AVG Automation 6" EZTouch I/O Flex, Manual, Page: 10 / 64

EZTouch I/O Flex
10 / 64
and electromechanical controls. If possible, the electromechanical
controls (motors, starters, solenoids, etc.) should be housed separately
or at the farthest point when enclosed within the cabinet.
We recommend that the unit has a minimum clear space of 2" on all sides
for adequate ventilation as shown in the image on the right.
Electrical Considerations
This section is designed to provide you with a very basic understanding of
electrical noise and how to keep it away from CPUs. Industrial plants have a
number of generators of electrical noise that are sometimes also referred to as
Radio Frequency Interference (RFI). Anytime an inductive load like a motor,
motor starter, or solenoid is turned off, it generates a burst of excess energy
that has to flow back to ground, just like electrical energy from a lightning storm
has to flow back to Earth. RFI is short bursts of electrical energy at very high
frequencies. Other sources include RF Welders or Radio Transmitters.
Effect of RFI on Electronic Automation Equipment
Electronic controls use faster and faster CPUs today. These CPUs are also
operating at 2.5V to 5VDC logic level power supply. RFI, if allowed to enter the
CPU inside, is a killer of logic. A CPU under this environment loses its brain and
behaves erratically. A smart industrial-grade CPU like the unit's card engine,
when faced with RFI, halts its operation instead of giving false outputs.
Types of RFI
RFI enters electronic controls in two ways: radiated RFI or conducted RFI. For
most practical purposes, electronic devices, unless sitting right next to a
powerful RFI transmitter, will not be affected by noise because air space severely
attenuates such interference. On the other hand, conducted RFI travels over
conductive surfaces such as power supply wires, electrical wiring of field devices,
and worst of all; improper ground planes.
Equipment cabinets usually incorporate one or two doors and/or hinged cabinet
panels. Relying on door hinges and swinging panels for a good metallic bond
between hinged parts and the main body of the cabinet does not insure
adequate grounding. Instead, the use of ground straps is recommended. It is
vital for the reliable operation of any electronic device to have any of its metallic
surfaces well grounded to Earth. This not only provides for safe operation, it will
also drain out any conducted RFI to Earth, away from the CPU's signal ground.