PRODUCED BY THE OPERATIONS DIRECTORATE OF ENERGY NETWORKS ASSOCIATION
Technical Specification 12-6
Issue 2 2014
Time fuse-links for use with current transformer
releases on circuit-breakers
www.energynetworks.org
PUBLISHING AND COPYRIGHT INFORMATION
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© 2014 Energy Networks Association
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system
or transmitted in any form or by any means, electronic, mechanical, photocopying, recording
or otherwise, without the prior written consent of Energy Networks Association. Specific
enquiries concerning this document should be addressed to:
Operations Directorate
Energy Networks Association
6th Floor, Dean Bradley House
52 Horseferry Rd
London
SW1P 2AF
This document has been prepared for use by members of the Energy Networks Association
to take account of the conditions which apply to them. Advice should be taken from an
appropriately qualified engineer on the suitability of this document for any other purpose.
First published, 1973
Revised, 2014
Amendments since publication
Issue
Date
Amendment
1
1973
First issue
2
2014
Major revision and updating of reference publications.
This issue includes the following principal technical changes.
Scope extended to cover protection of ENA TS 35-1 distribution
transformers with rated powers up to and including 1 500 kVA for use
on 11 kV and 20 kV systems.
Rated current of low voltage distributor fuse-links changed to align with
preferred values in ENA TS 12-08.
New Clause 4: Classification added with requirement for time fuse-links
to generally conform to BS EN 60269-1, BS 88-1 and BS HD 60269-2,
BS 88-2.
Clause 7.2: Mountings amended so covers have degree of protection not
less than IP3X in accordance with BS EN 60529.
Tables 2 & 3: LV distributor fuses with rated currents of 150 A, 300 A and
600 A changed to 200 A, 315 A and 630 A respectively to align with ENA
TS 12-08. Time fuse-link selections added for 1 250 kVA and 1 500 kVA
distribution transformers associated with 11 kV systems.
New Table 4 added with time fuse-link selections for distribution
transformers associated with 20 kV systems.
Details of all other technical, general and editorial amendments are
included in the associated Document Amendment Summary for this
Issue (available on request from the Operations Directorate of ENA).
ENA Technical Specification 12-6
Issue 2 2014
Page 3
Contents
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Foreword ................................................................................................................................ 5
1
Scope .............................................................................................................................. 7
2
Normative references ....................................................................................................... 7
3
Terms and definitions ....................................................................................................... 7
4
Classification .................................................................................................................... 8
5
Characteristics of fuses .................................................................................................... 8
6
5.1 Ratings ................................................................................................................... 8
5.2 Time/current characteristics .................................................................................... 8
5.3 Resistance .............................................................................................................. 8
Markings .......................................................................................................................... 9
7
Standard conditions for construction ................................................................................ 9
8
7.1 Construction and dimensions .................................................................................. 9
7.2 Mountings ............................................................................................................... 9
Tests ................................................................................................................................ 9
8.1
Type tests ............................................................................................................... 9
8.1.1 Temperature rise ......................................................................................... 9
8.1.2 Time/current characteristics ...................................................................... 10
8.1.3 Resistance ................................................................................................ 10
8.1.4 Duty .......................................................................................................... 10
Annex A (informative) Self Certification Conformance Declaration ...................................... 21
Annex B (informative) Selection of time fuse-links ............................................................... 23
B.0
B.1
B.2
B.3
B.4
General ................................................................................................................. 23
Inrush current of the distribution transformer ......................................................... 23
Overload rating of the distribution transformer ...................................................... 23
Faults in distribution transformer terminal zone ..................................................... 23
Discrimination between the low voltage distributor fuse-links and the
distribution transformer high voltage circuit-breaker .............................................. 23
Annex C (informative) Explanatory notes ............................................................................ 26
C.0
C.1
C.2
General ................................................................................................................. 26
Clause 5.3 ............................................................................................................ 26
Clause 11.4........................................................................................................... 26
Figures
Figure 1 — Time/current characteristics of time fuse-links.................................................... 17
Figure 2 — Dimensions of time fuse-links............................................................................. 18
Figure 3 — Circuit diagram for duty tests ............................................................................. 19
Figure 4 — Test rig for fuse-links.......................................................................................... 20
ENA Technical Specification 12-6
Issue 2 2014
Page 4
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Tables
Table 1 — Resistance of time fuse-links ................................................................................. 8
Table 2 — Selection of time fuse-link ratings for 11 kV systems ........................................... 12
Table 3 — Selection of time fuse-link ratings for 6.6 kV systems .......................................... 14
Table 4 — Selection of time fuse-link ratings for 20 kV systems ........................................... 15
ENA Technical Specification 12-6
Issue 2 2014
Page 5
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Foreword
This Technical Specification (TS) is published by the Energy Networks Association (ENA) and
comes into effect from date of publication. It has been prepared under the authority of the ENA
Engineering Policy and Standards Manager and has been approved for publication by the ENA
Electricity Networks and Futures Group (ENFG). The approved abbreviated title of this
engineering document is “ENA TS 12-6”.
