Allen-Bradley AADvance, Руководство пользователя

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Rockwell Automation Publication ICSTT-RM448J-EN-P - April 2018 81
Before You Begin Chapter 4
Estimating Center of Gravity
Information If it is necessary to calculate the location of the center of gravity of an
AADvance controller destined for a maritime or other shock-mounted
application, it is reasonable to assume the center of gravity of each assembly of
modules and their base unit is at the geometric center of the assembly.
Design Considerations for Electrical Grounding
All applications of the controller will require at least two separate ground
(earth) systems:
• An AC safety ground (sometimes called the 'dirty ground') to protect
people in the event of a fault. The ground stud on the T9100 processor
base unit, and all exposed metalwork such as DIN rails, will be bonded
to the AC safety ground.
• An instrument ground (sometimes called the 'clean ground' or the
'0 Vdc ground') to provide a good stable 0 V reference for the system.
Every signal return will be referenced to the instrument ground. The
instrument ground will be isolated from the AC safety ground.
The AC safety ground and the instrument ground will usually be made
available through bus-bars. Bus-bars must be of copper; they may be nickel
plated. For a small application, you may use ground studs instead of bus-bars.
Some field wiring, such as communications cables, will need shielded
(screened) cable. There may be a shield ground, in addition to the AC safety
and instrument grounds, to provide a common point to terminate shields of
such cables. The shield ground will usually be connected to the AC safety
ground; or, more rarely, to the instrument ground. In practice, the continuity
of the shield connections will be more important than the goodness of the
ground connection provided.
The controller input and output modules incorporate galvanic isolation.
Nevertheless, it is possible that a particular application will require the
provision of barrier strips with galvanic isolation, for example to provide
consistency with an existing installation. In these cases, there may be a separate
intrinsic safety ground as well.