Quantcast Grounding Through the Electrification System

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MIL-HDBK-1038 DC System Grounding.  DC systems which include wiring external to control
panel enclosures are required in NEC Section 250-3 to be grounded unless the
system is equipped with a ground detector, the system operates at 50 VDC or lower
or at 300 Volts or higher, or the system is derived from a rectifier.  On cranes,
DC systems generally do not have to be grounded as one of these exceptions is
applicable. Bonding.  All exposed, non-current carrying metal parts on cranes are
required in NEC Section 610-61 to be metallically joined together, by means of
mechanically fastening or welding uncoated surfaces, that is bonded, so that they
are grounded.  The equipment grounding conductor is used to connect the non-
current carrying parts of equipment to the system grounded conductor.  Although
various types of conductors, sheathing, and raceways are described in NEC Section
250-91(b), NCC policy is to require that the equipment grounding conductor be in
the form of a wire sized in accordance with NEC Table 250-95. Drum Grounding.  NCC policy is to require that wire rope drums of built-
up hoists on newer cranes be grounded by means of a copper ring, supported from
one drum end flange in such a manner that it is electrically bonded to the drum,
and a collector connected to the equipment grounding conductor system by means of
a #12 AWG copper wire. Grounding Through the Electrification System.  Regardless of the type of
equipment grounding conductor used elsewhere on a crane, an equipment grounding
conductor in the form of a wire must be included in all bridge crane festooned
conductor and cable reel electrification systems.  This is because none of the
other types of conductors described in NEC Section 250-91(b) can be used in such
electrification systems.  An equipment grounding conductor is included as one of
the conductors in rigid conductor systems.  This complies with NEC Section
250-91(b). Lightning Protection.  For lightning protection on newer outdoor cranes,
NCC policy is to require that bonding conductors be provided across all gudgeons
and the boom hinge and strut pins; the minimum size of these conductors is 2/0
AWG.  Also, it is required that a revolving crane's upperworks be electrically
bonded to the nonrotating portion utilizing size 2/0 AWG conductors and a
collector ring having a minimum cross-sectional area of 70 square millimeters.
Likewise, the equipment grounding conductor in festooned runway and bridge-to-
trolley electrification systems is required to be size 2/0 AWG (minimum) and in
rigid electrification systems is required to have a minimum cross-sectional area
of 70 square millimeters.
Reduced Voltage Starters.  Acceleration of a squirrel cage motor can be
made less abrupt through the use of reduced voltage starting.  By inserting
resistance or reactance in series with the motor windings, the voltage at the
motor terminal is reduced.  Current reduction is directly proportional to voltage
reduction.  Torque is reduced in proportion to the square of the voltage
Attached Safety Systems and Devices.  These standard commercial products
include load indicating devices (LIDs), load moment indicating devices (LMIs), and
anti-two-block devices (A2Bs).  On new mobile cranes the LMIs and A2Bs are
standard integral systems or available as options.  Some of the older mobile


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