This issue cancels and replaces Issue 1 1973.
This Specification has been revised with direction from the ENA Protection Assessment Panel
to ensure requirements for time fuse-links conform to relevant BS EN Standards for fuse-links.
The scope of this issue has been extended to cover protection of ENA TS 35-1 distribution
transformers with rated powers up to and including 1 500 kVA for use on 11 kV and 20 kV
systems.
Level 1 headings in this issue have been aligned with those in BS HD 60269-2, BS 88-2 for
the purposes on conformance declaration by manufacturers.
Where the term “shall” or “must” is used in this document it means the requirement is
mandatory. The term “may” is used to express permission.
NOTE: Commentary, explanation and general informative material is presented in smaller type, and does not
constitute a requirement.
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ENA Technical Specification 12-6
Issue 2 2014
Page 6
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ENA Technical Specification 12-6
Issue 2 2014
Page 7
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1 Scope
The time fuse-links covered by this Specification are intended to protect distribution
transformers covered by ENA TS 35-1 [N1] with rated powers from 200 kVA to 1 500 kVA and
to discriminate with low voltage distributor fuse-links covered by ENA TS 12-08 [N2] connected
to the transformer secondaries with current ratings up to 630 A.
NOTE: Distribution transformers with rated powers exceeding 1 500 kVA are intended to be protected by relays.
This Specification covers time fuse-links for use with current transformer release coils of circuitbreakers, where each protection element is fitted with one conventional a.c. release coil. They
are primarily for use with standard 6.6 kV, 11 kV and 20 kV distribution switchgear as covered
by ENA TS 41-36 [3] but may be used with other circuit-breakers.
2 Normative references
The following referenced documents, in whole or part, are indispensable for the application of
this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
Standards publications
BS 2572:1990, Specification for phenolic laminated sheet and epoxy cotton fabric laminated
sheet
BS EN 60269-1:2007+A1:2009, BS 88-1: 2007, Low-voltage fuses. General requirements
BS HD 60269-2:2013, BS 88-2: 2013, Low-voltage fuses. Supplementary requirements for
fuses for use by authorized person (fuses mainly for industrial application). Examples of
standardized systems of fuses A to K
BS EN 60529:1992+A2:2013, Degrees of protection provided by enclosures (IP code)
Other publications
[N1] ENA TS 35-1, Distribution transformers (from 16 kVA to 2 000 kVA)
[N2] ENA TS 12-08, The application of fuse-links to 11 kV/400 V and 6.6 kV/400 V underground
distribution networks
[N3] ENA TS 41-36, Switchgear for service up to 36 kV (cable and overhead conductor
connected)
NOTE: ENA Engineering Documents are revised when necessary by the issue either of revised pages or complete
new editions. It is important that users ascertain they are in possession of the latest issues.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
FLC
full load current
3.2
release coil
current operated coil that operates the trip mechanism of a circuit-breaker
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ENA Technical Specification 12-6
Issue 2 2014
Page 8
3.3
time fuse-link
fuse-link used to shunt a.c. release coils that have characteristics conforming to this
Specification
4 Classification
Time fuse-links shall generally conform to the requirements of BS EN 60269-1, BS 88-1 and
BS HD 60269-2, BS 88-2 system E (bolted) except where modified by this Specification.
5 Characteristics of fuses
5.1 Ratings
Time fuse-links shall have normal rated currents of 3 A, 5 A, 7.5 A, 10 A, 12.5 A and 15 A.
A time fuse-link shall be capable of carrying its rated current indefinitely without causing a
permanent change in the time/current characteristic and without exceeding the temperature
rise permitted.
5.2 Time/current characteristics
All time fuse-links shall operate within the boundaries of the time/current characteristics shown
in Fig. 1.
5.3 Resistance
The hot resistance of a time fuse-link, when measured in accordance with Clause 8.1.3 shall
not exceed the appropriate value given in Table 1.
