Download The basic function of switchgear is to control supply of electric power

Issued by : Switchgear Contracts Division LARSEN & TOUBRO LIMITED Powai Works, Bombay 400 072.
January-March 1996
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The basic function of switchgear is to
control supply of electric power and
to protect the equipment in the event
of abnormal conditions. Uninterrupted
supply of electric power is the need of
today's industry and is catered to by
the switchgear and controlgear in any
plant.
To meet the expectations of user
industry, switchgear has to be reliable
and safe. It should also be able to
withstand adequate number of
operations - in healthy conditions as
well as in abnormal/unhealthy
situations. Manufacturers as well as
customers, both concur on these
aspects.
However, reliability, adequacy are
subjective parameters. Unless they
are quantified, the expectations of
users and the capabilities of the
products cannot be matched.
Thus, defining reliability, safety and
adequacy in terms of quantifiable
parameters is essential from the
user's as well as the manufacturer's
point of view.
This activity is jointly done by a team
comprising representatives of user
industry, manufacturers and leading
technical institutes. The quantifiable
parameters are laid down in the form
of standards.
Development in technology, innovations, availability of better materials,
superior manufacturing processes
and techniques result in improvement
of products and their characteristic.
At the same time, field experience
gives a clue for further development.
It is realised that the field situations
are different from the laboratory
conditions and that product performance differs in actual working conditions. Expansions and increasing
loads demand higher source ratings,
which demand protective devices
with higher withstand capabilities.
All these demand continuous
upgradation of products.
To meet the demands of industry,
International Standards (formulated
by IEC) for Low Voltage Switchgear
and Controlgear were revised in
1988. Earlier, these standards were
having different numbers and some
of the terms were not clearly defined
or quantified. Now, standards for all
low voltage switchgear and
controlgear are grouped together
under a new specification - IEC-947.
Bureau of Indian Standards adopted
the new text in 1993 and a new
standard - IS 13947 was published
in 1993.
In this issue of L&T Current Trends,
we bring you an update on major
revisions carried out in the standards
on low voltage switchgear and
controlgear. Highlights of general
rules are covered in this issue. In the
next issue, product standards will be
covered.
BEST WISHES
FOR A
HAPPY
NEW
YEAR
Feature
Introduction to IS 13947 :
A new Indian Standard on
Switchgear and Controlgear Products
- H.T. Mistry, Senior Executive-Design, Switchgear Design and Product Development
In a major revision, International
Electrotechnical Commission (IEC)
introduced a new specification, IEC
947, on low voltage switchgear and
controlgear in 1988. Bureau of Indian
Standards also decided to adopt the
text of IEC 947 and consequently, in
1993, introduced a new dual number
specification IS 13947/IEC pub 947
on switchgear and controlgear
products. However, keeping in view
applicability of this specification in
Indian conditions, National Annex
has been added to IEC text to cover
typical requirements e.g. use of
aluminum conductors, ambient
conditions, preferred voltages, etc.
This specification is divided into two
documents.
(1) General rules (Part 1) :
All those parts of various products
standards which can be considered
as general have been consolidated in
this first part. These requirements are
applicable to all the products viz.
contactors, starters, air circuit breakers, moulded case circuit breakers,
switches and fuse combination units.
(2) Product standard :
Specific testing and performance
requirements of the products are
covered by relevant product standards. These are :
Part 2 : Circuit breakers
Part 3 :
Switches, disconnectors, switch
disconnectors
and fuse combination units
Part 4 : Contactors and motor
starters
(Section 1 : Electromechanical
contactors and starters)
Part 5 : Control circuit devices and
switching elements
(II) RATINGS AND LIMITING
VALUES :
Part 6 : Multiple function devices
Part 7 : Ancillary equipments
(i) Conventional thermal currents :
Following two new limiting currents have been introduced to
define thermal characteristics of
the product.
In pages to follow, we shall discuss
significant revisions that have been
introduced in IS 13947.
PART 1: GENERAL RULES :
This standard states those general
rules and requirements which are
common to low voltage switchgear
products e.g. definitions; characteristics; normal service, mounting and
transport conditions; information
supplied with the equipment; constructional and performance requirements and verification of these
requirements.
(a) Conventional free air thermal
current (lth)
(b) Conventional enclosed thermal
current (lthe)
These currents are defined as the
maximum values of currents to be
used for temperature rise tests on
unenclosed equipment (lth) or on
equipment in smallest enclosure
specified by the manufacturer (lthe).
These currents are not the ratings
and need not be marked on the
equipment.
Since a product standard is read in
conjunction with general rules, it may
not require and hence may omit a
general rule or it may add to it but will (ii) Rated impulse withstand voltage
(Uimp) :
not deviate from it.
