low, it often is interlocked with the other devices to ensure that master switch
handles are in the OFF position before the controls can be energized and also to
stop all motions in case of a motor overtemperature.
Timed delay relays are commonly used to ensure that the operation of
accelerating contactors is sequential. A timed delay relay is connected in the
control circuit in parallel with an accelerating contactor's coil and is energized
simultaneously. The contacts of the timed delay relay are connected in series
with the coil of the accelerating contactor for the next higher speed point and
their closing is delayed for a preset length of time. This prevents the
accelerating contactor from being energized for that length of time.
Conductors. The selection of conductors, both for size and insulation
type, is governed by the National Electrical Code. The application of conductors
must comply with the provisions of NEC Table 310-13. This table states the
suitability and maximum temperature rating for dry, damp, and/or wet locations.
The Table also describes the insulation and outer covering of each type of
18.104.22.168 Insulation. NCC policy is to require, for interconnecting wiring,
stranded copper conductors of any construction complying with NEC Table 310-13,
with the following exceptions:
Those containing asbestos in the insulation or outer covering are
Those having a thermoplastic insulation are prohibited for use in DC
circuits on outdoor cranes.
All conductors insulated with a thermoplastic are required to be
designated "/LS", that is low smoke type.
NCC requires that all conductors connected to or routed above resistors
have, with the exception of type SA and FEPB, insulation shown on NEC Table 610-
14(a) for 257 degrees (Fahrenheit) maximum temperature.
22.214.171.124 Gauges. The size designation of conductors is based upon the American
Wire Gauge (AWG), which was formerly known as Brown & Sharpe's (B&S) gauge. With
this system unique numbers are assigned to conductors of specific cross-sectional
areas; the lower the number, the larger the cross-sectional area. The ratio of
the cross-sectional area of any size conductor to the next smaller conductor is
1.261:1. The unit of measurement for cross-sectional area is the circular mil.
In the International Systems of Units (SI), cross-sectional area is measured in
square millimeters and is conventionally used to designate conductor size.
Raceways. Rigid metal conduit is the preferred raceway for electrical
conductors. It is fabricated from steel and protected from corrosion by a zinc
coating. For connections to equipment subject to vibration, liquid-tight flexible
metal conduit is used. The National Electrical Code addresses the installation of
both types of raceway, including conduit, elbows, couplings, bushings, and