Page 203 - Industrial Power Engineering and Applications Handbook

Special-purpose motors 7/183
lntrinsically safe category ‘ib’ for installations in Zones for the use of this equipment at such locations. This is
1 and 7,. The apparatus for this category must be of mandatory for the installation and operation of any
groups UA. nB. and nC (IEC 60079-0). electrical equipment in hazardous areas. These authorities
may also specify construction requirements for equipment
The main stipulation for such systems is that they will and issue regulations for its installation and operation.
not emit sparks in normal or fault conditions. This therefore In India, the Central Mining Research Institute, Dhanbad
restricts the use of only low-energy auxiliary and control carries out this testing and provides the necessary
circuits connecting instruments and devices at these certification for motors used in explosive atmospheres.
locations. Such instruments and devices may record, sense But for approval of the equipment, whether it is worthy
or monitor certain operating parameters of machines or of use in a particular hazardous area, there are accredited
apparatus installed at these locations or the surrounding agencies. Some of these are Directorate General Mines
atmosphere itself, through temperature detectors (RTDs). Safety, Dhanbad, Chief Controller of Explosives. Nagpur
instruments to measure humidity and pressure, and and Directorate General of Factory Advice Service and
vibration detectors. Labour Institute. Bombay.
To comply with the requirement of intrinsic safety, all
electrical circuits installed at such locations should be
safe and must produce no spark or heat under normal or 7.1 8 Additional requirements for
fault conditions, sufficient to cause ignition of the critical installations
wrrounding medium. The parameters of the circuit such
as b” I, R. L. and C, which can release heat energies by A process industry, a refinery, a petrochemical, a fertilizer
I’R. I’L and CV‘ have a recognisable bearing on the plant or a power station are installations that can be
ignitability of an explosive atmosphere. For an inductive classified as critical. They cannot afford a breakdown
circuit, for instance, with a magnetic or solenoid coil, V during normal operation and would prefer to incorporate
and L would be the most potential parameters. During a more safety features into the drive motors, even if these
fault condition the circuit may release high stored energy are expensive. These features can be one or more of the
by heating the wires or any other part of the device to following.
which it is connected. This energy may be sufficient to
cause ipnition of the surrounding atmosphere. 7.18.1 Powerhouse treatment of insulation
For the safety of these circuits, therefore. the main
consideration is defined by the level of this energy, which Special insulation and coating of stator windings and
should always be less than that required to ignite explosive overhangs are sometimes essential to ensure protection
gases. chemical vapour or volatile liquids in the against tropical weather, fungus growth, moisture, oil
wrrounding atmosphere. Accordingly, minimum safe abrasives, and acid and alkali fumes. Powerhouse
ignition currents (MICs) have been established by labo- treatment is one insulating process that can meet all these
ratory tests for different control voltages and R, L, and C requirements (see Section 9.3).
of the circuit. and are the basis of testing the circuits of
these devices and instruments at such locations to 7.18.2 Terminal boxes
determine their compliance for safe installation. Since
such circuits would be required for other zones and groups It is recommended that the motor should have separate
of gases also with different limiting temperatures, IEC terminal boxes for the main supply and for the accessories
60079- I 1 would also apply to all such zones and groups such as space heaters, embedded temperature detectors,
of gases. The test requirements. however, would vary bearing temperature indicators and moisture detector
for different locations, as stipulated. terminals, etc. In LT motors, however, if it is not possible
To contain the temperature of the electrical circuits to provide a separate terminal box for these accessories,
within safe limits for a particular temperature class of the main terminal box may be adequately spaced to
. the maximum current rating for a segregate these terminals from the main terminals within
a conductor is also stipulated in IEC the box.
60079- I I. The constructional requirements also stipulate In HT motors, however, these teiminal boxes are always
the rnininium clearances and creepage distances in air separate because two or more voltages (main and
between the conducting parts of all the intrinsically safe auxiliary). For main terminals there are normally two
electrical circuits. terminal boxes - one on one side of the stator to house
the main three-phase stator terminals and the second on
the other side to form the star point. These boxes are
7.1 7 Testing and certifying generally interchangeable to facilitate cable routing.
author it ies
7.1 8.3 Phase segregation
Installations categorized as hazardous locations, requiring It may be necessary to separate the live phases within
these types of motors, would be regarded as critical appli- the terminal box to eliminate any occurrence of a tlashover
cations. where stringent safety measures would be or a short-circuit and a subsequent explosion. Such explo-
mandatory. Normally, the government of a country would sions may be very severe, depending upon the short-circuit
authorize some agencies to independently grant approval level of the system and the surrounding atmosphere and