Following are the major revisions
introduced in part 1 :
This is a new concept for low voltage
switchgear products.
(I) DEFINITIONS :
So far attention was not given to the
nature and effect of overvoltages
originating from causes like load
fluctuations, switching operations,
resonances, faults and lightening
discharges on low voltage (LV)
systems.
Disconnector:
Definition of disconnector has been
enlarged. A disconnector has now
been defined as a mechanical
device which in the open position
complies with the requirements of
isolating function.
These overvoltages affect common
LV installations and equipment such
This definition differs from the
as motors, switchgears, appliances
present one (IEV 441-14-05) which
etc. Various utilities and manufacturrefers to only isolating distance. As
ers have come across failures of LV
per the new definition, a disconnector equipment attributable to these
needs to meet performance require- overvoltages. Increased applications
ments in addition to minimum isolat- of miniaturized electronics in indusing distance.
trial products underscore need to
consider overvoltage phenomena in
LV system.
Pollution degree 1:
No pollution or only dry, non-conductive pollution occurs.
In view of above, in order to evaluate
adequacy of design of low voltage
switchgear products to withstand
overvoltages, concept of rated
impulse withstand voltage has been
introduced.
Pollution degree 2:
Normally non-conductive pollution
occcurs. Occasionally, however, a
temporary conductivity caused by
condensation may be expected.
Preferred values of rated impulse
withstand voltage are specified in
new IS. These are reproduced in
table 1.
Pollution degree 3:
Conductive pollution occurs or dry,
non-conductive pollution occurs
which becomes conductive due to
TABLE I
PREFERRED VALUES OF IMPULSE WITHSTAND VOLTAGE
MAXIMUM VALUE OF
RATED OPERATIONAL
VAOLTAGE TO EARTH
IV
ORIGIN OF
INSTALLATION
LEVEL
III
DISTRIBUTION
CIRCUIT
LEVEL
II
LOAD LEVEL
I
SPECIALLY
PROTECTED
LEVEL
50
1.5
0.8
0.5
0.33
100
2.5
1.5
0.8
0.5
150
4
2.5
1.5
0.8
300
6
4
2.5
1.5
600
8
6
4
2.5
1000
12
8
6
4
Overvoltage category for the equipment is decided by its location in the
installation. (Origin of installation,
distribution, load etc.).
Preferred value of impulse withstand
voltage reduces as the equipment is
located further away from the source
of energy.
To be suitable for application
for a given location/overvoltage
category, rated impulse withstand
voltage of the equipment should be
equal to or above the preferred
value for that location/overvoltage
category.
(III) NORMAL SERVICE
CONDITIONS :
Pollution degrees :
In order to define environmental
conditions more precisely and to
evaluate clearance and creepage
distances following four degrees of
pollution are established.
(b) Terminals :
Constructional requirements e.g.
strength, size etc. for terminals
suitable for unprepared copper
conductors are evaluated through
following tests.
(i) Tests for verification of mechanical strength
(ii) Flexion test (Test for damage to
conductors and their accidental
loosening)
(iii) Pull out test
(iv) Test for insertability of conductors
Requirements for terminals suitable
for aluminum conductors are under
consideration.
PREFERRED VALUES OF IMPULSE WITHSTAND
VOLTAGE in kV (1.2/50 S)
OVER VOLTAGE CATEGORY
V
of the material (hot wire ignition and
arc ignition tests).
(c) Clearance and creepage distances : Clearance:
By definition, clearance is the distance between two conductive parts
along a string stretched the shortest
way between these conductive parts.
The requirement of clearance in air is
determined by transient overvoltages
and micro environment.
condensation.
Pollution degree 4:
The pollution generates persistent
conductivity caused, for instance, by
conductive dust or rain or snow.
Unless otherwise specified equipment for industrial application is
generally suitable for use in pollution
degree 3 environment.
(IV) CONSTRUCTIONAL REQUIREMENTS :
Constructional requirements as
regards insulating materials, terminals, clearance and creepage
distances have now been made more
detailed.
(a) Materials :
The suitability of insulating materials
is verified with respect to its resistance to abnormal heat and fire
through elaborate tests. These tests
are either conducted on the equipment (glow wire test) or on specimen
Minimum clearance values have
been specified in new IS on the basis
of
- rated impulse withstand voltage and
TABLE II
MINIMUM CLEARANCE IN AIR
RATED IMPULSE
WITHSTAND
VOLTAGE
Uimp, kV (1.2/50 S)
0.33
0.5
0.8
1.5
2.5
4
6
8
12
MINIMUM CLEARANCE, in mm
POLLUTION DEGREE
1
0.01
0.04
0.1
0.5
1.5
3
5.5
8
14
2
0.2
0.2
0.2
0.5
1.5
3
5.5
8
14
3
0.8
0.8
0.8
0.8
1.5
3
5.5
8
14
4
1.6
1.6
1.6
1.6
1.6
3
5.5
8
14
- pollution degree
Stipulated minimum clearance values
for various rated impulse withstand
voltages are given in table II.
