Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 17)

Publication SGI-1.1 - August 2009
Section 3: Application Guidelines 17
Comments: 3.6.1 — Temperature
Operation above the maximum rated temperature will usually result in
many failures in a short time. Nuisance type malfunctions can also be
encountered as a result of elevated ambient temperature. These
malfunctions, when they occur, are usually temporary and normal
operation resumes when temperatures are lowered.
Some solid-state devices temporarily cease to function when ambient
temperature is below their minimum rated operating temperature.
Operation in cold environments should be avoided or heaters should be
installed in the equipment enclosures to bring the system up to the
minimum specified operating temperature before applying power to the
system.
Air circulating in a non-ventilated enclosure with equipment operating will
be at a higher temperature than the room in which it is installed. A
temperature differential of 10...20º C can be expected in a typical industrial
installation. See also section 2.1.
3.6.2 Contaminants
Moisture, corrosive gases and liquids, and conductive dust can all have adverse
effects on a system that is not adequately protected against atmospheric
contaminants.
If these contaminants are allowed to collect on printed circuit boards, bridging
between the conductors may result in malfunction of the circuit. This could
lead to noisy, erratic control operation, or at worst, a permanent malfunction.
A thick coating of dust could also prevent adequate cooling on the board or
heat sink, causing malfunction. A dust coating on heat sinks reduces their
thermal efficiency.
Preventive measures include a specially conditioned room or a properly
specified enclosure for the system.
Comments: 3.6.2 — Contaminants
Modules for solid-state systems usually consist of electronic devices
mounted on printed circuit boards with relatively close spacing between
conductors. Moisture in the form of humidity is one of the atmospheric
contaminants which can cause failure. If moisture is allowed to condense
on a printed circuit board, the board metallizations could "electroplate"
across the conductor spacings when voltage is applied. In low-impedance
circuits, this conductive path would immediately burn open, then reform
to be burned open again. This action can lead to erratic operation. In high
impedance circuits, a short circuit may appear resulting in a permanent
malfunction. Specifications for equipment often include a relative
humidity exposure limit, but appropriate precautions should be taken to
prevent condensation. Failures due to moisture are often accelerated in the
presence of corrosive gases or vapors. These increase the conductivity of
the moisture layer allowing electromigration to occur more rapidly and at
lower potentials.