इंटरनेट मानक

Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to
information for citizens to secure access to information under the control of public authorities,
in order to promote transparency and accountability in the working of every public authority,
and whereas the attached publication of the Bureau of Indian Standards is of particular interest
to the public, particularly disadvantaged communities and those engaged in the pursuit of
education and knowledge, the attached public safety standard is made available to promote the
timely dissemination of this information in an accurate manner to the public.

“जान1 का अ+धकार, जी1 का अ+धकार” “प0रा1 को छोड न' 5 तरफ”

Mazdoor Kisan Shakti Sangathan Jawaharlal Nehru
“The Right to Information, The Right to Live” “Step Out From the Old to the New”

IS 7421 (1988): Porcelain bushings for alternating voltage

up to and including 1000 V [ETD 6: Electrical Insulators
and Accessories]

“!ान $ एक न' भारत का +नम-ण”

Satyanarayan Gangaram Pitroda
“Invent a New India Using Knowledge”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता ह”

है”
ह
Bhartṛhari—Nītiśatakam
“Knowledge is such a treasure which cannot be stolen”
 IS:7421 -1988
((Reaffirmed
Reaffirmed 1994 )
2004 )
Indian Standard
SPECIFICATIONFOR
PORCELAIN BUSHINGSFORALTERNATING
vOLTAGESUPTOANDINCLUDING10OOV
( First Revision )
First Reprint DECEMBER 1997

UDC 621’315’626 : 621’315’612’2

@ Copyright 1989

BUR’EAU OF INDIAN STANDARDS

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002

Gr 4 August 1989
 IS : 7421L 1988

Indian Standard
SPECIFICATIONFOR
PORCELAINBUSHINGSFORALTERNATING
VOLTAGES UP TO AND INCLUDING 1OOOV
( First Revision )
0. FOREWORD

0.1 This Indian Standard ( First Revision ) was 0.4 The dimensions of pbrcelain parts and
adopted by the Bureau of Indian Standards on metal parts for porcelain transformer bushings
26 October 1988, after the draft finalized by the rated up to 1 000 V for use in lightly polluted
Electrical Insulators and Accessories Sectional atmospheres are covered in IS : 3347 ( Part l/
Committee had been approved by the Electro- Set 1 )-1979” and IS : 3347 (Part l/Set 2)-1979-t,
technical Division Council. respectively.
0.2 This standard has been revised in the light 0.5 The dimensions of porcelain bushings
of experience gained in the manufacture and use ( tubes ) for terminals of rotating electrical
of porcelain bushings in the country. The machines, electrical apparatus and capacitors
following changes have been incorporated in this are covered in IS : 10314-1982$.
revision: 0.6 For the purpose of deciding whether a
particular requirement of this standard is comp-
a) The bushing of 800 A current rating has
lied with, the final value, observed or calculated,
been added, and
expressing the result of a test, shall bz rounded
b) The electrical characteristics of the bush- off in accordance wtth IS : 2-1960s. The
ings, the requirements of temperature-rise number of significant places retained in the
and rated short time current have been rounded off value should be the same as that of
defined clearly. the specified value in this standard.

0.3 This standard covers the performance *Dimensions for porcelain transformer bushings for
requirements of the porcelain bushings for ac use in normal and lightly polluted atmospheres :Part 1
voltages u~p to and including 1000 V but the UD to and including 1 kV, Section 1 Porcelain parts
performance requirements of the porcelain (first revision ).
( ceramic ) bushings for ac voltages above i-Dimensions for porcelain transformer bushings for
use in normal and lightly polluted atmospheres : Part 1
1 000 V are covered in iS : 2099-1986*. Up to and including 1 kV, Section 2 Metal parts (first
revision ).
*Specification for bushings for alternating voltages t;Dimensions for ceramic bushings for terminals.
above 1 000 volts ( second revision ). gRules for rounding off numerical values ( revised).

