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2 ELECTRICAL NOISE
Any unwanted extraneous electrical signal in an electronic
system is known as noise. Noise can be caused by EMI or
RFI. Electrical noise-producing equipment, such as rotating
machinery, relays, power lines, and handheld two-way radios
are examples of sources of both EMI and RFI.
The effects of high field strength EMI and RFI, such as from
two-way radios, are unpredictable and depend on many
factors
including
field
frequency, direction, installation-related factors and the
uncontrolled use of radio transmission devices. All of these
factors can disrupt the correct operation of any electrical
equipment's performance and reliability.
The C1900 range is designed to meet international standard
requirements for Electro Magnetic Compatibility (EMC),
however, any system can be subjected to levels of EMI above
the prescribed limits, especially under uncontrolled conditions.
In order to ensure correct operation in industrial environments,
it is essential that correct installation methods are followed.
Handheld radios operated in close proximity to any electronic
equipment can produce high and unpredictable field strengths
beyond the normal reasonable levels. In such circumstances
the use of a radio transmission exclusion zone must be
considered.
2
strength,
separation
distance,
3 SHIELDING
The importance of shielding and grounding on electrical
control and instrumentation equipment and wiring cannot be
over-emphasized, because it determines to a large degree
how the electrical control systems will behave in the presence
of EMI/RFI emissions.
A shield is a metal construction of aluminium foil, aluminium
conductors, mylar, or copper conductors surrounding the
interior of a wire or cable.
interference or noise from entering the wire, and helps to
prevent signals from leaving the wire and interfering with
adjacent wires and signals. Foil shields, while less flexible
than braided shields, offer the best protection by providing 100
percent wire coverage.
If strong magnetic induced noise is present, a double-shielded
wire or cable may be used, as shown in Figure 3.1.
Fig. 3.1 Double-shielded
Cable
Shielded twisted-pair wire, which will reduce the effects of
electromagnetic and electrostatic noise coupling, should be
used for analog signal connections, as shown in Figure 3.2.
A twisted-pair comprises two wires, each insulated and twisted
around each other. Any induced interference in each wire
(from outside magnetic fields around the two wires), tends to
be equal and in opposite directions, causing the interference
to cancel.
Cable trays and conduit are useful for both cable separation
and shielding. All trays and conduit should be metal and
solidly grounded. The cables and wires should be grouped
according to their signal level, keeping power and relay
switching lines in a separate conduit away from signal input or
communication lines. When trays or conduits of unlike signals
must cross, they should cross at 90-degree angles. If trays or
conduits are non-metallic or open, a grounded steel barrier
should be placed between the two at the crossover point.
The shield helps to prevent
—
+
Fig. 3.2 Twisted-pair
Cable
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