EN 50068:2018

High-Voltage Switchgear and Controlgear - Gas-filled wrought steel enclosures

This document applies to wrought steel enclosures and their welding. These enclosures are pressurized with dry air, inert gases, for example sulphur hexafluoride or nitrogen or a mixture of such gases, used in indoor and outdoor installations of high-voltage switchgear and controlgear with rated voltages above 1kV, where the gas is used principally for its dielectric and/or arc-quenching properties with rated voltages: - above 1 kV and up to and including 52 kV concerning gas-filled compartments with design pressure higher than 300 kPa relative pressure (gauge); - above 52 kV concerning all gas-filled compartments. The enclosures comprise parts of electrical equipment not necessarily limited to the following examples: - circuit-breakers; - switch-disconnectors; - disconnectors; - earthing switches; - current transformers; - voltage transformers; - surge arrestors; - busbars and connections; - etc. The scope also covers enclosures of pressurized components such as the centre chamber of live tank switchgear, gas-insulated current transformers, etc.

Hochspannungs-Schaltgeräte und Schaltanlagen - Gasgefüllte Kapselungen aus Schmiedestahl

Appareillage électrique haute tension - Enveloppes sous pression en acier corroyé et en alliage d'acier

Le présent document s’applique aux enveloppes en acier et leur soudage. Ces enveloppes sont sous pression d'air sec et de gaz inertes, par exemple l'hexafluorure de soufre ou l'azote ou un mélange de tels gaz, utilisées dans les installations intérieures et extérieures d'appareillage à haute tension sous des tensions assignées supérieures à 1 kV. Le gaz est utilisé principalement pour ses propriétés diélectriques et/ou d'extinction de l'arc avec des tensions assignées: - supérieures à 1 kV et inférieures ou égales à 52 kV, en ce qui concerne les compartiments à remplissage de gaz dont la pression de calcul est supérieure à une pression relative (manométrique) de 300 kPa; - supérieures à 52 kV en ce qui concerne tous les compartiments à remplissage de gaz. Exemples de parties d'équipements électriques contenues dans les enveloppes: - disjoncteurs; - interrupteurs-sectionneurs; - sectionneurs; - sectionneurs de terre; - transformateurs de courant; - transformateurs de tension; - parafoudres; - jeux de barres et barres de raccordement; - etc. Le domaine d'application couvre également les enveloppes de composants sous pression tels que le carter d'appareillage à mécanisme sous tension, les transformateurs de courant à isolation gazeuse, etc.

Visokonapetostne stikalne in krmilne naprave - S plinom polnjena ohišja iz gnetljivega jekla

General Information

• Status: Published
• Publication Date: 29-Nov-2018
• ICS: 29.130.10 - High voltage switchgear and controlgear
• Technical Committee: CLC/TC 17AC - High-voltage switchgear and controlgear
• Drafting Committee: CLC/TC 17AC - High-voltage switchgear and controlgear
• Current Stage: 9093 - Decision to confirm - Review Enquiry
• Start Date: 19-Aug-2020
• Completion Date: 23-Sep-2025
• Ref Project: SIST EN 50068:2019 - High-voltage switchgear and controlgear - Gas-filled wrought steel enclosures

Relations

• Replaces: EN 50068:1991/A1:1993 - Wrought steel enclosures for gas-filled high-voltage switchgear and controlgear
• Effective Date: 07-Jun-2022

Overview

EN 50068:2018 - published by CLC/CENELEC - specifies requirements for gas-filled wrought steel enclosures used in high-voltage switchgear and controlgear. It applies to indoor and outdoor equipment with rated voltages above 1 kV (with specific design-pressure thresholds up to 52 kV) where the filling gas (dry air, inert gases, SF6, N2 or mixtures) is used mainly for dielectric and/or arc-quenching functions. Typical equipment covered includes circuit‑breakers, disconnectors, current and voltage transformers, surge arresters, busbars and related pressurized components.

