Circuit Isolation Requirements - Parker H Series Installation & Service Instructions Manual

Safety Guidelines for the Application, Installation and Maintenance of Solid State Control

3.2 Circuit Isolation Requirements

3.2.1 Separating Voltages
Solid state logic uses low level voltage (e.g., less than 32
volts DC) circuits. In contrast, the inputs and outputs are
often high level (e.g., 120 volts AC) voltages. Proper
design of the interface protects against an unwanted
interaction between the low level and high level circuits;
such an interaction can result in a failure of the low voltage
circuitry. This is potentially dangerous. An input and
output circuitry incorporating effective isolation techniques
(which may include limiting impedance or Class 2 supplied
circuitry) should be selected.
3.2.2 Isolation Techniques
The most important function of isolation components is to
separate high level circuits from low level circuits in order
to protect against the transfer of a fault from one level to
the other.
Isolation transformers, pulse transformers, reed relays, or
optical couplers are typical means to transmit low level
logic signals to power devices in the high level circuit.
Isolation impedance means also are used to transmit
logic signals to power devices.
3.3 Special Application Considerations
3.3.1 Converting Ladder Diagrams
Converting a ladder diagram originally designed for
electromechanical systems to one using solid state control
must account for
electromechanical and solid state devices.
replacing each contact in the ladder diagram with a
corresponding solid state "contact" will not always produce
the desired logic functions or fault detection and response.
For example, in electromechanical systems, a relay having
a mechanically linked normally open (NO) and normally
closed (NC) contact can be wired to check itself. Solid
state components do not have a mutually exclusive NO-
NC arrangement. However, external circuitry can be
employed to sample the input and "contact" state and
compare to determine if the system is functioning properly.
the differences between
Simply
C.3.2.1 Separating Voltages
For specifications of Class 2 circuitry, refer to Article 725
of the National Electrical Code, NFPA 70.
C.3.2.2 Isolation Techniques
In addition to utilizing the various components discussed
in the left column, specific wiring techniques should be
applied to assure separation of power circuit wires from
logic circuit wires. If at all possible, logic wires should be
run in a conduit that is segregated for that purpose only.
Multiple conductors in a shielded cable is an appropriate
substitute for separate conduits. Another common practice
is to run the logic signals through twisted pairs of wire.
Regardless of the circumstances, wires carrying logic
signals should never be wrapped in the same bundle with
wires that carry power signals.
C.3.3.1 Converting Ladder Diagrams
The example cited in this section of the NEMA Standard
illustrates only one of a number of reasons for special
care in converting an electromechanical (relay) ladder
diagram to a programmable controller (PC) program.
Some other basic considerations are:
• A PC program is an instruction to the PC's central
processing unit to enable it to perform the logic functions
and sequences for a particular application. Typically,
the PC's logic level components are electrically isolated
from the actual input, sensor and actuator devices, as
contrasted with electromechanical controls which usually
include contacts and coils of the actual plant floor
devices in the control schematic. Therefore a PC
program normally functions as open-loop control, unless
feedback loops from the plant floor devices to separate
inputs of the PC are provided and programmed to cause
corrective action if inconsistencies are detected.
Programmers of PC systems should evaluate functional
and safety implications of all control paths and provide
appropriate feedback arrangements as needed.
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