Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 12)

Publication SGI-1.1 - August 2009
12 Section 3: Application Guidelines
Programmers of PC systems should evaluate functional and safety
implications of all control paths and provide appropriate feedback
arrangements as needed.
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 PC’s 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 PC’s 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.
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.
Comments: 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.