Key Topics

• Scope and definitions: enclosure, design pressure, design temperature, manufacturer responsibilities.
• Materials: requirements for wrought steel selection and material identification consistent with pressure‑vessel practice.
• Design methods: evaluation of mechanical strength by:
  • Design by formula
  • Design by analysis
  • Design by burst test
  • Special treatment of flanges and bolted connections
• Welding and fabrication: welding procedure specifications, welder qualification, assembly/welding sequence, preheating, repair of welding defects and workmanship rules.
• Inspection, testing and certification:
  • Type tests (including burst and strain measurement tests)
  • Routine pressure tests
  • Non‑destructive testing (radiographic, ultrasonic, penetrant) and NDT personnel qualification
  • Documentation, stamping, and final inspection requirements
• Quality assurance and service conditions: production QA, normal and special environmental/service conditions.
• Pressure relief devices: requirements for bursting discs, self‑closing and non‑self‑closing relief devices.
• Exclusion from PED: EN 50068 provides a European specification and clarifies the exclusion of high‑voltage enclosures from Directive 2014/68/EU (pressure equipment).

Applications

EN 50068:2018 is intended for:

• Manufacturers and designers of gas‑insulated switchgear (GIS) and pressurized HV equipment
• Steel fabricators and welding shops producing enclosures
• Test laboratories performing burst, strain and NDT tests
• Certification and inspection bodies assessing conformity and issuing certificates
• Specifiers, procurement teams and asset owners requiring standards‑based enclosure design and documentation

Using this standard helps ensure safe mechanical integrity, reliable dielectric performance, correct welding quality, and traceable testing for gas‑filled HV enclosures.

Related Standards

Relevant referenced documents include:

• EN 13445-2 / EN 13445-3 (unfired pressure vessels - materials & design)
• EN ISO 15614-1, EN ISO 9606-1 (welding procedure and welder qualifications)
• EN ISO 17636, EN ISO 17640, EN ISO 3452 (NDT methods)
• EN 62271-1 (HV switchgear common specifications)
• EN ISO 898-1 / EN ISO 898-2 (fasteners)
• EN ISO 9712 (NDT personnel certification)

Keywords: EN 50068:2018, gas-filled wrought steel enclosures, high-voltage switchgear, SF6, welding, design pressure, burst test, non-destructive testing, gas-insulated switchgear.

Standards Content (Sample)

SLOVENSKI STANDARD
01-februar-2018
1DGRPHãþD
SIST EN 50068:1995
SIST EN 50068:2015
Visokonapetostne stikalne in krmilne naprave - S plinom polnjena ohišja iz
gnetljivega jekla
High-voltage switchgear and controlgear - Gas-filled wrought steel enclosures
Hochspannungs-Schaltgeräte und Schaltanlagen - Gasgefüllte Kapselungen aus
Schmiedestahl
Appareillage électrique haute tension - Enveloppes sous pression en acier corroyé et en
alliage d´acier
Ta slovenski standard je istoveten z: EN 50068:2018
ICS:
29.130.10 Visokonapetostne stikalne in High voltage switchgear and
krmilne naprave controlgear
77.080.20 Jekla Steels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN 50068
NORME EUROPÉENNE
EUROPÄISCHE NORM
November 2018
ICS 29.130.10 Supersedes EN 50068:1991
English Version
High-Voltage Switchgear and Controlgear - Gas-filled wrought
steel enclosures
Appareillage électrique haute tension - Enveloppes sous Hochspannungs-Schaltgeräte und Schaltanlagen -
pression en acier corroyé et en alliage d'acier Gasgefüllte Kapselungen aus Schmiedestahl
This European Standard was approved by CENELEC on 2018-08-27. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden,
Switzerland, Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2018 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN 50068:2018 E
Contents Page
European foreword .4
Introduction .5
1 Scope .6
2 Normative references .6
3 Terms and definitions .7
4 Quality assurance .9
5 Normal and special service conditions .9
6 Materials . 10
7 Design . 10
7.1 General . 10
7.2 Calculation methods . 10
7.2.1 General . 10
7.2.2 Evaluation of mechanical strength using “Design by Formula” . 11
7.2.3 Evaluation of mechanical strength using “Design by Analysis” . 11
7.2.4 Evaluation of mechanical strength using “Design by Burst test” . 12
7.2.5 Flanges . 14
7.2.6 Bolted connections . 14
7.3 Inspection and access openings . 15
8 Manufacture and workmanship . 15
8.1 Material identification . 15
8.2 Order of completion of weld seams . 15
8.3 Cutting of materials . 15
8.4 Forming of shell sections and end plates . 15
8.5 Assembly tolerances . 15
8.6 Welded joints . 15
8.7 Assembly for welding. 16
8.8 General welding requirements . 16
8.9 Preheating . 16
8.10 Surface finish . 16
9 Repair of welding defects . 16
10 Inspection, testing and certification . 17
10.1 Type tests . 17
10.1.1 General . 17
10.1.2 Burst test procedure . 17
10.1.3 Strain measurement test . 17
10.2 Inspection and routine tests . 17
10.2.1 General . 17
10.2.2 Routine pressure tests . 18
10.3 Welding procedure specifications . 18
10.4 Welder performance tests . 18
10.5 Non-destructive testing . 18
10.5.1 Amount of testing of welded joints . 18
10.5.2 Test methods for weld seams . 19
10.5.3 Surface conditions and preparations for testing . 19
10.5.4 Marking of the enclosure welds . 20
10.5.5 Reporting . 20
10.5.6 Minimum acceptance levels . 20
10.5.7 Assessment of imperfections . 20
10.6 Design specification, drawings and data sheets . 22
10.7 Certificate . 22
10.8 Stamping . 22
10.9 Final inspection . 22
11 Pressure relief devices . 23
11.1 General . 23
11.2 Bursting discs . 23
11.3 Self-closing pressure relief valves . 23
11.4 Non-self-closing pressure relief devices . 23
Annex A (informative) A-deviations . 25
Bibliography . 26