Table 1 — Resistance of time fuse-links
Time fuse-link rating
(A)
Resistance
(Ω)
3
0.073
5
0.043
7.5
0.031
10
0.028
12.5
0.020
15
0.017
The impedance of release coils which are to be used in conjunction with the time fuse-links
covered by this Specification should conform to the following in order that operation of the
release may not occur due to spill current through the coil, so defeating discrimination in the
event of a high fault current.
a) Release coils which are set to operate at 2 A should have an impedance of 1.5 ohm or
more when the release is in a reset condition.
b) Release coils which are set to operate at 5 A should have an impedance of 0.24 ohm or
more when the release is in a reset condition.
ENA Technical Specification 12-6
Issue 2 2014
Page 9
6 Markings
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Each time fuse-link shall be marked with the following.
a) Name or identity of manufacturer.
b) Rated current.
c) Manufacturer's list number.
d) The word 'time fuse-link'.
The rated current marking shall be in the position shown in Fig. 2.
7 Standard conditions for construction
7.1 Construction and dimensions
Each time fuse-link shall be fitted with terminal tags and shall conform to Fig. 2. It shall be of
sealed construction and suitable for withstanding the conditions of vibration and humidity
experienced in service (see ENA TS 41-36 [3] for normal service conditions).
The terminal tags shall be suitably plated to ensure a consistent low resistance connection.
7.2 Mountings
Mountings for time fuse-links shall be designed to accept the standard link shown in Fig. 2.
Securing of a link to its mounting, for both mechanical and electrical purposes, shall be by
means of screws which engage with the terminal tags of the link. The screws shall be captive
to the mounting and designed for manipulation by a screwdriver.
The mounting shall be such that, when a time fuse-link is assembled to it, the link shall conform
to Clause 5.2 regarding its time/current characteristic and Clause 8.1.1 regarding temperature
rise.
The mounting may be either front or back connected. If designed for front connection, should
the assembly be external to the switchgear, the mounting shall be provided with a cover which
completely shields the connections and the time fuse-links to degree of protection not less than
IP3X in accordance with BS EN 60529.
The material and the design of the mounting shall be such that the assembly is suitable for
normal service conditions regarding both normal and fault current carrying capacity, vibration
and humidity.
Any terminals, connection screws or current carrying studs which are of a size less than M6
shall be made of phosphor-bronze or stainless steel and be not less in size than M5.
8 Tests
8.1 Type tests
8.1.1 Temperature rise
Each rating of time fuse-link shall be tested for temperature rise at rated current in surroundings
free from external draughts and mounted vertically in a test rig conforming to Fig. 4 in an
ambient temperature between 15 °C and 40 °C inclusive. The temperature rise shall be
measured at the point indicated on Fig. 4.
Connections between the terminals of the test rig and the source of supply shall be by copper
cables, pvc-insulated, size 7/0.67 mm. Each cable shall have a minimum length of 1 m.
ENA Technical Specification 12-6
Issue 2 2014
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The temperature rise shall be measured by thermocouples attached by a low melting point
alloy or by some equally effective means of attachment.
After the fuse-link on test has carried its rated current continuously for sufficient time to allow
its temperature to become steady, the temperature rise shall not exceed 40 °C.
8.1.2 Time/current characteristics
Tests to prove compliance of the fuse-links with the time/current characteristics shown in Fig.
1 shall be conducted as follows.
The fuse-link shall be tested in surroundings free from external draughts and mounted
vertically in a test rig conforming to Fig. 4 in an ambient temperature between 15 °C and 40 °C
inclusive.
Connections between the terminals of the test rig and the source of supply shall be by copper
cables, pvc-insulated, size 7/0.67 mm. Each cable shall have a minimum length of 1 m.
At the beginning of each test, the connections, test rig and fuse-link shall each be at a
temperature within plus or minus 10% of ambient temperature.
Not less than six off time fuse-links of each current rating shall be tested at various values of
current such that the results, when plotted, can be shown to result in a characteristic curve
which lies within the appropriate envelope shown in Fig. 1.
8.1.3 Resistance
The resistance of each rating of time fuse-link shall be measured at 60% of the melting time
when carrying a direct current which causes melting in 0.2 seconds. The resistance values so
measured shall not exceed the values given in Table 1.
8.1.4 Duty
Time fuse-links conforming with this Specification shall be capable of operating satisfactorily
under all conditions of fault which may occur in service. The tests for duty described in this
clause are arranged to simulate actual service conditions.
The tests shall be made from a single phase a.c. supply of frequency 50 Hz ± 2.5 Hz and the
test circuit shall conform to Fig. 3.