1. SCOPE bushings, if not covered by relevant Indian

Standard, shall 6e subjected to special agreement
1.1 This standard covers the requirements, between the purchaser and the supplier.
characteristics and methods of tests for porcelain
bushings for indoor and outdoor use, suitable 2. TERMINOLOGY
for electrical equipment of nominal system 2.0 For the purpose of ~this standard, the follow-
voltages up to and including 1 000 V and fre- ing definitions, in addition to those covered in
quency between 15 and 60 Hz. IS : 2099-1986* and IS : I885 ( Part 54 )-19807
1.2 This standard also applies to bushings shall apply
supplied separately for use in the construction 2.1 Bushings - A structure carrying one or
of electrical apparatus, transformers, electrical several conductors through a partition, such as,
installations and to bushings supplied as an a wall or tank, etc, and insulating it or them
integral part of equipment when required by the therefrom, incorporating the means of attach-
relevant equipment specifications. ment ( flange or othe.r fixing device ) to the
partition.
1.3 This standard does not apply to bushings
for use with rectifiers, rotating machines, testing *Specification for bushings for alternating voltages
transformers, dc converter transformers or to above 1 000 volts ( second revision ).
terminal?, for power cables ( pot heads ). Such tElectrotechnica1 vocabulary : Part 54 Insulators.
IS : 7421- 1988

The conductor may form an integral part of as aluminizing, may be used subject to agreement
between the purchaser and the supplier. The details
the bushing or be drawn through. on aluminizing are given in IS : 6697-1972*.
2.2 Fixing Flange - The metal portion of a 5. RATED VALUES
bushing normally at earth potential, by which
the bushing is fixed to its associated apparatus. 5.1 Rated Voltage - The standard value of
rated voltage of bushings may be any value
2.3 Porcelain - Any ceramic material used in between alternating voltages up to and including
the construction of a bushing. 1000 v.
2.4 Immersion Level - The immersion level is
the minimum level to which the bushing shall be 5.2 Rated Current - The standard values of
immersed in oil or other medium in order to rated current of the bushing shall be:
obtain electrical characteristics specified in the 250, 400, 630, 800, 1000, 2 000 and 3 150 A.
standard. The immersion level is specified by NOTE - The rated current of the bushing need not
the manufacturer. necessarily be identical with the rated current of
the complete apparatus. It may be higher or lower,
3. STAlVDARD ATMOSPHERIC depending upon how t& actual service conditions
CONDITIONS deviate from the standard test conditions for the
bushing. The general rule for the permissible service
3.1 The reference atmospheric conditions at current should be such that it shall not give rise to
which bushing characteristics shall be expressed higher actual conductor temperatures than the tem-
for the purpose of comparison shall be as given perature-rise permitted in 7, added to the temperature
of the relevant cooling medium.
in 3.5.2 to 3.5.2.2 of IS : 2071 (Part 1 )-1974*.
NOTE l- A pressure of 1 013 m bar is equivalent to 5.3 Rated Thermal Short Time Current ( Ith )
a pressure of 760 mm Hg at 0°C. If the height of 5.3.1 Unless otherwise stated, the rated
mercury in the barometer is h mm and the tempera-
ture of the mercury is r”C. the atmospheric pressure duration ( tth ) shall be 1 second except for
in millibars is: transformer bushings. The transformer bushings
shall be rated for 2 seconds.
P=!g(l - 1.80 x lo-4t )
5.3.2 Minimum Values - IJnless otherwise
NOTE 2 - The standard reference temperiture of stated, the minimum value of rated short time
27°C and corresponding humidity have not been
specified because of the non-availability of test current of a bushing is 25 times the rated current
voltage values and correction factors for these condi- for the rated duration.
tions. These conditions will replace those specified NOTE - Experience has shown that if a bushing is
above when the corresponding test voltage values and -correctly designed for its rated current, it is not
correction factors are available. generally necessary to prove, by test, its thermal and
NOTE 3 - Except by agreement between the manu- mechanical performance under a short time current
facturer and purchaser, tests shall not be made if loading of 25 times the rated current for one second.
the relative humidity exceeds 85 percent. If, exceptionally, a higher short time current is
required, the need for a test should be decided by
No-4 - Corrections of test voltage for atmos: agreement between the purchaser and the supplier.
pheric conditions differing from 3.1 shall be applied
in accordance with Appendix A. 5.4 Rated Frequency - The value of rated
frequency of the bushings may be any value
4. GENERAL REQUIREMENTS between 15 and 60 Hz.
4.1 The porcelain shall be sound, free from 6. CREEPAGE DISTANCE
defects, thoroughly vitrified and smoothly glazed
6.1 The minimum specific creepage distance ( in
and shall comply with the appropriate class
millimetres per units of UN ) for bushings, one
given in IS : 5621-19807.
or both ends of which are intended to be, out-
4.2 The design of the bushing shall be such that doors shall be as given below:
the stress due to expansion or contraction in a) Lightly polluted atmospheres 16 mm/kV
any part of the bushing shall not lead to deterio-
b) Medium polluted atmospheres 20 mm/kV
ration.
c) Heavily polluted atmospheres 25 mm/kV
4.3 The glaze, unless otherwise specified, shall d) Very heavily polluted 31 mm/kV
be brown in colour. The glaze shall cover all the atmospheres
exposed porcelain parts of the bushing except Nore - The definition of various polluted atmos-
those areas which serve as supports during firing pheres are under consideration.
or required to be left unglazed.
7. TEMPERATURE--RISE
4.4 All the ferrous metal parts except the threads
of the nuts shall be hot-dip galvanized. The 7.1 The bushing shall be so designed as to
threads of the nuts shall be cut after galvaniza- ensure that in continuous operation at rated
current and frequency, the temperature-rise does
tion.
not exceed the value given in 8 of IS : 2099-
NOTE - In view of the scarcity of zinc which is
used for galvanizing other types of protection, such 1986-f.

