuses upstream of circuit-breakers . 28
9.4.1 General . 28
9.4.2 Verification of selectivity for operating time ≥ 0,1 s . 28

9.4.3 Verification of selectivity for operating time < 0,1 s . 28
9.4.4 Verification of total selectivity . 28
10 Short-circuit damage protection . 30
10.1 General . 30
10.2 Short-circuit current paths . 30
10.3 Current limitation . 31
10.4 Rated conditional short-circuit current, rated breaking capacity . 31
11 Protection of power factor correction capacitors . 31
12 Transformer protection . 32
12.1 Distribution transformers with a high-voltage primary . 32
12.2 Distribution transformers with a low-voltage primary . 33
12.3 Control circuit transformers . 33
13 Motor circuit protection . 33
13.1 General . 33
13.2 Fuse and motor-starter coordination . 34
13.3 Criteria for coordination at the rated conditional short-circuit current I . 34
q
13.4 Criteria for coordination at the crossover current I . 35
co
13.5 Criteria for coordination at test current “r” . 35
14 Circuit-breaker protection in a.c. and d.c rated voltage circuits . 36
15 Protection of semiconductor devices in a.c. and d.c. rated voltage circuits . 36
16 Fuses in enclosures . 38
16.1 General . 38
16.2 Limiting temperature of utilization category gG fuse-links according to
IEC 60269-2 – System A . 38
16.3 Other fuse-links . 38
17 DC applications . 38
17.1 General . 38
17.2 Short-circuit protection . 38
17.3 Overload protection . 39
17.4 Time-current characteristics . 40
18 Automatic disconnection for protection against electric shock for installations in
buildings . 40
18.1 General . 40
18.2 Principle of the protection . 41
18.3 Examples . 42
19 Photovoltaic (PV) system protection . 43
19.1 General . 43
19.2 Selection of PV fuse-links . 44
19.2.1 Fuse utilization category . 44
19.2.2 PV string fuses . 44
19.2.3 Fuse replacement . 44
19.2.4 Unearthed or Ungrounded PV Systems . 44
19.2.5 Functional earthing fuses . 44
19.2.6 PV array and PV sub-array fuses . 45
19.2.7 Fuse monitoring . 45
19.2.8 Breaking capacity . 45
19.2.9 Voltage of gPV fuses . 45
19.2.10 Rated current of gPV fuses . 45

– 4 – IEC TR 60269-5:2014 © IEC 2014
20 Protection of wind mills . 45
Annex A (informative) Coordination between fuses and contactors/motor-starters . 47
A.1 General . 47
A.2 Examples of suitable fuse-links used for motor protection . 47
A.3 Values of I t and cut-off current observed in successful tests of fuse-
link/motor-starter combinations worldwide . 48
A.4 Criteria for coordination at the rated conditional short-circuit current I . 51
q
A.4.1 General . 51
A.4.2 Maximum operating I t and cut-off current . 51
A.4.3 Guidance for choosing the maximum rated current of an alternative fuse
type . 52
A.4.4 Further guidance . 52
A.5 Criteria for coordination at test current "r" . 53
A.6 Types of coordination . 54
Bibliography . 57

Figure 1 – Typical fuse-link according to IEC 60269-2. 13
Figure 2 – Typical fuse-link according to IEC 60269-2. 14
Figure 3 – Current-limiting fuse operation . 15
Figure 4 – Fuse operation on overload . 16
Figure 5 – Time current characteristic for fuse-links . 17
Figure 6 – Currents for fuse-link selection . 23
Figure 7 – Selectivity – General network diagram . 25
Figure 8 – Verification of selectivity between fuses F and F for operating time t ≥
2 4
0,1 s . 26
Figure 9 – Verification of selectivity between circuit-breaker C and fuses F and F . 27
2 5 6
Figure 10 – Verification of selectivity between fuse F and circuit-breaker C for
2 3
operating time t ≥ 0,1 s . 29
Figure 11 – Verification of selectivity between fuse F and circuit-breaker C for
2 3
operating time t < 0,1 s . 30
Figure 12 – Fuse and motor-starter coordination . 35
Figure 13 – DC circuit . 39
Figure 14 – DC breaking operation . 39
Figure 15 – Fuse operating time at various d.c. circuit time constants . 40
Figure 16 – Time-current characteristic . 42
Figure A.1 – Collation of cut-off currents observed in successful coordination at I . 49
q
Figure A.2 – Pre-arcing and operating I t values of fuses used in successful
coordination tests as a function of contactor rated current AC3 . 50
Figure A.3 – Pre-arcing and operating I t values of fuses used in successful
coordination tests as a function of fuse rated current I . 51
n
Figure A.4 – Illustration of the method of selection of the maximum rated current of a
fuse for back-up protection of a contactor of rating I = X amperes . 54
e
Figure A.5 – Withstand capabilities of a range of contactors and associated overload
relays at test current "r" . 55
Figure A.6 – Illustration of a method of deriving curves of maximum peak current at
test current "r" as a function of fuse rated current . 56