European foreword
This document (EN 50068:2018) has been prepared by CLC/TC 17AC, “High-voltage switchgear and
controlgear”.
The following dates are fixed:
• latest date by which this document has (dop) 2019-08-27
to be implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2021-08-27
standards conflicting with this document
have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 50068:1991, EN 50068:1991/corrigendum Aug. 2007 and
EN 50068:1991/A1:1993.
This document has been revised by CENELEC Technical Committee 17AC “High-voltage switchgear and
controlgear”. It supplements the relevant product standards on gas-insulated switchgear and controlgear in
that it provides specific requirements for pressurized high-voltage switchgear and controlgear.
The present document has been written to get a European specification for the design, construction, testing,
inspection and certification of pressurized enclosures used in high-voltage switchgear and controlgear.
In this respect, this document constitutes the exclusion of HV switchgear from the scope of the Directive
2014/68/EU (superseding 97/23/EC) concerning pressure equipment. Article 1, 2. (l) excludes “enclosures
for high-voltage electrical equipment such as switchgear, controlgear, transformers, and rotating machines”
from the scope of the Directive.
This document deals with gas-insulated switchgear enclosures of wrought steel and their welding. For
different enclosure materials, other European Standards are available.
Introduction
This document covers the requirements for the design, construction, testing, inspection and certification of
gas-filled enclosures for use specifically in high-voltage switchgear and controlgear, or for associated gas-
filled equipment.
Special consideration is given to these enclosures for the following reasons.
(a) The enclosures usually form the containment of electrical equipment, thus their shape is determined by
electrical rather than mechanical requirements.
(b) The enclosures are installed in restricted access areas and the equipment is operated by instructed,
authorized persons only.
(c) As the thorough drying of the inert, non-corrosive gas-filling medium is fundamental to the satisfactory
operation of the electrical equipment, the gas is periodically checked. For this reason, no internal corrosion
allowance is required on the wall thickness of these enclosures.
(d) The enclosures are subjected to only small fluctuations of pressure as the gas-filling density will be
maintained within close limits to ensure satisfactory insulating and arc-quenching properties. Therefore, the
enclosures are not liable to fatigue due to pressure cycling.
(e) The operating pressure is relatively low.
Due to the foregoing reasons and to ensure maximum service continuity as well as to reduce the risk of
moisture and dust entering the enclosures which could endanger safe electrical operation of the switchgear,
no pressure tests should be carried out after installation and before placing in service and no periodic
inspection of the enclosure interiors or pressure tests should be carried out after the equipment is placed in
service.
1 Scope
This document applies to wrought steel enclosures and their welding. These enclosures are pressurized with
dry air, inert gases, for example sulphur hexafluoride or nitrogen or a mixture of such gases, used in indoor
and outdoor installations of high-voltage switchgear and controlgear with rated voltages above 1kV, where
the gas is used principally for its dielectric and/or arc-quenching properties with rated voltages:
— above 1 kV and up to and including 52 kV concerning gas-filled compartments with design pressure
higher than 300 kPa relative pressure (gauge);
— above 52 kV concerning all gas-filled compartments.
The enclosures comprise parts of electrical equipment not necessarily limited to the following examples:
— circuit-breakers;
— switch-disconnectors;
— disconnectors;
— earthing switches;
— current transformers;
— voltage transformers;
— surge arrestors;
— busbars and connections;
— etc.
The scope also covers enclosures of pressurized components such as the centre chamber of live tank
switchgear, gas-insulated current transformers, etc.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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.
EN 13445-2:2014, Unfired pressure vessels — Part 2: Materials
EN 13445-3, Unfired pressure vessels — Part 3: Design