*Specification for hot dip aluminium coating on

*Methods of high voltage testing : Part 1 General ferrous pa$s ( other than sheet, strip and wire) for
definitions and test requirements (first revision ). general pur‘poges.
fspecification for hollow insulators for use in abJvyyificatio n for b Ush ings for alternating volt&ges
electrical equipment (first revision ). ‘bs-y olts ( second revision ).

2
 IS:7421-1988

7.2 Alternatively, where the bushing is supplied Rated thermal short time current with-
as an integral part of the switchgear or similar stand test ( 11.9 ),
apparatus, the thermal performance shall be Pressure test on liquid filled and liquid
such that it shall meet the thermal requirements insulated bushings ( 11.10 ), and
for the apparatus as demonstrated by the tem-
perature-rise test on complete apparatus with Tightness test at the flange or other fixing
the bushing fitted when such a test is carried out. device ( 11.11).
NOTE 1 - Type tests are normally made once and
7.3 Temperature-rise measurement shall be unless otherwise agreed to, the test certificate giving
carried out in accordance with 11.8. the results of type tests made on not less than two
bushings identical in all essential details with those
8. ELECTRICAL CHARACTERISTICS to be supplied are regarded as evidence of com-
pliance. -Both the bushings shall be subjected to
8.1 Dry Power-Frequency Withstand Voltage tests ( a ) to ( f ), after which one shall be subjected
( for Indoor Bushings) - The bushings designed to test ( g ) and the other to test (h ).
for indoor application shall withstand, without NOTE 2 - The type tests shall be made and certi-
failure, a test voltage of 5 kV when tested in fied by the manufacturer or by an agreed independent
accordance with 11.4. authority.
8.2 Wet Power-Frequency Withstand Voltage 11.1.2 Acceptance Tests - The test Samples
( for Outdoor Bushings ) - The bushings after‘having withstood the routine tests shall be
designed for outdoor application shall withst subjected to the following acceptance tests in the
and, without failure, a test voltage of 5 kV order indicated below:
when tested in accordance with 11.5. a) Verification of dimensions ( 11.3 ),
9. MARKING b) Dry power-frequency voltage withstand
9.1 Each bushing shall be legibly
and indelibly test ( 11.4 ),
marked to show the following: c) Temperature cycle test ( 11.6 ),
a) Name or trade-mark of the manufacturer, d) Porosity test ( 11.7 ),
b) Year of_manufacture, and e) Pressure test on liquid filled and liquid
c) Country of manufacture. insulated bushings ( ll.lO), and
9.1.1 Marking on porcelain shall be printed f) Tightness test at the flange or other fixing
and shall be applied before firing. device ( 11.11 ).
9.1.2 The porcelain bushings may also be 11.1.2.1 For carrying out acceptance tests
marked with the Standard Mark. specified in this standard, the sampling proce-
NOTE - The use of the Standard Mark is governed dure and the criterion of conformity to the
by the provisions of the Bureau of Indian Standards requirements of acceptance tests shall be as given
Act, 1986 and the Rules and Regulations made there- in Appendix B.
under. The Standard Mark on products covered by
an Indian Standard conveys the assurance that they 11.1.3 Routine Tests - The following tests
have been produced to comply with the requirements shall constitute routine tests:
of that Standard under a well defined system of
inspection, testing and quality control which is a) Visual examination ( 11.2 ),
devised and supervised by BIS and operated by the
producer. Standard marked products are also b) Voltage test ( 11.4 and 11.5 ),
continuously checked by BIS for conformity to that c) Pressure test on liquid filled and liquid
standard as a further safeguard. Details of conditions
-under which a licence for the use of the Standard insulated bushings ( 11.10 >, and
Mark may be granted to manufacturers or producers
may he obtained from the Bureau of Indian
d) Tightness t&t at the flange or other
Standards fixing device ( 11.11).
11.2 Visual Examination - A visual examina-
10. PACKING
tion shall be made on each bushing. The
10.1 The bushings may be packed in baskets or mounting of metallic parts on the bushing shall
crates or boxes suitable for rough handling. be in accordance with agreed drawings. The
11. TESTS
colour of the porcelain bushing shall approxi-
mate the colour specified in the drawing. Some
11 .l General variation in colour shade is permitted and shall
not justify rejection of the bushing.
11.1.1 Type Tests -The following tests shall
constitute the type tests: 11.2.1 The areas specified as glazed on the
a>
Visual examination ( 11.2 ), drawing shall be covered by a smooth hard
glaze, free from cracks and other defects pre-
b) Verification of dimensions ( 11.3 ), judicial to satisfactory performance in service.
c>
Dry power-frequency voltage withstand 11.2.2 On those areas sp’ecified as glazed on
test ( for indoor bushings ) ( 11.4 ), the drawing, the total area, not covered by the
d) Wet power-frequency voltage withstand glaze shall not -exceed:
test ( for outdoor bushings ) ( 11.5 ), DF
1+mmm2
4 Temperature-rise test ( 11.8 ),
IS : 7421 - 1988