The test circuit consists of a current transformer the primary winding of which is energised from
the test supply; the secondary winding being connected to a time fuse-link and coil connected
in parallel.
The coil shall have an impedance of 7.2 ohm minimum at a power factor between 0.5 and 0.7.
If a release coil is used, these values shall apply in the reset condition.
The characteristics of the current transformer and the magnitude of the primary test current
shall be such that:
a) when the time fuse-link is replaced by a solid link, the secondary current of the current
transformer has a minimum steady state peak value of 1 400 A;
b) when the time fuse-link is removed, the steady state voltage applied to the coil has a
minimum peak value of 950 V. If a release coil is used, it must be held in the reset condition.
The overall power factor of the test supply shall be between 0.2 and 0.3 whilst the voltage need
not exceed that necessary to circulate the value of current required to satisfy conditions a) and
b) above.
ENA Technical Specification 12-6
Issue 2 2014
Page 11
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Each test shall be made with the test supply set to conform to the foregoing requirements and
the primary circuit shall be closed at an instant between 0° and 60° on a rising voltage wave.
The test current shall flow for not less than 0.25 seconds.
Three tests shall be made each with a time fuse-link of maximum current rating inserted in a
test rig conforming to Fig. 4. In each test the fuse-link must operate without any of the following
occurring.
a) Damage sufficient to render unserviceable any parts of the assembly other than the time
fuse-link itself.
b) External arcing or bursting of the time fuse-link.
c) Reduction of the insulation resistance below 100 000 ohms, when measured at 250 V
within 3 minutes of the conclusion of the test.
Two of the three tests shall be made with the metal cover of the test rig connected to one of
the terminals of the secondary winding of the current transformer, the remaining test to be
made with the metal cover connected to the other terminal of the secondary winding.
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ENA Technical Specification 12-6
Issue 2 2014
Page 12
Table 2 — Selection of time fuse-link ratings for 11 kV systems
1
2
3
4
5
6
7
8
9
10
Power
Transformer
Rating
Transformer
Primary
FLC
Current
Transformer
Ratio
Maximum
Sustained
Current in
Time
Fuse-Link
(150%
Transformer
Rating)
Time
FuseLink
Rating
Primary
Current
(0.6 x
11.5 x
FLC)
Current
in Time
FuseLink
kV
kVA
A
–
A
A
A
A
Min.
11
200
10.5
40/5
2.0
3
72.4
9.05
50/5
1.6
3
72.4
11
300
15.8
40/5
3.0
7.5
108
50/5
2.4
5
108
40/5
3.1
7.5
50/5
2.5
40/5
11
11
11
11
11
315
500
750
800
1 000
16.6
26.2
39.4
42
52.5
121)
131)
Discrimination with Distributor Fuse-Links1)
for Phase/Phase Fault
Phase/neutral fault
in secondary terminal zone
System
Nominal
Voltage
11
Operating
Time of Time
Fuse-Link
(Seconds)
Minimum 1
Second
Operating
Current of Time
Fuse-Link
Secondary
Current
Corresponding
to Column 10
Maximum
Distributor
Fuse-Link
Rating which
may be used to
achieve
Discrimination
Maximum 1
Second
Operating
Current of
Distributor
Fuse-Link
Max.
A
A
A
A
0.36
0.66
6.9
1 215
160
1 200
7.2
0.75
3.5
6.9
1 520
200
1 500
13.5
1.00
4.0
13.8
2 430
315
2 300
10.8
0.80
5.0
10.5
2 310
315
2 300
114
14.2
0.85
1.9
13.8
2 430
315
2 300
5
114
11.4
0.62
1.6
10.5
2 310
315
2 300
4.9
12.5
181
22.6
0.74
2.0
22.0
3 870
400
3 600
50/5
3.9
10
181
18.1
0.74
1.7
17.0
3 740
400
3 600
80/5
3.7
10
272
17.0
0.95
3.0
17.0
6 000
630
5 800
100/5
3.0
7.5
272
13.6
1.00
4.1
13.8
6 070
630
5 800
80/5
3.9
10
290
18.1
0.74
1.7
17.0
6 000
630
5 800
100/5
3.2
7.5
290
14.5
0.65
2.0
13.8
6 070
630
5 800
80/5
4.9
12.5
362
22.6
0.74
2.0
22.0
7 740
630
5 800
100/5
3.9
10
362
18.1
0.74
1.7
17.0
7 480
630
5 800
11
1 250
65.6
100/5
4.9
12.5
453
22.6
0.74
2.0
22.0
9 860
630
5 800
11
1 500
78.7
100/5
5.9
12.5
543
27.1
0.34
0.62
22.0
9 680
630
5 800
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ENA Technical Specification 12-6
Issue 2 2014
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NOTE 1: 300 kVA and 750 kVA power transformers and 40/5A and 80/5A current transformer ratios are included to cater for existing installations only.
