Safety Guidelines for the Application, Installation and Maintenance of Solid State Control E116P
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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.
C.3.1.2 Power Up/Power Down Considerations
Response of a system during power up/power down can
create hazards not encountered during normal operation.
Erratic operation of solid state outputs due to the changing
voltage of DC power supplies during start up is one
example. To avoid unpredictable outputs, many power
supplies incorporate a power turn-on time delay circuit.
This allows power supply output voltage to reach its
specified value before being applied to solid state logic
and output circuits. If this protection is not part of the DC
supplies for a system, a timing circuit external to the
power supply can be added to delay the application of
power to output devices.
Removing all power or losing all power from a system
simultaneously usually does not result in a hazard since
the power for machine operation is also being removed.
However, when power other than electrical power is being
controlled, a power interlock circuit may be required to
protect against unexpected machine motion. Power
interlocks with automatic shutdown should be included if
erratic or hazardous operation results due to loss of one
power supply in a system with multiple supplies. Automatic
power supply sequencing should be employed in systems
that require the application or removal of power in a
specific sequence. If the STOP or E-STOP sequence
normally employs dynamic braking, alternative safeguards,
such as automatic mechanical braking upon loss of power,
should be provided if coasting stops are hazardous.
If hazardous operation can result from unexpected
restoration of power during a power outage or a system
shutdown, the system should include a feature which
requires a deliberate operator action before power is
reapplied to the system.
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