Also the area of~any single glaze defect shall not occur after the application of test voltage
not exceed: for one minute.
DF mm2 11.6 Temperature Cycle Test
o’5 + 20 OiJO
11.6.1 The porcelain parts of the bushings
where shall be quickly and completely immersed in a
D = greatest diameter of the bushing in mm, water bath maintained at a temperature of 70°C
and above-that of the cold water and left submerged
F = creepage distance of the bushing in mm. for a period of 15 minutes. It shall then be
withdrawn and quickly immersed completely
11.3 Verification of Dimensions - Unless other- without being placed in an intermediate con-
wise specified, the dimensions of the porcelain tainer, in a bath of cold water for the same
parts shall conform to those of relevant drawing period of 15 minutes.
of the bushing subject to a tolerance of
11.6.2 This heating and cooling cycle shall be
rt ( 0’03 d + 0‘3 ) mm, d being the dimension in
millimetres. performed three times in+succession.
11.6.3 After the completion of the immersion,
11.4 Dry Power-Frequency Voltage Withstand the bushings shall be examined to verify that
Test (Applicable to Indoor Bushings) the bushing parts have not cracked and that
the glaze is undamaged.
11.4.1 The bushing shall be tested dry under
the conditions prescribed in Appendix C. 11.7 Porosity Test
11.4.2 The bushings shall be subjected to test 11.7.1 Porcelain fragments from the bushings
voltage given in 8.1 ( subject to adjustment for shall be immersed in orie percent alcoholic
atmospheric conditions at the time of test given solution of fuchsin ( 1 g fuchsin in 100 g of
in Appendix A ) for a period of one minute. methylated spirit ) under a pressure of not less
than 15 x 10”iU/m2 for a period such that the
11.4.3 A test voltage of about 7.5 percent of product of test duration in hours and test
the specified voltage as determined in 11.4.2 pressure in N/m” is not less than 180 X 10s.
shall be applied and then increased gradually to
reach the specified test voltage in a time not less 11.7.2 The fragments shall then be removed
than five seconds. Flashover or puncture shall from the solution, washed,~dried and broken.
not occur [ see IS : 2071 ( Part 1 )-1974* ] -after 11.7.3 Examination with naked eye of the
the application of the voltage for one minute. freshly broken surface shall not reveal any dye
penetration. Penetration into small cracks
11.5 Wet Power-Frequency Voltage Withstand rormed during the initial breaking shall be
Test ( for Outdoor Bushings ) neglected.
11.5.1 The bushing shall be arranged as pres- 11.8 Temperature-Rise Test
cribed in Appendix C. Before the commence-
11.8.1 Temperature-rise test is not required
ment of the test, the bushings shall be exposed
to artificial rain produced in accordance with for bushings used with transformers whose
metal parts are manufactured in accordance
IS : 2071 ( Part 2 2-1976-r and having the follow-
with IS : 3347 ( Part ~/SW 2 j-1982*. For other
ing characteristics:
type of bushings whether the test is applicable
a) Rainfit intensity 3 mm/min or not, may be decided by mutual agreement
f 10 percent between the purchaser and the supplier.
b) Tolerance on intensity
10 000 ohm cm 11.8.2 Test Procedure and Measurement of
c) Water resistivity
Temperature -- The test procedure for the
d) Tolerance on resistivity i 10 percent measurement of temperature-rise of bushings
The water temperature shall not differ by shall be in accordance with 11.6.3 of IS : 2099-
more than 15°C from that of the bushing. 1986t.
11.9 Thermal Short Time Current Withstand
11.5.2 The artificial rain shall impinge on tl:c Test - IF rcquircd by the purchaser, the test
bushing for at least five minutes immediately shall be carried out in accordance with the
before the test voltage is applied ( .ree procedure given in Il.8.of IS : 2@99-1986t.
Appendix C ) and shall be maintained through-
out the test. The test voltage shall be asspecified 11.10 Pressure Test on Liquid Filled aud Liquid
in 8.2 after corrections for atmospheric Insulated Bushings ‘- The pressure test for
conditions ( see Appendix A >. The tesst shall leakage of internal filling is applicable only to
bc made in accordance with IS : 2071 ( Part 1 >- liquid filled and liquid insu’lated ( see 2 of
1974” and duration of application of test voltage 1s : 2099-1986t for definitions ) bushings except
shall be one minute. Flashover or puncture shall
*Dimensions for porcelain transformer bushings for
use in normal and lightly polluted atmospheres : Part 3
*Methods of high voltage testing : Part 1 General ~12 and 17.5 kV bushings, Section 2 Metal parts (first
definitions and test requirements (fir.rt revision ). revhion ).
tMethods of high voltage testing : Part 2 Tesl -ISpecification for bushings for alternating voltages
procedures ( first revision ). above 1 000 volts ( second revision ).

