Transient Overvoltage - Parker H Series Installation & Service Instructions Manual

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

2.7 Transient Overvoltage

Solid state controls may be affected by transient
overvoltages which are in excess of those specified by
the manufacturer. Voltage limiting means should be
considered and may be required.
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 components or logic
devices, require that the user be concerned with the safety
of personnel and the protection of the electronics.
It is recommended that circuits which the user considers
to be critical to personnel safety, such as "end of travel"
circuits and "emergency stop" circuits, should directly
control their appropriate functions through an
electromechanical device independent of the solid state
logic. Such circuits should initiate the stop function through
deenergization rather than energization of the control
device. This provides a means of circuit control that is
independent of system failure.
C.2.7 Transient Overvoltage
Solid state devices are especially sensitive to excessive
voltage. When the peak voltage rating is exceeded, even
for a fraction of a second, permanent damage can occur.
The crystalline structure of the device may be irretrievably
altered and the device may no longer be able to turn OFF.
The external symptom of this situation is exactly the same
as that of an electromechanical device with welded contacts.
Minimum Holding Current
Another characteristic of concern is the minimum holding
current requirement for triacs and SCRs. When the load
current falls below the minimum value, typically 25 – 100
mA, the triac or SCR ceases conduction and passes only
off-state current until again triggered. Thus, it may not be
possible for the circuit to turn-on or conduct full load
current for very light loads. In these instances, a load
resistor called a bleeder resistor may be connected to the
output to provide the minimum load. In some equipment
special circuitry is provided to overcome this problem.
C.3.1.1 Circuit Considerations
The predominant failure mode of solid state devices is in
the ON condition. This failure mode and the other types
of failures mentioned in the NEMA Standard are the
reasons for the precautions that are recommended for
safety-critical circuits on systems that control potentially
hazardous processes or machine operations.
Alternatively, if solid-state is used for circuits designated
as safety-critical, the circuits should be designed to provide
safety equivalent to the recommended "hard-wired"
electromechanical circuits. In such cases consideration
should be given to techniques such as: redundancy,
feedback loops, diagnostics, interlocking and read-only
memory for critical parts of a program.
De-energization rather than energization of the control
device should be specified for STOP circuits so broken
wires or corroded contacts do not go undetected. E-stop
push buttons or pull cords should be installed at
appropriate locations on a machine to provide operators
with a rapid and convenient means for removing power
from devices which control machine motion.
Alternate methods of stopping such as dynamic braking
and regenerative braking may be available. In order to
accomplish the dynamic braking and regenerative braking
function, the control circuit, or part of it, in the equipment
must remain energized during the stopping mode.
However, operator interface components such as stop
push buttons or pull cords, must still be connected through
normally closed contacts so broken wires or corroded
contacts do not go undetected.
regenerative braking is used for emergency stopping,
provision should be made to interrupt the power circuit as
soon as the machine comes to a stop.
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Also, if dynamic or