Table 1 – Derating factors for different altitudes . 18
Table 2 – Definitions and symbols of switches and fuse-combination units . 19
Table 3 – Fuse application . 20
Table 4 – Maximum operational voltage of a.c. fuse-links . 21
Table 5 – Typical operational voltage ratings of d.c. fuse-links . 21
Table 6 – Fuse selection for power factor correction capacitors (fuses according to
IEC 60269-2, system A) . 32
Table 7 – Conventional non fusing current . 37
Table 8 – Time constants of typical d.c. circuits . 40
Table A.1 – Examples of typical fuse-link ratings used for motor-starter protection
illustrating how the category of fuse-link can influence the optimum current rating . 48
Table A.2 (Table 12 of IEC 60947-4-1:2009) – Value of the prospective test current
according to the rated operational current . 53
Table A.3 – Types of coordination . 54

– 6 – IEC TR 60269-5:2014 © IEC 2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW-VOLTAGE FUSES –
Part 5: Guidance for the application of low-voltage fuses

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60269-5, which is a technical report, has been prepared by subcommittee 32B: Low-
voltage fuses, of IEC technical committee 32: Fuses.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous
edition:
a) recommendations for fuse operations in high altitudes added
b) more details for operational voltages added
c) recommendations for photovoltaic system protection added
d) numerous details improved
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
32B/621A/DTR 32B/624/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60269 series, under the general title: Low-voltage fuses, can be
found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 8 – IEC TR 60269-5:2014 © IEC 2014
INTRODUCTION
Fuses protect many types of equipment and switchgear against the effects of over-current
which can be dramatic:
– thermal damage of conductors or bus-bars;
– vaporisation of metal;
– ionisation of gases;
– arcing, fire, explosion,
– insulation damage.
Apart from being hazardous to personnel, significant economic losses can result from
downtime and the repairs required to restore damaged equipment.
Modern fuses are common overcurrent protective devices in use today, and as such provide
an excellent cost effective solution to eliminate or minimize the effects of overcurrent.

LOW-VOLTAGE FUSES –
Part 5: Guidance for the application of low-voltage fuses

1 Scope
This technical report, which serves as an application guide for low-voltage fuses, shows how
current-limiting fuses are easy to apply to protect today's complex and sensitive electrical and
electronic equipment. This guidance specifically covers low-voltage fuses up to 1 000 V a.c.
and 1 500 V d.c. designed and manufactured in accordance with IEC 60269 series. This
guidance provides important facts about as well as information on the application of fuses.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary. Available from
http://www.electropedia.org/
IEC/TR 60146-6, Semiconductor convertors – Part 6: Application guide for the protection of
semiconductor convertors against overcurrent by fuses
IEC 60269 (all parts), Low-voltage fuses
IEC 60269-1:2006, Low-voltage fuses – Part 1: General requirements
IEC 60269-2, Low-voltage fuses – Part 2: Supplementary requirements for fuses for use by
authorized persons (fuses mainly for industrial application) – Examples of standardized
systems of fuses A to K
IEC 60269-3, Low-voltage fuses – Part 3: Supplementary requirements for fuses for use by
unskilled persons (fuses mainly for household or similar applications) – Examples of
standardized systems of fuses A to F
IEC 60269-4:2009, Low-voltage fuses – Part 4: Supplementary requirements for fuse-links for
the protection of semiconductor devices
IEC 60269-6, Low-voltage fuses – Part 6: Supplementary requirements for fuse-links for the
protection of solar photovoltaic energy systems
IEC 60364-4-41:2005, Low-voltage electrical installations – Part 4-41: Protection for safety –
Protection against electric shock
IEC 60364-4-43:2008, Low-voltage electrical installations – Part 4-43: Protection for safety –
Protection against overcurrent
IEC 60364-5-52, Low-voltage electrical installations – Part 5-52: Selection and erection of
electrical equipment – Wiring systems