4
 is,:-7421 - 1988

those bushings of which the liquid filling has a at a pressure of 1’0 f 0’1 bar above
viscosity equal to or above 5 X lo-* m2/s at maximum operating pressure. The bushings shall
20°C. Any one of the two alternative test Qroce- be in air at an ambient temperature of not less
Jures given in 11.10.1 and f1.10.2 may be used than 10°C. The duration of the test shall be
at the choice of the supplier. 15 minutes.
NOTE -- If the design of the bushing is such that The bushing shall be considered effectively
the tests as specified below are impracticable, the test sealed if there is no evidence of leakage of gas.
procedure shall be subject to agreement between
the purchaser and the manufacturer. 11.11 Tightness Test at the Flange and Other
Fixing Device
11.10.1 Test with Oil or Insulating Liquid at
Ambient Temperature - The bushing complete 11.11.1 The test shall be .aQplicable to all
as for normal service shall be filled to the bushings of indoor-immersed, outdoor-immersed
normal level with oil or insulating liquid of the and completely immersed types, intended to
quality specified by the manufacturer. The -be used as an integral part of an apparatus,
bushings shall be placed in air at ambient tem- such as switchgear or transformer, where the
perature of not less than 10°C. The temperature bushings contribute to the sealing of the com-
of the oil or insulating liquid shall be the plete apparatus.
ambient temperature.
11.11.2 The bushing shall be assembled as far
The following test pressure shall be produced as necessary for the test. The end for immersion
inside the bushings and maintained for at least shall be mounted on a tank as for normal
12 hours: operation. The tank shall be filled with air and
any suitable gas at a gauge pressure of 1’5 f 0’1
a) For bushings opera- To be tested at a bar and maintained for 15 minutes.
ting at a pressure pressure of 1’0 f
above the attios- 0’1 bar above the 11.11.3 The bushing shall be considered to
pheric pressure maximum operat- have passed the test if there is no evidence of
between 0’6 and 2 ing pressure leakage.
bar NOTE - The test shall be a type test only in the
case of bushings with gaskets of which the final
b) For bushings opera- To be tested at a -placing is not carried out by the supplier, for example,
ting at a pressure pressure of 1’0 f. the top cap gasket of draw-through conductor trans-
above the atmos- 0’1 bar above the former bushings.
pheric pressure ~atmospheric The test may be omitted for transformer and
between 0 and 0’6 pressure circuit breaker bushings fitted with a one-piece
bar metal flange, provided the flange has been
The bushing shall be considered to be effec- subjected to a preliminary tightness test and the
tively sealed if there is no evidence of leakage bushing has been tested according to IS : 2099-
of oil or insulating liquid. 1986* or the end to be immersed does not
include any gaskets.
11.10.2 Test with Compressed Gas - The
bushing, complete as for normal service, shall *Specification for bushings for alternating voltages
be filled with air or any suitable gas maintained above 1 000 volts ( second revision ).

