Page 462 - Industrial Power Engineering and Applications Handbook

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The values thus shown in Table 14.7 take cognisance of obtained on a regular basis. This has been possible through
the recommendations of IEC 60298 to maintain the leakage seismographs and accelerographs installed at the various
current at less than 0.5 mA for all HT systems. strategic locations throughout the world (see Section
14.6.2). Availability of new information on aspects on
The dielectric test may be limited to a power frequency the behaviour of the earth’s body is making it possible to
voltage withstand test. This test is considered neither update and improve the above test methods and facilities.
necessary nor advisable at site if already conducted at With such information, it is also hoped that our scientists
the manufacturer’s works unless there is a major may soon be able to predict an earthquake, its intensity
modification or repairs at site in the existing switchgear and location well in advance.
assembly. Repeated application of a high voltage may The likely ‘responses’ of buildings, structures and
degrade the properties of the insulation system used. If installations to such events are now available. These help
such a test becomes necessary at site, it may be carried us to study earthquakes more closely and their effects
out at a reduced test voltage of 85% for LT systems as in and enable us to take more authentic and appropriate
IEC 60439-1 and at 80% of the test values for HT systems preventive measures at the design stage. We offer an
as prescribed in note 2 of Table 32.1 (a) as in IEC 60298, introduction to this subject with a view to make the
for series I voltage systems or at 75% as prescribed in students and the engineers more conversant with and
note 2 of Table 14.2 for series I1 voltage systems, as in aware of these geological phenomena and to be more
ANSI C 37.20C. concerned about safety for life and property. A few
When the required test voltage is not available at site, examples may be houses, buildings, hospitals, industrial
the reduced voltage power frequency withstand test may plants, power generating and distributing systems, dams,
be carried out at still lower test voltages, depending upon bridges and handling of hazardous materials. These and
the voltage availability at site. Then the duration of the similar systems should be given constructional and design
test must be increased as shown in Table 14.8, and IS considerations to make them reasonably safe against such
10118-3 and BS 159. effects. In further discussions we consider only the
In this case also verification of insulation resistance secondary systems that are supported on the primary
or the measurement of leakage current will be carried system and consist mainly of the electrical and mechanical
out before and after the HV test. machines, devices and components. The primary systems,
which include houses, buildings and main structures,
Table 14.8 Duration for dielectric test at reduced test voltages (columns, beams, trusses, floors, walls), dams and bridges
etc., fall within the purview of civil and structural
Rated test voltage % Test duration in minutes engineering and are not discussed here.
Our present discussions relate only to the laboratory
100 testing of safety-related secondary systems, as are
83.5 employed in critical areas such as areas of emergency
15 power supply and reactor power control supply etc. of
70 a nuclear power plant (NPP) according to IEEE 344
66.6 and IEC 60980. There are other codes also but IEEE 344
60
51.7 is referred to more commonly. Basically, all such codes
are meant for an NPP but they can be applied to other
critical applications or installations that are prone to
earthquakes.
14.6 An introduction to earthquake It should be noted in subsequent discussion that the
engineering and testing design of structures and foundations for machines and
methods equipment play a vital role in absorbing or magnifying
seismic effects. A proper design consideration in such
areas at the initial stage can save the primary (and, in all
The consequences of an earthquake on life and property probability the secondary) systems from such effects to
have caused increasing concern among scientists, a large extent. Testing may be essential only for the
engineers and educational institutions. All are becoming critical equipment, such as being used in the critical
more conscious about preventive measures for buildings areas of an NPP or similar, more critical installations.
and critical installations against possible earthquakes to
mitigate, if not eliminate, their devastating effects. It is 14.6.1 Seismic disturbances
becoming common practice to conduct seismic studies
on a region where a large and/or critical project is to be Random vibrations, such as those caused by an earthquake,
located before commencing work and locating the more cause shocks and ground movements and are termed
important buildings in seismically safer areas. Buildings, seismic disturbances. Shocks and turbulence caused by
other structures and important machines are designed to a heavy sea, landslides and volcanic eruptions are also
withstand earthquakes. Analytical methods and laboratory examples of shocks that may cause vibrations and result
test facilities have also been developed to demonstrate in tremors, not necessarily earthquakes. Nevertheless,
the suitability of structures to withstand scismic cvents. they may requir-e design considerations similar to those
Additional information on newer aspects on the for an earthquake, depending upon the application (e.g.
behaviour of the earth during an earthquake has been naval applications, hydro projects, dams and bridges),