NOTE 2: The same time fuse-link selections may be used for 1 600 kVA distribution transformers as for 1 500 kVA units.
1)
Based on distributor fuse-links having the most unfavourable time/current characteristics specified in ENA TS 12-08.
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ENA Technical Specification 12-6
Issue 2 2014
Page 14
Table 3 — Selection of time fuse-link ratings for 6.6 kV systems
1
2
3
4
5
6
7
8
9
10
11
121)
Discrimination with Distributor Fuse-Links1)
for Phase/Phase Fault
Phase/neutral fault
in secondary terminal zone
System
Nominal
Voltage
Power
Transformer
Rating
Transformer
Primary
FLC
Current
Transformer
Ratio
Maximum
Sustained
Current in
Time
Fuse-Link
(150%
Transformer
Rating)
Time
FuseLink
Rating
Primary
Current
(0.6 x 11.5 x
FLC)
Current
in Time
FuseLink
kV
kVA
A
–
A
A
A
A
Min.
Max.
A
A
A
A
6.6
200
17.5
40/5
3.3
7.5
121
15.1
0.64
1.4
13.8
1 460
160
1 200
50/5
2.6
5
121
12.1
0.63
1.6
10.5
1 385
160
1 200
6.6
300
26.2
40/5
4.9
12.5
181
22.6
0.75
2.0
22.0
2 320
315
2 300
50/5
3.9
10
181
18.1
0.72
1.6
17.0
2 245
250
1 900
40/5
5.2
12.5
190
23.8
0.62
1.4
22.0
2 320
315
2 300
50/5
4.2
10
190
19.0
0.60
1.2
17.0
2 245
250
1 900
80/5
4.1
10
302
18.9
0.63
1.3
17.0
3 590
400
3 6002)
100/5
3.3
7.5
302
15.1
0.64
1.4
13.8
3 640
400
3 600
80/5
6.2
15
453
28.3
0.90
2.8
29.0
6 140
630
5 800
100/5
4.9
12.5
453
22.6
0.75
2.1
22.0
5 810
630
5 800
80/5
6.6
15
482
30.1
0.78
2.0
29.0
6 140
630
5 800
100/5
5.3
12.5
482
24.1
0.58
1.2
22.0
5 810
630
5 800
80/5
8.2
15
605
37.8
0.40
0.66
29.0
6 140
630
5 800
100/5
6.6
15
605
30.2
0.94
1.7
29.0
7 650
630
5 800
6.6
6.6
6.6
6.6
6.6
315
500
750
800
1 000
27.6
43.8
65.5
70.0
87.6
Operating
Minimum 1
Time of Time Second Operating
Fuse-Link
Current of Time
(Seconds)
Fuse-Link
Secondary
Current
Corresponding
to Column 10
Maximum
Maximum 1
Distributor Fuse- Second Operating
Link Rating which
Current of
may be used to
Distributor Fuseachieve
Link
Discrimination
NOTE: 200 kVA, 300 kVA and 750 kVA power transformers and 40/5A and 80/5A current transformer ratios are included to cater for existing installations only.
1)
2)
131)
Based on distributor fuse-links having the most unfavourable time/current characteristics specified in ENA TS 12-08.
The maximum 1 second operating current of a 400 A BS 88 Part 5 400 A J Type fuse is less than 3 600 A and has been shown to grade with a 10 A TLF for practical purposes.
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ENA Technical Specification 12-6
Issue 2 2014
Page 15
Table 4 — Selection of time fuse-link ratings for 20 kV systems
1
2
3
4
5
6
7
8
9
10
Power
Transformer
Rating
kV
kVA
A
20
300
8.7
20
20
20
20
20
315
500
750
800
1 000
Transformer
Current
Primary
Transformer
FLC.
Ratio
9.1
14.4
21.6
23.1
28.9
Maximum
Sustained
Current in
Time
Fuse-Link
(150%
Transformer
Rating)
Time
FuseLink
Rating
Primary
Current
Current
in Time
(0.6 x 11.5 x FuseFLC)
Link
–
A
A
A
A
Min.
