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Enclosure & Device Design
Enclosure Design Requirements
1. All windows must be shatterproof and sized as small as
possible.
2. All NFPA 496 required markings must be placed on or
near all enclosure doors and covers.
3. The enclosure must withstand an internal pressure of
ten (10) inches of water without sustaining permanent
deformation and resist all corrosive elements in the sur-
rounding atmosphere.
4. All lightweight objects in the enclosure, such as paper
or insulation, must be firmly secured.
5. The enclosure should be constructed from materials
such as metal or non-static polycarbonate to meet or
exceed NEMA 4 or 12 performance requirements, but
does not require third party approval.
6. The installation of obstructions or other barriers which
block or impede the flow of protective gas must be
avoided.
7. The creation of air pockets or other areas which trap
flammable gases within the enclosure or devices must
be avoided.
8. The enclosure should be located in an area where impact
hazards are minimal.
9. If the enclosure is nonmetallic and contains equipment
which utilizes or switches power loads greater than 2500
VA, it must be constructed from substantially noncom-
bustible materials, such as materials designed to meet
or exceed ANSI/UL94 ratings of 94 V-0 or 94 5V.
adjacent Enclosures
1. Adjacent enclosures must be protected by one of the
following means:
a) purged or pressurized in series with the protected
enclosure;
b) purged or pressurized separately; or
c) protected by other means; e.g. explosion proof
enclosures, hermetically sealed devices or intrinsic
safe circuits.
2. Adjacent purged or pressurized enclosures must be de-
signed to meet all construction requirements above.
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total Volume Calculation
1. The total volume of all pressurized enclosures, devices
and wireways must be considered.
2. All enclosure, device and wireway volumes must be
calculated without consideration of internally consumed
space.
Device Ventilation
1. Enclosed devices within the protected enclosure which
do not exceed 1.22 cubic inches of free volume do not
require ventilation to the protected enclosure.
2. If the free volume of an internal device exceeds 1.22
cubic inches it must be protected by one of the following
means:
a) ventilated on the top and bottom sides with one (1)
square inch of opening for each four hundred (400)
cubic inches of volume within the internal protected
enclosure, at a minimum diameter of one (1) quarter
inch;
b) purged in series with the protected enclosure or be
purged separately; or
c) protected by other means; e.g. explosion proof
enclosures, hermetically sealed devices or intrinsic
safe circuits.
temperature limitations
1. The enclosure must have no surface area which exceeds
80 percent of the flammable or ignitable substance's
auto-ignition temperature.
2. Internal devices which exceed this temperature must be
protected by one of the following manners.
a) The device is enclosed in a chamber that is cULus
or FM listed as a hermetically sealed device which
prohibits the entrance of flammable or ignitable
substance, and maintains a surface temperature
below temperature limits.
b) It can be proven by testing that the devices will not
ignite the substance involved.
c) The device is purged in a separate enclosure that
bears an ETW (Enclosure Temperature Warning
Nameplate). Devices may only be accessed after
power has been removed and the device has been
allowed to cool to safe temperature, or the area is
positively known to be nonhazardous.
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