APPENDIX A

[ C~~uLw 3.1 ( Note 4 ), 11.4.2, 11.5.2 and C-l.5 ]

CORRECTION OF TEST VOLTAGES FOR ATMOSPHERIC CONDITIONS

A-l. GENERAL should be made to certain of the test voltages

A-l.1 Variations in barometric pressure and -air by the application of the following correction
humidity of the atmosphere cause variation in factors in accordance with Table 1:
electric strength of the air and hence also in the a) Correction Factor for Air Density d:
flashover voltage of insulators exposed to the
air, under oil, flashover and puncture strength, d = s ( d lies between 0’95 and 1’05 )
however, are not significantly affected by these
changes. where

A-2. CORRECTION FACTORS p = atmospheric pressure in millibars, and

t = temperature in “C.
A-2.1 When the atmospheric conditions in the
neighbourhood of the insulator during the test For a wider range of density and for higher
differ from the reference conditions, adjustments accuracy, instead of d, the factor k should be

5
 ..-_

IS : 74il- 1988

used. The values of k corresponding to the obtained from Fig. 1 for any deviation of
factor d are given below: ambient atmospheric pressure from the
d k standard value of 1 013 millibars. This
0’70 0’72 correction should be obtained from Fig. 2.
0’75 0’77 For corrected value of absolute humidity thus
0’80 0’82 obtained, the correction factor h shall be deter-
0’85 0’86 mined from Fig. 3.
0’90 0’91
TABLE 1 CORRECTION OF VOLTAGES FOR
0’95 0’95
ATMOSPHERIC CONDITIONS
1’00 1’00 ( Clause A-2.1 )
1’05 1’05 TEST ADJUSTMENTREQUIRED
1’10 1’09
(1) (2)
1’15 1’13 a) Dry one-minute power Voltage applied shall be
b) Correction Factor for Humidity ( h ) - frequency withstand appropriate value speci-
test fied in 8.1 multiplied by
Figure 1 gives the absolute humidity value
k and divided by h
for wet and dry-bulb temperature ( when
b) Wet one-minute power Voltage applied shall be
the velocity of air over the wet bulb frequency withstand the appropriate value
exceeds 3 m/s ) for the standard atmos- te5t specified in 8.2 multi-
pheric pressure of 1 013 millibars. For plied by k
better accuracy, a correction should be c) Power-frequency punc- No adjustment required
applied to absolute humidity value ture withstand test

DRY BULB THERMOMETER READING [‘Cl

FIG. 1 DETERMINATION OF ABSOLUTE HUMIDITY

6
 IS : 7421- 1988

1.06

0 2 4 6 6 10 12 14 16 18 20 22 24 26 28 50

ABSOLUTE HUMlDlTY(g/m3)

FIG. 3 HUMIDITY CORRECTIONFACTOR

Locate the point corresponding to the devia-

tion of the ambient atmospheric (pressure from
1 013 millibars on the left hand side of Fig. 2B
and join it with right hand side top ‘corner by
a straight line. Then locate the point on the
curve in Fig. 2A corresponding to the observed
value of the difference of dry and wet bulb
temperature. Draw a vertical line through this
point to intersect the straight line drawn in
Fig. 2B. Read the correction to be applied to
humidity from the right hand side of Fig. 2B
corresponding to the point of intersection. This
correction is positive for a positive deviation
and negative for negative deviation from the
standard atmospheric pressure.