– 10 – IEC TR 60269-5:2014 © IEC 2014
IEC 60947 (all parts), Low-voltage switchgear and controlgear
IEC 60947-3:2008, Low-voltage switchgear and controlgear – Part 3: Switches, disconnectors,
switch-disconnectors and fuse-combination units
IEC 60947-4-1:2009, Low-voltage switchgear and controlgear – Part 4-1: Contactors and
motor-starters – Electromechanical contactors and motor-starters
IEC/TR 61912-1:2007, Low-voltage switchgear and controlgear – Overcurrent protective
devices – Part 1: Application of short-circuit ratings
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
switch (mechanical)
mechanical switching device capable of making, carrying and breaking currents under normal
circuit conditions, which may include specified operating overload conditions and also
carrying, for a specified time, currents under specified abnormal conditions such as those of
short-circuits
Note 1 to entry: A switch may be capable of making but not breaking, short-circuit currents.
[SOURCE: IEC 60050-441:1984, 441-14-10]
3.2
disconnector
mechanical switching device that, in the open position, complies with the requirements
specified for isolating function
Note 1 to entry: Some disconnectors may not be capable of switching load.
[SOURCE: IEC 60050-441:1984, 441-14-05, modified (modified definition and Note 1 to entry
added)]
3.3
fuse-combination unit
combination of a mechanical switching device and one or more fuses in a composite unit,
assembled by the manufacturer or in accordance with his instructions
[SOURCE: IEC 60050-441:1984, 441-14-04, modified (Note removed)]
3.4
switch-fuse
switch in which one or more poles have a fuse in series in a composite unit
[SOURCE: IEC 60050-441:1984, 441-14-14]
3.4.1
single-break and double-break
switch-fuse must be single break (it opens the circuit on one side of the fuse link) or double
break (it opens the circuit on both sides of the fuse link)
3.5
fuse-switch
switch in which a fuse-link or a fuse-carrier with fuse-link forms the moving contact

[SOURCE: IEC 60050-441:1984, 441-14-17]
3.5.1
single-break and double-break
fuse-switch must be single break (it opens the circuit on one side of the fuse link) or double
break (it opens the circuit on both sides of the fuse link)
3.6
Switching device SD
device designed to make or break the current in one or more electric circuits
Note 1 to entry: A switching device may perform one or both of these operations.
[SOURCE: IEC 60050-441:1984, 441-14-01, modified (Note 1 to entry added)]
3.7
short-circuit protective device SCPD
device intended to protect a circuit or parts of a circuit against short-circuits by interrupting
them
3.8
overload protection
protection intended to operate in the event of overload on the protected section
[SOURCE: IEC 60050-448:1995, 448-14-31]
3.9
overload
operating conditions in an electrically undamaged circuit, which cause an over-current
[SOURCE: IEC 60050-441:1984, 441-11-08]
3.10
overcurrent
current exceeding the rated current
[SOURCE: IEC 60050-442:1998, 442-01-20]
3.11
rated conditional short-circuit current (of a switching device)
I
q
prospective current that a switching device, protected by a short-circuit protective device, can
satisfactorily withstand for the operating time of that device under test conditions specified in
the relevant product standard
3.12
selectivity of protection
ability of a protection to identify the faulty sections and/or phase(s) of a power system
Note 1 to entry: Whereas the terms “selectivity” and “discrimination” have a similar meaning according to the IEV
definitions, this report prefers and uses the term “selectivity” to express the ability of one over-current device to
operate in preference to another over-current device in series, over a given range of over-current. The effect of
standing load current on selectivity in the overload zone is also considered.
[SOURCE: IEC 60050-448:1995, 448-11-06, modified (Note 1 to entry added)]

– 12 – IEC TR 60269-5:2014 © IEC 2014
4 Fuse benefits
The current-limiting fuse provides complete protection against the effects of overcurrents by
protecting both, electric circuits and their components. Fuses offer a combination of
advantageous features, for example:
a) High breaking capacity (high current interrupting rating).
b) No need for complex short-circuit calculations.
c) Easy and inexpensive system expansion in case of increased fault currents.
d) High current limitation (low I t values).
e) Mandatory fault elimination before reenergizing.
Fuses cannot be reset, thus forcing the user to identify and correct the fault condition
before re-energizing the circuit.
f) Reliability.
No moving parts to wear out or become contaminated by dust, oil or corrosion. Fuse
replacement ensures protection is restored to its original level when the fuse is replaced.
g) Cost effective protection.
Compact size offers low cost overcurrent protection at high short-circuit levels.
h) No damage for starters and contactors (type 2 protection according to IEC 60947-4-1).
By limiting short-circuit energy and peak currents to extremely low levels, fuses are
particularly suitable for type 2 protection without damage to components in motor circuits.
i) Safe, silent operation.
No emission of gas, flames, arcs or other materials when clearing the highest levels of
short-circuit currents. In addition, the speed of operation at high short-circuit currents
significantly limits the arc flash hazard at the fault location.
j) Easy coordination.
Standardized fuse characteristics and a high degree of current limitation ensure effective
coordination between fuses and other devices.
k) Standardized performance.
Fuse-links designed and manufactured in accordance with IEC 60269 series ensure
availability of replacements with standardized characteristics throughout the world.
l) Improved power quality.
Current-limiting fuses interrupt high fault currents in a few milliseconds, minimizing dips or
sags in system supply voltage.
m) Tamperproof.
Once installed, fuses cannot be modified or adjusted thus preserving their level of
performance and avoiding malfunction.
n) No maintenance.
Properly sized fuses require no maintenance, adjustments or recalibrations. They can
remain in service providing originally designed overcurrent protection levels for many
decades.
o) High level of energy efficiency.
The resistance and therefore the power dissipation of the fuse is very low compared with
other protection devices. The magnitude of power loss compared to the power transmitted
by rated current is much less than 0,1%.
p) Excellent personnel and equipment protection in case of arc flash.
Properly sized current limiting fuses operating in their current limiting range interrupt
currents due to arcing fault in a few milliseconds, keeping arc energy well below
hazardous and damaging levels.

