120
P a r r I n s t r u m e n t C o m p a n y
Explosion Proof Apparatus, Continued
Explosion Proof Heaters
The easiest way to provide
an explosion proof heater is to
use a steam or hot oil jacket
and ensure that the highest
temperature that can be
reached in the jacketed cylin-
der is well below the minimum
ignition temperature for the
specific hazardous atmosphere
in which it will be installed. An
aluminum block heater can be
considered explosion proof
if it has explosion proof wiring,
and if it is operated with an
auxiliary controller that will
hold the surface temperature
below a safe maximum.
Electric heaters purged with
clean air can also be consid-
ered explosion proof, but it
is doubtful that seals can be
maintained in a purged heater
to provide true protection over
a long period of time.
Please see page 102 for
additional information on
heater selection.
Explosion Proof Wiring
In an explosion proof
system, all electric wiring with
significant voltage or current
carrying capability must be
routed in approved sealed
conduit or in specially sealed
flexible cables. All termina-
tions and switches must be
contained in approved boxes
or housings. The user must
provide all local wiring and
connections to a power sup-
ply, and must ensure that the
installation meets all require-
ments of the local electrical
code.
Certain sensors, such as
thermocouples, pressure trans-
ducers and tachometer pickups
carry such low electric loads
that they are a potential igni-
tion source only in the event
of a most unusual failure. In
many installations these low
hazard components are not
seen as a problem. They can,
however, be protected with
isolation barriers which will
make them intrinsically safe
even in an unusual failure.
These energy limiting elec
tronic barriers can be provided
where required.
Explosion Proof Controllers
The most commonly used
method for dealing with the
ignition hazard introduced by
a temperature or process
controller is simply to locate
the controller outside of
the hazardous atmosphere.
Another choice is to install the
controller in a cabinet which
can be purged with clean air
within the hazardous location.
Special Systems
Parr can furnish systems
approved for use in hazardous
locations up through Class I,
Division 1, Group B in which
specific hazardous gases will
be present. Each of these
formally approved systems
must be designed and built
on a custom basis, with all
current carrying wiring and
fittings installed in accordance
with the requirements
discussed above.
Air Motor
