Countering The Effects Of Off-State Current - Parker H Series Installation & Service Instructions Manual

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

3.5 Countering the Effects of Off-State Current

3.5.1 Off-State Current
Solid state components, such as triacs, transistors, and
thyristors, inherently have in the off-state a small current
flow called "off-state current".
Off-state current may also be contributed by devices used
to protect these components, such as RC snubbers.
3.5.2 Off-State Current Precautions
Off-state currents in a device in the off-state may present
a hazard of electrical shock and the device should be
disconnected from the power source before working on
the circuit or load.
Precautions should be taken to prevent the off-state current
of an output device which is in the off-state from energizing
an input device.
C.3.5.1 Off-State Current
See section 2.3.
C.3.5.2 Off-State Current Precautions
The off-state current of a power switching device such as
a solid state motor controller can be lethal. Simply switching
off power via a stop push button in a control circuit is not a
sufficient precaution, since off-state current will continue
to flow through solid state devices which remain connected
to the supply. Good practice requires disconnection of all
power from equipment before working on or near exposed
circuit parts. (See NFPA 70E, Part II.)
It should not be assumed that a shock hazard does not
exist simply because a solid state circuit operates at low
voltage levels. Standing on a wet floor or working in a
damp location can lower a person's body impedance to
the extent that offstate current from low voltage also
presents an electrical shock hazard.
If it is necessary to work on energized equipment, the
guidelines detailed in section 5.2 for Preventive
Maintenance should be followed. In addition to the specific
procedures for personnel safety, care is needed when
making measurements in energized systems. First, there
is a possibility of damage to delicate instruments due to
off-state current. Second, the off-state current can lead
to false conclusions when using sensitive instruments to
check for "contact continuity."
When a device (solid state or electromechanical) that can
produce a leakage current in the off-state is used to
provide the input to a solid state control, the precautions
explained in section C.2.3 apply.
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