5 Fuse construction and operation
5.1 Components
A fuse is a protective device comprising
• the fuse-link,
• the fuse-base,
• the fuse-carrier or replacement handle.
These components may be integrated in a fuse combination unit.
5.2 Fuse-construction
5.2.1 Fuse link
Figures 1 and 2 show the design of typical low-voltage fuse-links for industrial application.
Such fuse-links are commonly called current-limiting or high breaking capacity fuse-links.
Fuse-links according to IEC 60269-2 (fuses for industrial application) are available in current
ratings up to 6 000 A.
Fuse-links according to IEC 60269-3 (fuses for household application) are available in current
ratings up to 100 A.
The fuse-element is usually made of flat silver or copper with multiple restrictions in the cross-
section, called notches. This restriction (or notch) pattern is an important feature of fuse
design, normally achieved by precision stamping.
M-effect (see 5.3.3) material is added to the fuse-element to achieve controlled fuse operation
in the overload range. The purity of the fuse-element materials and their precise physical
dimensions are of vital importance for reliable fuse operation.

Key
1 Blade contact
2 Fuse-elements
3 Fuse body
4 End cap
5 Filler
Figure 1 – Typical fuse-link according to IEC 60269-2

– 14 – IEC TR 60269-5:2014 © IEC 2014

Key
1 Blade contact
2 Fuse-element
3 Fuse body
4 Endplate (with gripping lug)
5 Indicator wire
6 M-effect material
7 Filler
8 Indicator
Figure 2 – Typical fuse-link according to IEC 60269-2
5.2.2 Fuse-link contacts
Fuse-link contacts provide electrical connection between the fuse-link and fuse-base or fuse
carrier. The contacts are made of copper or copper alloys and are typically protected against
the formation of non-conductive layers by plating.
5.2.3 Indicating device and striker
Some fuses are equipped with indicators or strikers for rapid recognition of fuse-link operation.
Fuses equipped with strikers also provide means for mechanical actuation (e.g. for a switch of
remote signalling) as well as a visual indication.
5.2.4 Fuse-base
The fuse-base is equipped with the matching contacts for accepting the fuse-link, connecting
means for cables or busbars and the base insulator.
5.2.5 Replacement handles and fuse-holders
Replacement handles or fuse-carriers, where applicable, enable changing fuse-links in a live
system under specified safety rules. They are made of insulating material and subjected to
tests as required for safety tools. For some systems, fuse-carriers are an integral part of the
fuse-holder, eliminating the need for an external replacement handle.

5.3 Fuse operation
5.3.1 General
Fuses are designed to operate under both short-circuit and overload conditions. Typically
short-circuits are current levels at or above 10 times the fuse’s rating, and overloads are
current levels below 10 times the fuse’s rating.
5.3.2 Fuse operation in case of short-circuit
During a short-circuit, the restrictions (notches) all melt simultaneously forming a series of
arcs equal to the number of restrictions in the fuse element. The resulting arc voltage ensures
rapid reduction in current and forces it to zero. This action is called “current limitation”.
Fuse operation occurs in two stages (see Figures 3a and 3b):
• the pre-arcing (melting) stage (t ): the heating of the restrictions (notches) to the melting
m
point and associated vaporization of the material;
• the arcing stage (t ): the arcs begin at each notch and are then extinguished by the filler.
a
The operating time is the sum of the prearcing time and arcing time.
The energies generated by the current in the circuit to be protected during pre-arcing time and
2 2
t and operating I t values, respectively.
operating time are
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