ircon Modline 4, Installation And Operation Manual

Page: 81 / 88

Appendix
A–2
Section 3: APPLICATION GUIDELINES
3.1 GENERAL APPLICATION PRECAUTIONS
3.1.1 Circuit Considerations
The consequences of some malfunctions such as
those caused by shorted output devices, alteration,
loss of memory, or failure of isolation within compo-
nents or logic devices require that the user be con-
cerned with the safety of personnel and the protec-
tion of the electronics.
It is recommended that circuits which the user con-
siders to be critical to personnel safety, such as
“end of travel” circuits and “emergency stop” cir-
cuits, should directly control their appropriate func-
tions through an electromechanical device indepen-
dent of the solid state logic. Such circuits should ini-
tiate the stop function through deenergization rather
than energization of the control device. This pro-
vides a means of circuit control that is independent
of system failure.
3.1.2 Power Up/Power Down Considerations
Consideration should be given to system design so
that unsafe operation does not occur under these
conditions since solid state outputs may operate
erratically for a short period of time after applying or
removing power.
3.1.3 Redundancy and Monitoring
When solid state devices are being used to control
operations, which the user determines to be critical,
it is strongly recommended that redundancy and
some form of checking be included in the system.
Monitoring circuits should check that actual
machine or process operation is identical to con-
troller commands; and in the event of failure in the
machine, process, or the monitoring system, the
monitoring circuits should initiate a safe shutdown
sequence.
3.1.4 Overcurrent Protection
To protect triacs and transistors from shorted loads,
a closely matched short circuit protective device
(SCPD) is often incorporated. These SCPD’s
should be replaced only with devices recommend-
ed by the manufacturer.
3.1.5 Overvoltage Protection
To protect triacs, SCR’s and transistors from over-
voltages, it may be advisable to consider incorpo-
rating peak voltage clamping devices such as varis-
tors, zener diodes, or snubber networks in circuits
incorporating these devices.
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) volt-
ages. 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 compo-
nents 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 the differences between
electromechanical and solid state devices. Simply
replacing each contact in the ladder diagram with a
corresponding solid state “contact” will not always
produce the desired logic functions or fault detec-
tion and response. For example, in electromechani-
cal systems, a relay having a mechanically linked
normally open (NO) and normally closed (NC) con-
tact can be wired to check itself. Solid state compo-
nents 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 function-
ing properly.