EN ISO 15614-1, Specification and qualification of welding procedures for metallic materials — Welding
procedure test — Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys (ISO
15614-1)
EN ISO 17636 (all parts), Non-destructive testing of welds — Radiographic testing (ISO 17636)
EN ISO 17640, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and
assessment (ISO 17640)
EN 62271-1:2017, High-voltage switchgear and controlgear — Part 1: Common specifications for alternating
current switchgear and controlgear (IEC 62271-1:2017)
EN ISO 898-1:2013, Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts,
screws and studs with specified property classes — Coarse thread and fine pitch thread (ISO 898-1:2013)
EN ISO 898-2:2012, Mechanical properties of fasteners made of carbon steel and alloy steel — Part 2: Nuts
with specified property classes — Coarse thread and fine pitch thread (ISO 898-2:2012)
EN ISO 3452 (all parts), Non-destructive testing — Penetrant testing (ISO 3452)
EN ISO 9606-1, Qualification testing of welders — Fusion welding — Part 1: Steels (ISO 9606-1)
EN ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel (ISO 9712)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
enclosure
part of gas-insulated metal-enclosed switchgear retaining the insulating gas under the prescribed conditions
necessary to maintain safely the rated insulation level, protecting the equipment against external influences
and providing a high degree of protection to personnel
3.2
manufacturer
organization that is responsible for the design of the enclosure and the production of the gas-insulated
switchgear. In this standard this is the switchgear manufacturer
3.3
design pressure
pressure, expressed in relative terms (gauge), used to determine the thickness of the enclosure
Note 1 to entry: It is at least equal to the maximum pressure in the enclosure at the highest temperature that the gas
used for insulation can reach under specified maximum service conditions.
3.4
design temperature
design temperature of an enclosure
maximum temperature that the enclosures can reach under specified maximum service conditions
Note 1 to entry: This is generally the upper limit of ambient air temperature increased by the temperature rise due to
the flow of rated normal current.
Note 2 to entry: Solar radiation should be taken into account when it has a significant effect on the temperature of the
gas and on the mechanical properties of materials. Similarly, the effects of low temperatures on the properties of some
materials should be considered.
[SOURCE: EN 62271-203:2012, 3.112, modified – Note 1 to entry and Note 2 to entry have been added]
3.5
design stress
maximum permissible stress on the enclosure imposed by conditions of operation, environment or test that
determine the (material) characteristics of an enclosure
3.6
normal load
load whose occurrence and level can be planned or predicted
3.7
exceptional load
load whose probability of occurrence during the lifetime of product is very small or accidental
3.8
alloy
substance having metallic properties and composed of two or more elements so combined that they cannot
readily be separated by physical means
[SOURCE: EN 12258-1:2012, 2.2.1]
3.9
weld defect
imperfections in metallic fusion welds
3.9.1
lack of fusion
lack of union between the weld metal and the parent material or between the successive layers of weld metal
[SOURCE: EN ISO 6520-1:2007, Reference No. 401]
3.9.2
overlap
excessive weld metal covering the parent material surface but not fused to it
[SOURCE: EN ISO 6520-1:2007, Reference No. 506]
3.9.3
undercut
irregular groove at a toe of a run in the parent material or in previously deposited weld metal
[SOURCE: EN ISO 6520-1:2007, Reference No. 5011]
3.9.4
hot crack
hot tear
crack formed in a cast metal or in a welding because of internal stress developed upon cooling at the solidus
temperature or slightly above
[SOURCE: EN 12258-1:2012, 5.2.8]
3.9.5
inclusion
extraneous material accidentally entrapped into the liquid metal during melting or melt treatment or
entrapped into the metal surface during hot or cold working
[SOURCE: EN 12258-1:2012, 5.5.7]
3.9.6
blister
raised spot whose inside is hollow, that forms on the surface of products and is caused by the penetration of
a gas into a subsurface zone typically during thermal treatment
[SOURCE: EN 12258-1:2012, 5.5.10]
Note 1 to entry: A void resulting from a blister that has ruptured is often termed “blow hole”.
3.10
thermal treatment