FIG. 2 CORRECTIONTO ABSOL~JTE

HUMIDITY FOR VARIATION
IN PRESSURE

APPENDIX B

( Ckwe 11.1.2.1 )
SAMPLING PROCEDURE FOR ACCEPTANCE TESTS

B-l. LOT bushings as given in Table 2 shall be selected at

B-l.1 In a consignment, all the bushings of random and subjected to acceptance tests
same rating manufactured from the same material specified in 11.1.2.
under similar conditions of production shall be
grouped together to constitute a lot. B-2.2 The number of bushings to be selected
from the lot shall depend u~pon the size of the
B-3.. SELECTION OF SAMPLE lot and shall be in accordance with co1 1 and 2
B-2.1 From each lot, a certain number of of Table 2.

7
IS : 7421- 1988

B-3. CRITERIA FOR CONFORMITY

TABLE 2 SAMPLE SIZE AND CRITERIA FOR
CONFORMITY B-3.1 The bushings as selected in B-2.2 shall be
subjected to the acceptance tests. A bushing
( Clauses B-2.1, B-2.2, and B-3.1 ) failing to meet any one or more of the require-
LOT SIZE FIRST SECOND ACCEPTANCE FIRST SECOND ments of accep_tance tests shall be termed as
SAMPLE SAMPLE NUMBER REJEC- REJEC- defective. The lot shall be considered as con-
SIZE SIZE TION TlON forming to the requirements of the acceptance
NUMBER NUMBER
tests if the nufiber of defectives found in the
(f") ( n1 1 ( n2 1 ( al ) ( r1 > ( r2 I first sample is less than or equal to the accep-
(1) (2) (3) (4) (5) (6) tance number given in co1 4 of Table 2. The lot
up to 300 5 5 0 2 2 shall be rejected if the number of defectives is
301 to 500 8 8 0 2 2 greater than or equal to the first rejection
50.1 to 1 000 13 13 0 2 2 number given in co1 5 of Table 2. If the number
1001 and 20 20 0 3 4 of defectives is between the acceptance number
above and the first rejection 1 number, the second
sample of the same size ( 11~) shall be selected
B-2.2.1 The bushings in the sample shall be from the lot at random and subjected to the
selected at random from ~the lot. For the acceptance tests. The lot shall b-e declared as
purpose of random selection, reference may be conforming to the requirements of the accep-
made to IS : 4905-1968*. tance tests if the number of defectives in the
two samples combined is less than the second
*Methods for random sampling. rejection number, otherwise not.

APPEN DIX C
( Clauses 11.4.1, 11.5.1 and 11.5.2 )
HIGH VOLTAGE TESTS

C-l. ARRANGEMENT OF BUSHING unless otherwise stated, the earthed fixing

flange.
C-l.1 The bushing shall be clean and dry.
C-1.5 Reference atmospheric conditions are
C-l.2 -High voltage tests shall be carried out given in 3 and correction factors for high voltage
only on bushings eomplete with their fixing tests are given in Appendix A.
flanges or other fixing devices and all other C-2. POWER-FREQUENCY TESTS
accessories with which they are fitted when in
C-2.1 Power-frequency’tests shall be made in
use but with protective gap, if any, removed.
accordance with IS : 2071 ( Part 1 )-1974*.
C-l.3 For bushings intended for immersion, the C-2.1.1 The transformer test circuit shall be
immersed end of a bushing designed to function such that when the test object i’s short-circuited
in an insulating medium, shall be immersed to at the test voltage, the current is not less than
the immersion level of the bushing except that 0’1.4 if~dry tests are to be made, and not less
where there is earthed metal extending to this than 0’5A if the wet tests are to be made.
level, the depth of immersion may be increased
up to the fixing flange. Bushings shall be tested C-3. EXCESSIVE HUMIDITY
in the medium in which they are intended to C-3.1 Precautions shall be taken to avoid con-
function in service or, by agreement, in oil densation on the surface of the bushing, specially
complying with IS : 335-1983”. The insulating when the relative humidity is high. For example,
medium shall be at room temperature. Bushings the bushing shall be maintained at the ambient
need not be immersed in oil when their dimen- temperature of the test location for sufficient
sions permit the tests to be made in air. time for equilibrium to be reached before the
test commences. Except by agreement between
C-l.4 High voltage tests shall be made by the manufacturer and the purchaser, tests shall
applying a voltage between the conductor not be made if the relative humidity exceeds
(either integral with the bushing or a substitute 85 percent.
lead inserted for the purpose of the test ) and
*Methods of high voltage testing : Part 1 General
*Specification for new insulating oils ( third revisiofl ). definitions and test requirements (first revision ).

8
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