Parker H Series, Инструкция по установке и обслуживанию

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Safety Guidelines for the Application, Installation and Maintenance of Solid State Control E116P
10
3.3.2 Polarity and Phase Sequence
Input power and control signals should be applied with
polarity and phase sequence as specified by the
manufacturer. Solid state devices can be damaged by the
application of reverse polarity or incorrect phase sequence.
C.3.3.1 (continued)
• In an electromechanical implementation of a ladder
diagram, power is available to every rung at all times,
so that the logic of the various rungs is executed
continually and simultaneously, limited of course by the
operating delays inherent in the electromechanical
devices. By contrast, a typical PC examines the status
of input devices (I/O scan), then executes the user
program in sequence (program scan), then changes
outputs accordingly in the next I/O scan. Therefore, the
sequential order of a PC program can be of more
importance and significance than in its
electromechanical counterpart, particularly when special
instructions such as “immediate” inputs or outputs are
programmed as some PCs permit. Also, differences in
response characteristics of components, differences in
system architecture, and the scan time associates with
a PC system can combine to change timing
characteristics of a circuit significantly. In particular,
care must be taken in handling momentary or rapidly
changing inputs to a PC system which might be missed
between scans. Simple transfer of a ladder diagram
without consideration of these characteristics of PCs
may produce unintended and possibly hazardous results.
Programmers should consult the user’s manual in order
to understand the characteristics of the particular PC
being used, and provide appropriate features in the
program to accommodate them.
• Another concern is the operating mode of devices
connected to input terminals. Input signals must be
arranged so loss of signal due to a broken wire or
corroded contact does not go undetected and create a
hazardous condition. In particular, stop functions should
be initiated by opening a normally closed external circuit
rather than closing a normally open circuit even though
the system is capable of being programmed to accept
either type of input.
The considerations described in this section apply to the
creation of “new” programs as well as conversion of
existing ladder diagrams.
C.3.3.2 Polarity and Phase Sequence
Additionally, incorrect polarity or phase sequence
connection may cause erratic response by solid state
controls, with potential hazards to personnel. Frequently,
such a system contains a detection circuit that illuminates
an indicator when incorrect phase sequence is applied.
Phase sequence may be corrected by interchanging any
two system input power leads. It is advisable to check
rotation of motors whenever input power leads are
disconnected and reconnected in a system.