heating, holding at elevated temperature and cooling of the solid metal in such a way as to obtain desired
metallurgical structure or properties
[SOURCE: EN 12258-1:2012, 3.6.1]
Note 1 to entry: The term “heat treatment” is used for the same concept as a synonym.
3.11
ductility
ability of a material to deform plastically before fracturing
[SOURCE: EN 12258-1:2012, 4.3.15]
3.12
fatigue
tendency for a metal to break under conditions of repeated cyclic stressing considerably below the tensile
strength
[SOURCE: EN 12258-1:2012, 4.3.23, modified – Note 1 to entry has been removed]
3.13
tensile strength
ratio of maximum load before rupture in a tensile test to original cross-sectional area
[SOURCE: EN 12258-1:2012, 4.3.3, modified – Note 1 to entry has been removed]
3.14
yield stress
stress necessary to produce a defined small plastic deformation in a material under uniaxial tensile or
compressive load
[SOURCE: EN 12258-1:2012, 4.3.4, modified]
3.15
test piece
two or more parts of material welded together in accordance with a specified weld procedure, in order to
make one or more test specimens
3.16
test specimen
portion detached from a test piece, in specified dimensions, finally prepared as required for testing
4 Quality assurance
It is the intention of this document that the switchgear manufacturer shall be responsible for achieving and
maintaining a consistent and adequate quality of the product.
Sufficient examinations shall be made by the enclosure manufacturer to ensure that the materials,
production and testing comply in all respects with the requirements of this document.
Inspection by the user`s inspectors shall not absolve the switchgear manufacturer from his responsibility to
exercise such quality assurance procedures as to ensure that the requirements and the intent of this
document are satisfied.
5 Normal and special service conditions
Clause 2 of EN 62271-1:2017 is applicable.
6 Materials
Any suitable steel is permissible. A list of recommended materials is given in EN 13445-2:2014.
General requirements are defined in EN 13445-2:2014, 4.1.
NOTE Contact with more noble metals can lead to heavy galvanic corrosion.
7 Design
7.1 General
The rules for the design of enclosures of gas-insulated switchgear and controlgear prescribed in this clause
take into account that these enclosures are subjected to particular operating conditions (refer to Introduction)
which distinguish them from compressed air receivers and similar storage vessels. Examples of such
enclosures are listed in Clause 1.
As part of the validation process of the enclosure the mechanical strength of an enclosure shall be proven by
a type test according to subclause 10.1.
An enclosure can be designed by two alternative methods:
— Design by Formula (DbF)
— Design by Analysis (DbA)
The geometry of an enclosure is determined by electrical rather than mechanical considerations. Moreover,
constraints in shape can be enforced by the casting process used. These constraints can result in an
enclosure geometry which cannot be calculated by DbF. In such cases DbA shall be applied.
When designing an enclosure, account shall be taken of the following, if applicable:
a) the evacuation of the enclosure as part of the filling process;
b) the full differential pressure across the enclosure wall or partition;
c) superimposed loads and vibrations by external effects, e.g. as they are caused by thermal or seismic
effects.
The enclosures are filled in service with a non-corrosive thoroughly dried gas. Therefore no internal
corrosion allowance is necessary.
7.2 Calculation methods
7.2.1 General
This part provides calculation rules, design stresses and boundary conditions for the design of enclosures. In
7.2.5 and 7.2.6 boundary conditions for the design of flanges and bolts are given. For the design of the
enclosure itself three methods may be used to proof appropriate design stress (Table 1): the preferred
method, Design by Formula is given in 7.2.2. The alternative methods, Design by Analysis or Design by
Burst test are given in 7.2.3 and 7.2.4.
Table 1 — List of design methods
1 Design by Formula preferred
7.2.2
2 Design by Analysis
7.2.3
3 Design by Burst test
7.2.4
7.2.2 Evaluation of mechanical strength using “Design by Formula”
When the wall and flange thicknesses of the enclosure are calculated, the formulas from established
specifications such as the following codes shall be taken, using the design pressure, the design temperature
as defined in 3.3 and 3.4 and the safety factor as defined in this subclause:
