Page 202 - Industrial Power Engineering and Applications Handbook

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Perfectly sealed joint for 7.14 Motors for Zone 2 locations
protection against dust,
moisture corrosion
and water Locations falling within this category can employ motors
Special winding that are more economical than the other types discussed
for 10°C lower above. IEC 60079-14 has defined the basic requirements
for such enclosures which are obviously less stringent
than the others. In addition to maintaining specified
creepages and clearances between the rotating and the
stationary parts, the following are the two main
requirements specified for such enclosures:

1 They should produce no arcing during a start-up or
run.
2 Surface temperature should not exceed the ignition
temperature noted in Table 7.6 for a particular
temperature class under any conditions of operation.
There is no limit to the temperature rise to the
and rotor permissible limits for a particular class of insulation
Anti-loosening, anti-rotating type of windings or other parts of the machine, except the
terminal arrangement with specified limiting surface temperature as in Table 7.6. For such
clearance and creepage distances
an application, a normal 1P:SS enclosure may also be
employed.
7.14.1 Non-sparking motors, type Ex. ‘n’
A subsequent study of construction features of motors
for Zone 2 locations, resulted in the development of non-
sparking, type ‘n’ motors. The basic design consideration
for such motors is similar to that of type ‘e’ motors but
Figure 7.16 General requirements for an increased safety now there is no restriction in the limiting temperature,
motor by 10”C, as in type ‘e’ motors. Frame sizes for these
motors are generally the same as for general-purpose
motors. Thus they tend to be smaller and less expensive
than type ‘e’ motors for the same output.

to about 75 mm water-gauge above that of the atmosphere 7.15 Motors for mines, collieries and
surrounding the enclosure. The enclosure is sealed and quarries
the leakage rate controlled to about 250 litredmin. Some
of the basic safety features and devices are noted below.
Such enclosures are recommended only when there are Special provisions are laid down in IEC 60079-0 and
a number of these machines installed at the same location IEC 60079-1 for motors required for such locations in
so that a common pressurizing and piping system may view of fluctuating degrees of humidity and temperature.
be employed to reduce costs on piping network and Such locations are defined with a surface temperature
pressurizing equipment and its accessories, in addition limit of 150°C where coal dust can form a layer, or
to its regular maintenance. 450°C where it is not expected to form a layer. Otherwise,
other details are generally the samc as for flameproof
motors type Ex ‘d’, according to IEC 60079-1. For
Safety features variations in length of paths, gaps, widths, creepage and
The equipment is usually fitted with interlocks to ensure clearance distances, the reader should consult these
that if the internal pressure or flow rate of air or inert Standards.
gas falls below a certain minimum level, the supply to
the motor is cut off. 7.1 6 Intrinsically safe circuits,
Pressure-measuring device: This is provided for the
operation of alarm or trip devices in the event the type Ex ‘i’
pressure within the casing falls below the permitted
minimum. IEC 60079-14 specifies two categories of intrinsically
Safe-starting device: This is provided to ensure that safe circuits and apparatus, i.e.
no apparatus within the enclosure is energized until
the initial atmosphere within the casing has been 0 Intrinsically safe category ‘ia’ as in IEC 60079-1 1 for
completely displaced. installations in Zone 0, and