Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 15)

Publication SGI-1.1 - August 2009
Section 3: Application Guidelines 15
If the installation site will be subjected to this type of noise, thorough
testing should be performed to assure that the solid-state system has
sufficient noise immunity for the expected levels of radio frequency
energy. Corrective measures should be taken if necessary. These include
shielding of solid-state circuits and/or connected wiring and the
establishment of restrictions to provide safe operating distances between
the solid-state equipment and the RF sources.
Grounding practices in industry are frequently misunderstood and often
ignored. Poor grounding can lead to many problems in solid-state systems.
Intentionally grounding one circuit conductor of any electrical supply
system is widely accepted and is generally required by electrical codes.
However, the non-current carrying parts of a system which enclose
equipment and conductors must also be grounded. In addition to
complying with various codes and standards, proper equipment grounding
achieves several desirable objectives:
1. It reduces the potential difference between conductive surfaces to
minimize electric shock hazard exposure for personnel.
2. It provides a path for passage of fault current to operate protective
devices in the supply circuit.
3. It attenuates the electrical noise and transients that can reach enclosed
equipment and also reduces the electrical noise which the equipment
can contribute to its surroundings.
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.
Comments: 3.5.1 — Off-State Current
See section 2.3.
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.
Comments: 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