40/5
1.6
3
60
7.5
50/5
2)
-
2)
-
2)
-
2)
40/5
1.7
3
50/5
-2)
40/5
121)
131)
Discrimination with Distributor Fuse-Links1)
for Phase/Phase Fault
Phase/neutral fault
in secondary terminal zone
System
Nominal
Voltage
11
Minimum 1
Second Operating
Current of Time
Fuse-Link
Secondary
Current
Corresponding
to Column 10
Max.
A
A
A
A
0.65
4
6.9
2 208
200
1 500
-
2)
-
2)
-
2)
-
2)
-
2)
-
-2)
63
7.9
0.48
1.2
6.9
2 208
200
1 500
-2)
-2)
-2)
-2)
-2)
-2)
-2)
-2)
-2)
2.7
5
99
12.4
0.52
1.2
10.5
3 360
315
2 300
50/5
2.2
3
9.9
0.27
0.48
6.9
2 760
315
2 300
80/5
2.0
3
9.3
0.33
0.64
6.9
4 416
400
3 600
100/5
1.6
3
7.4
0.68
5
6.9
5 520
400
3 600
80/5
2.2
3
9.9
0.27
0.48
6.9
4 416
400
3 600
100/5
1.7
3
7.9
0.48
1.2
6.9
5 520
400
3 600
80/5
2.7
5
12.4
0.52
1.2
10.5
6 720
630
5 800
100/5
2.2
3
9.9
0.27
0.48
6.9
5 520
400
3 600
149
159
199
Operating
Time of Time
Fuse-Link
(Seconds)
Maximum
Maximum 1
Distributor Fuse- Second Operating
Link Rating which
Current of
may be used to
Distributor Fuseachieve
Link
Discrimination
20
1 250
36.1
100/5
2.7
5
249
12.4
0.52
1.2
10.5
8 400
630
5 800
20
1 500
43.3
100/5
3.2
7.5
299
14.9
0.62
1.4
13.8
11 040
630
5 800
NOTE 1: 300 kVA and 750 kVA power transformers and 40/5A and 80/5A current transformer ratios are included to cater for existing installations only.
NOTE 2: The same time fuse-link selections may be used for 1 600 kVA distribution transformers as for 1 500 kVA units.
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ENA Technical Specification 12-6
Issue 2 2014
Page 16
NOTE 3: The time/current characteristics of 3 A time fuse-links are not compatible with faults in the secondary terminal zone of 200 kVA distribution transformers.
1)
Based on distributor fuse-links having the most unfavourable time/current characteristics specified in ENA TS 12-08.
2)
The time/current characteristics of 3 A time fuse-links are not compatible with faults in the secondary terminal zone of 300 kVA and 315 kVA distribution transformers with CT ratio of 50/5.
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ENA Technical Specification 12-6
Issue 2 2014
Page 17
Figure 1 — Time/current characteristics of time fuse-links
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ENA Technical Specification 12-6
Issue 2 2014
Page 18
Figure 2 — Dimensions of time fuse-links
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ENA Technical Specification 12-6
Issue 2 2014
Page 19
Figure 3 — Circuit diagram for duty tests
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ENA Technical Specification 12-6
Issue 2 2014
Page 20
Figure 4 — Test rig for fuse-links
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ENA Technical Specification 12-6
Issue 2 2014
Page 21
Annex A
(informative)
Self Certification Conformance Declaration
ENA TS 12-6 Technical Specification for Time Fuse-Links (for use with Current Transformer Releases on Circuit Breakers)
CLAUSE BY CLAUSE CONFORMANCE WITH ENA TS 12-6
Time fuse-links covered by ENA TS 12-6 shall conform with the clauses in this Specification.
This check sheet identifies the clauses in ENA TS 12-6.
The manufacturer shall declare conformance or otherwise, clause by clause, using the following levels of conformance declaration codes.
Conformance declaration codes*
Instructions for completion
NA
Clause is not applicable or appropriate to the product
Cs1
the product is fully conforms with the requirements of this clause
Cs2
the product partially conforms with the requirements of this clause
Cs3
the product does not conform with the requirements of this clause
Cs4
the product does not currently conform with the requirements of this clause, but the
manufacturer proposes to modify and test the product in order to comply.
When Cs1 code is entered no remark is
necessary
When any other code is entered the reason
for non-conformance shall be entered
Manufacturer:
Product Reference:
Name:
Check sheet follows…..