EN 13445-3
AD 2000 Regelwerk [2]
ASME Code [3]
CODAP [4]
Raccolta VSR [5]
SVTI [6]
The formulas in the specifications are equivalent to each other; the choice is left to the manufacturer.
The design stress (fd) at the design pressure including the safety factor of the appropriate formulae is given
by:
R
e
f ⋅ν
d
1,5
where
Re is yield stress of the material at the design temperature taken from the material standard for the
chosen alloy;
1,5 is safety factor;
ν is welding factor to be taken as 0,75 or 1 depending of the situation.
Selection of the welding factor according to 10.5.1.
7.2.3 Evaluation of mechanical strength using “Design by Analysis”
7.2.3.1 Normal loads
f
dn_
The design stress ( ) for normal loads is given by:
R
e
f ⋅ν
dn_
S
n
where
is maximum permissible design stress for normal loads
f
dn_
Re is yield stress of the material at the design temperature taken from the material standard for the
chosen alloy
Sn is safety factor for normal loads Sn = 1,05
ν is welding factor to be taken as 0,75 or 1 depending of the situation
Selection of the welding factor according to 10.5.1.
=
=
Examples for normal loads includes:
— gas pressure;
— temperature (ambient, current);
— dead load;
— erection load (transportation and handling on site);
— ice and/or wind;
— tension loads (cable, overhead lines).
Combinations of different loads shall reflect the operating conditions on site. Load combinations do not
change the overall safety factor.
7.2.3.2 Exceptional loads
f
d _e
The design stress ( ) for exceptional loads is given by:
R
m
f ⋅ν
d _e
S
e
where
is maximum permissible design stress for exeptional loads
f
d _e
R is minimum tensile strength of the material at the design temperature taken from the
m
material standard for the chosen alloy
S is safety factor for exceptional loads S = 1,05
e e
ν is welding factor to be taken as 1
Examples for exceptional loads include:
— earthquakes;
— extreme wind and/or ice;
— short-circuit tensile loads (overhead lines, cable).
Combinations of different loads shall reflect the operating conditions on site. Load combinations do not
change the overall safety factor.
7.2.4 Evaluation of mechanical strength using “Design by Burst test”
7.2.4.1 General
When the thickness of the pressure parts are not calculated or where doubt exists regarding the accuracy of
the calculations, a proof test shall be carried out with one enclosure of a particular design. One of the
following proof tests is applicable:
a) burst test;
b) strain measurement test.
When proof tests are carried out on enclosures which are subjected to significant static superimposed loads
in service, the effect of these loads shall be simulated during the tests.
=
The proof test may be used for the purpose of establishing the design pressure of enclosures or enclosure
parts only when the wall thickness is not determined by means of the design rules given in this document.
The design pressure of all other parts shall be determined by means of the applicable design rules.
7.2.4.2 Burst test procedure
This procedure is to be used for enclosures or enclosure parts under internal pressure. The design pressure
of any part of the enclosure tested by this method shall be established by a pressure test.
The design pressure (p) for which an enclosure meets the requirements of this document shall be calculated
according to:
p σ
R a
pv  ⋅⋅
2,3 σ
t
where
p is design pressure;
pR is burst pressure;
ν is welding factor (refer to 10.5.1);
σa is permissible design stress at design temperature;
σ is permissible design stress at test temperature;
t
2,3 is safety factor against bursting strength.
7.2.4.3 Strain measurement test procedure
Before the test commences or any pressure has been applied to the enclosure, strain gauges of electrical
resistance or other types shall be affixed to both the inside and the outside surfaces of the enclosure. The
type and number of gauges, their positions and their directions shall be chosen so that principle strains and
stresses can be determined at all points of importance to the integrity of the enclosure. The type of gauge
and the cementing technique shall be chosen so that strains up to 1 % can be determined.
The pressure shall be applied gradually in steps of approximately 20 % of the expected design pressure and
shall be unloaded after each step. Strain readings shall be taken during the loading and unloading cycle.
An indication of localized permanent set may be disregarded provided ther
...