3
Inductive coils: - MOVs are recommended for tran-
sient suppression in inductive coils, connected in par-
allel and as close as possible to the coil. Additional
protection may be provided by adding an RC network
across the MOV.
Figure 5. Contacts: - Arcing may occur across con-
tacts when they open and close. This results in electri-
cal noise as well as damage to the contacts. Connect-
ing 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.
2.6 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: In a liquid media, the
most agitated area. 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 (10 ft.). Use of three
wire RTDs is strongly recommended.
2.7 Panel Wiring
In general, all wiring connections are made to the in-
strument after it is installed. Copper wires must be
used for all connections (except thermocouple signal
wires).
To avoid electrical shock, AC power wiring
must not be connected to the source distribu-
tion panel until all wiring procedures are com-
pleted.
Check the information label on the case to de-
termine the correct voltage before connecting
to a live supply.
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