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 MIL-HDBK-1038
ventilated or nonventilated. Ventilated Type 1 enclosures should be used for
constant potential DC controllers to permit the migration of ozone from the
enclosure. Otherwise, ozone, generated from the arcing across opening contacts,
builds-up and has a highly corrosive effect upon all equipment within the
enclosure. A Type 12 enclosure is also for indoor applications; it is
nonventilated and protects against dust, falling dirt, and dripping noncorrosive
liquids.
A Type 4X enclosure is for indoor or outdoor applications; it is
nonventilated, fabricated from corrosion resistant material, and protects against
windblown dust and rain and against splashing or hose-directed water. NCC policy
is to specify Type 4X enclosures, fabricated from type 304 stainless steel, for
outdoor applications.
There are also "explosionproof" and "dust-ignition-proof" types of
enclosures (Type 7 and Type 9, respectively) for use in locations classified as
hazardous due to the presence of certain gasses or dusts.
4.5.22
Limit Switches. Switches are supplementary devices used to establish the
limits for movement of a crane drive. Limit switches are not to be used for
positioning a crane, boom, or hoist.
4.5.22.1 Motion/Position Limits (for OET, Gantry, and Semi-Gantry Cranes). On a
bridge crane, limit switches are used for the upper and lower travel of the hook.
The lower limit switch, a geared limit switch, prevents the hook from touching the
floor. This switch also protects against unspooling the wire rope from the drum.
Two upper limit switches are used. This is because of how important it is the
prevent two-blocking. Two-blocking occurs when the load block comes into contact
with the upper block; a catastrophic failure would result from the severing of the
wire rope. On newer AC powered cranes the first upper limit switch is the geared
limit switch. The back-up upper limit switch is block actuated, utilizing a
counter-weighted arm or paddle mechanism. The geared limit switch contacts
operate in the control circuit and the direction of motor rotation can be reversed
if the switch is actuated. The block actuated limit switch de-energizes a line
contactor in the motor branch circuit. If this switch is actuated, the motor
cannot be operated in either direction and the crane must be inspected for any
damage; the switch must be manually overridden to permit the lowering of the hook.
Bridge and trolley motions on newer bridge cranes can also be limited by
the use of switches operating in their respective control circuits. Slow-down
switches are often used to reduce the speed at which the bridge or trolley is
travelling prior to reaching the point at which motion is interrupted. Travel in
the direction away from a limit switch is not affected.
4.5.22.2 Motion/Position Limits (for Portal and Floating Cranes). The limit
switch arrangement on a portal or floating crane is more complicated than on a
bridge crane because protection must also be afforded against two-blocking caused
by the boom being lowered. The NCC arrangement of limit switches, as described
hereinafter, prevents the hooks from being raised high enough that the boom, in
its normal operating range, could be lowered into a hook block.
The luffing drive has a geared limit switch to stop the motor when the
boom goes up to the angle which puts the main hook at its minimum radius, goes
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