Signature:
Date:
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ENA Technical Specification 12-6
Issue 2 2014
Page 22
Table A.1 — Self Certification Conformance Declaration
Clause
Clause Title
4
Classification
5
Characteristics of fuses
5.1
Ratings
5.2
Time/current characteristics
5.3
Resistance
6
Markings
7
Standards conditions for construction
7.1
Construction and dimensions
7.2
Mounting of time fuse-links
8
Tests
8.1
Type tests
8.1.1
Temperature rise
8.1.2
Time/current characteristics
8.1.3
Resistance
8.1.4
Duty
Conformance code
ENA TS 12-6
Conformance code
BS EN 60269-21)
1) In addition, the manufacturer is to state the equivalent clause(s) in BS EN 60269-2, where they do not align with ENA TS 12-6.
Remarks
ENA Technical Specification 12-6
Issue 2 2014
Page 23
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Annex B
(informative)
Selection of time fuse-links
B.0
General
Tables 2, 3 and 4 have been prepared to cover the normal conditions which are met in service
where time fuse-links are applied. The following conditions were considered in preparing these
tables.
B.1
Inrush current of the distribution transformer
The choice of time fuse-link in Tables 2, 3 and 4 takes account of available information
concerning the inrush currents of distribution transformers. For this purpose a time fuse-link is
deemed satisfactory if it withstands without deterioration, the equivalent of ten times
transformer primary full load current for 0.1 second.
B.2
Overload rating of the distribution transformer
In each case the selected time fuse-link permits the distribution transformer to carry 150% of
rated load without the continuous normal current rating of the time fuse-link being exceeded.
B.3
Faults in distribution transformer terminal zone
The selected time fuse-link will operate in less than 5 seconds with a phase/neutral or
phase/earth fault in the low voltage terminal zone of the transformer, when the cable sheaths
are directly bonded to the neutral.
This is the criterion which sets an upper limit to the rating of the fuse-link selected in Tables 2,
3 and 4. All other aspects could be satisfied with a time fuse-link of higher current rating than
is listed in Tables 2, 3 and 4, i.e:
a) withstanding inrush currents;
b) permitting transformer overloads;
c) discriminating with distributor fuses.
It is, of course, essential that a terminal zone earth fault on the secondary side of the
transformer is cleared quickly by the high voltage circuit-breaker even with the earth fault
current restricted by source impedance, fault impedance and low voltage cable impedance to
a value less than that determined from the distribution transformer impedance alone. A value
of 0.6 has been used to calculate the values stated in column 7.
B.4
Discrimination between the low voltage distributor fuse-links and the
distribution transformer high voltage circuit-breaker
A phase/phase fault on a low voltage distributor results in the most onerous condition. This
condition gives a maximum phase/phase current on the low voltage side of the transformer
which is 0.866 times the symmetrical 3-phase fault value. The corresponding currents in the
high voltage lines have ratios of 2:1:1 the maximum value being equal to the symmetrical 3phase fault value. Thus with this type of fault a time fuse-link in one phase may pass a current
corresponding to a 3-phase symmetrical fault whereas the distributor fuse-link passes only
0.866 of the symmetrical fault value.
ENA Technical Specification 12-6
Issue 2 2014
Page 24
Consider now the headings of Tables 2, 3 and 4.
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Column 5 is the secondary current for the current transformer when the distribution transformer
is on 150% load.
Column 7 gives the current in the high voltage line of a phase/neutral (or phase/earth) fault in
the low voltage terminal zone of the transformer.
The value is based on:
A transformer impedance of 5% for a delta/star connected transformer, this results
in a fault current equal to
100
5√3
= 11.5 times
A factor of 0.6 is introduced to take account of the estimated average low voltage
earth fault impedance.
Column 8 lists the current passed through the time fuse-link and corresponds to column 7
multiplied by the inverse of the current transformer ratio.
Column 9 gives the limits of time within which the time fuse-link will melt when passing the
currents of column 8.
Columns 10 to 13 are concerned with the performance of the maximum rated distributor fuselink in relation to the chosen time fuse-link. The distributor fuse-link ratings conform with the
requirements of ENA TS 12-08 and are listed in column 12. The time fuse-link rating is selected
so that the transformer secondary current corresponding to the minimum one-second
operating current of the time fuse-link has a safety margin over the maximum one-second
operating current of the distributor fuse-link. This is based upon the assumption that the
characteristic of the distributor fuse-link approaches that of the time fuse-link most closely at
the one-second ordinate. The arcing time of the power fuse-link is negligible at this melting
time.
Column 10 is the minimum current which will melt the time fuse-link in 1 second; it is read
directly from Fig. 1.
Column 11 is the corresponding current in the low voltage circuit and is derived by multiplying
column 10 by the current transformer ratio and the current transformation factor of the
distribution transformer.
