6
If any wires need to run parallel with any other lines,
maintain a minimum space of 150mm between them.
If wires MUST cross each other, ensure they do so at
90 degrees to minimize interference.
Use of Shielded Cable
All analog signals must use shielded cable. This will
help eliminate electrical noise induction on the wires.
Connection lead length must be kept as short as pos-
sible keeping the wires protected by the shielding. The
shield should be grounded at one end only. The pre-
ferred grounding location is at the sensor, transmitter
or transducer.
Noise Suppression at Source
Usually when good wiring practices are followed, no
further noise protection is necessary. Sometimes in
severe electrical environments, the amount of noise is
so great that it has to be suppressed at source. Many
manufacturers of relays, contactors, etc., supply ‘surge
suppressors,’ which mount on the noise source. For
those devices that do not have surge suppressors sup-
plied, Resistance-Capacitance (RC) networks and/or
Metal Oxide Varistors (MOV) may be added.
Inductive coils: MOVs are recommended for transient
suppression in inductive coils, connected in parallel and
as close as possible to the coil. Additional protection may
be provided by adding an RC network across the MOV.
Figure 9
Transient suppression with inductive coils
Contacts: Arcing may occur across contacts when
they open and close. This results in electrical noise as
well as damage to the contacts. Connecting a properly
sized RC network can eliminate this arc.
For circuits up to 3 amps, a combination of a 47 ohm
resistor and 0.1 microfarad capacitor (1000 volts) is
recommended. For circuits from 3 to 5 amps, connect
two of these in parallel.
Figure 10
Contact noise suppression
Sensor Placement (Thermocouple or RTD)
If the temperature probe is to be subjected to corrosive
or abrasive conditions, it must be protected by an ap-
propriate thermowell. The probe must be positioned to
reflect true process temperature:
1. In a liquid media - the most agitated area
2. In air - the best circulated area
The placement of probes into pipe work some
distance from the heating vessel leads to trans-
port delay, which results in poor control.
For a two wire RTD a wire link should be used in place
of the third wire. Two wire RTDs must only be used with
lead lengths less than 3 meters. Use of three wire RTDs
is strongly recommended.
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