Clearance is verified through a type
test for verification of dielectric
properties.
TABLE V
SIZE OF CONDUCTORS CORRESPONDING TO RATED/TEST CURRENT
Creepage distance : Creepage
distance is defined as the shortest
distance along the surface of an
insulating material between two
conductive parts.
RANGE OF CURRENTS
Recommendations for creepage
distances are based on a considerable amount of empirical data.
To account for deterioration of
properties of insulating materials
minimum creepage distances specified consider following fundamental
factors
-
Rated insulation voltage or
working voltage
Environmental conditions or
pollution degree
Insulating materials used
Insulating materials can be roughly
characterized according to the
damage they suffer due to flow and
interruption of leakage current on
contaminated surfaces. Though no
direct relationship exists, it has been
found by tests and experience that
insulating materials with higher
comparative tracking index (CTI)
have higher relative performance.
Hence for the purpose of specifying
minimum creepage distance insulating materials have been classified
into four material groups as given in
table III.
A
-
8
12
15
20
25
32
50
65
85
100
115
130
150
175
200
225
250
275
300
350
400
500
630
800
1000
1250
1600
2000
2500
* 2 Cables of 150 mm2 OR 2 links of 30 mm x 5 mm cross section
The value of the current shall be greater than first value and less than
or equal to the second value in first column.
TABLE III
MATERIAL GROUPS
MATERIAL GROUP I
MATERIAL GROUP II
MATERIAL GROUP IIIa
MATERIAL GROUP IIIb
0
8
12
15
20
25
32
50
65
85
100
115
130
150
175
200
225
250
275
300
350
400
500
630
800
1000
1250
1600
2000
SIZE OF CONDUCTOR, mm2
COPPER
ALUMINIUM
(IEC TEXT)
(NATIONAL ANNEX)
1
1.5
1.5
1.5
2.5
2.5
2.5
4
4
6
6
10
10
16
16
25
25
35
35
50
35
50
50
70
50
70
70
95
95
150
95
150
120
185
150
240
185
240
185
240
240
300
2X150 or 2x30x5*
2x32x8
2x185 or 2x40x5
2x40x8
2x240 or 2x50x5
2x50x8
2x60x5
2x50x10
2x80x5
2x63x12
2x100x5
4x50x8
3x100x5
3x100x10
4x100x5
4x100x10
600 <= CTI
400 <= CTI < 600
175 <= CTI < 400
100 <= CTI < 175
Minimum creepage distances for low
voltage switchgear products are
given in table IV.
Creepage distance is verified by
actual measurement as a part of the
type test for verification of dielectric
properties.
TABLE IV
MINIMUM CREEPAGE DISTANCES
RATED
INSULATION
VOLTAGE, V
250
320
400
500
630
800
1000
CREEPAGE DISTANCE, in mm
POLLUTION DEGREE
2
Material group
I
II
IIIA
IIIB
1.25
1.8
2.5
2.5
1.6
2.2
3.2
3.2
2
2.8
4
4
2.5
3.6
5
5
3.2
4.5
6.3
6.3
4
5.6
8
8
5
7.1
10
10
POLLUTION DEGREE
3
Material group
I
II
IIIA
3.2
3.6
4
4
4.5
5
5
5.6
6.3
6.3
7.1
8
8
9
10
10
11
12.5
12.5
14
16
IIIB
4
5
6.3
8
10
-
POLLUTION DEGREE
4
Material group
I
I
IIIA
IIIB
5
6.3
8
6.3
8
10
8
10
12.5
10
12.5
16
12.5
16
20
16
20
25
20
25
32
-
(v) VERIFICATION OF PERFORMANCE REQUIREMENTS :
General test conditions e.g. mounting
arrangements, test circuits, permissible variations on test parameters etc.
for all nature of tests are given in this
part.
In national annex aluminum conductors to be used for various tests are
included. The specified cross sections are corresponding to copper
sections specified in IEC text. These
are compared in table v.
For further information, please contact Switchgear Contracts Division, Larsen & Toubro Limited, Saki-Vihar Road, Bombay 400 072.
Edited, printed and pubilshed by Cerena de Souza for Larsen & Toubro Limited. Published from L&T House, Ballard Estate, Bombay 400 001,
and printed at Uma Offset, A to Z Indl. Estate, G. Kadam Marg, Lower Parel, Bombay 400 013. Associate Editor: Luis S. R. Vas.