As mentioned earlier the condition considered is a phase/phase fault on the low voltage side
of the transformer since this is the most onerous. Under this condition the transformation ratio
of the distribution transformer for current is equal to the voltage ratio multiplied by 0.866, e.g.
for an 11 kV/433 V transformer the fault current in the low voltage circuit is equal to the fault
current in the high voltage line which has maximum value multiplied by the current
transformation factor
11 000 × 0.866
= 22
433
Using line 1 as an example, column 11 is derived:
6.9 A ×
40
× 22 = 1 215 A
5
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ENA Technical Specification 12-6
Issue 2 2014
Page 25
Column 13 states the current which causes a distributor fuse-link of maximum rating given in
column 12, having the most unfavourable time/current characteristics specified in ENA TS 1208, to melt in one second. In all cases this value is less than the minimum secondary current
required to operate the time fuse-link in one second, thus ensuring that the distributor fuse-link
clears before the time fuse-link melts. If a greater margin of discrimination is required, then a
distributor fuse-link of less than maximum rating should be fitted and this can be chosen by
comparing the value given in column 11 with the time/current characteristic envelopes of the
appropriate distributor fuse-link given in ENA TS 12-08.
ENA Technical Specification 12-6
Issue 2 2014
Page 26
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Annex C
(informative)
Explanatory notes
C.0
General
The following notes are an explanation of the reasons underlying the form of the Specification.
C.1
Clause 5.3
This clause is concerned with ensuring that the high voltage circuit-breaker does not open on
the occurrence of a fault beyond the low voltage distributor fuse-link.
The condition to be avoided is current spilling through the release coil whilst it is still shunted
by an intact time fuse-link. Excess spill can occur so opening the circuit-breaker unless the
relative impedances of the fuse-link and the release coil are suitably matched.
Maximum spill occurs immediately prior to clearance of the low voltage distributor fuse-link
since the time fuse-link is increasing its temperature, and hence its resistance, during the
period of the fault.
Danger of mal-operation through spill current is present only at high values of fault current
approaching the maximum throughput of the distribution transformer. Under these conditions
a typical low voltage distributor fuse-link selected in accordance with this Specification clears
at a time when approximately 60% of the melting time of the time fuse-link has expired.
The choice of a current which causes melting in 0.2 second (see Clause 9.3) is arbitrary since
the time is not critical provided it is sufficiently short to prevent appreciable heat loss from the
time fuse-link element during the melting period. The resistance is measured under these
conditions by oscillographic means.
It will be noted that the impedances quoted are for a release coil which has a burden of at least
6 VA in a reset condition irrespective of whether it has a rating of 2 A or 5 A. For release coils
which have a burden below 6 VA, special consideration may be necessary.
The higher operating voltage of the 2 A rated coil compared with the 5 A coil results in better
performance of the 2 A coil from the point of view of spill current on high through faults. The 5
A coil with the minimum impedance 0.24 ohm, if shunted by a 5 A time fuse-link, is liable to
receive under maximum through fault conditions a spill current in the region of 50% in excess
of its setting; it is considered, however, that the rapid clearance of the low voltage distributor
fuse-link would result in a current pulse of a time duration insufficient to operate the circuitbreaker release coil.
C.2
Clause 11.4
This clause is intended to ensure the ability of the fuse-link to operate satisfactorily under
maximum fault conditions.
Since the output of typical current transformers under maximum fault conditions consists
mainly of spikes of voltage and current which are very large in crest value when compared with
the knee point voltage and associated current of the current transformer, the fuse-links must
be tested for interruption under voltage and current conditions which are typical as to
waveshape and which have values equivalent to or greater than those to be expected in
practice.
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ENA Technical Specification 12-6
Issue 2 2014
Page 27
It is considered that measurement of the peak value of the current and voltage is the best
criterion of their severity rather than r.m.s. or other values. It is not considered practicable to
determine sinusoidal values of current and voltage which would be equivalent, so far as the
fuse-link is concerned, to given spikes of these quantities.
The peak values of current and voltage stipulated (1 400 A and 950 V) are derived from tests
and calculations of typical current transformers.
When stipulating the peak current and voltage which the time fuse-link must handle, a current
transformer primary current of 21.9 kA was considered. The specification is thus applicable on
systems having fault levels up to 350 MVA at 11 kV or 250 MVA at 6.6 kV.
In performing the duty tests, a release coil of 7.2 ohm impedance when in the non-operated
(or reset) condition is specified. This is considered to be the maximum impedance likely to be
encountered in practice and hence gives the maximum recovery voltage with which the fuselink has to deal.