Micromod MOD: 30ML Installation - Manual (Page 44)

MOD 30ML Multiloop Controller
POWER, GROUNDING, AND BUILT-IN I/O CONNECTIONS
3.4 GROUND CONNECTIONS
On Model C instruments, connect ground terminal on the lower right side (Fig 3-1a) directly to
the plant safety ground system. On Models A & B, a protective ground terminal (green metal
stud) is provided at the bottom of the terminal blocks near the power connections (Figure 3-
1b). Connect this terminal directly to the plant safety ground system. This terminal is to be
used only for the protective ground conductor. Keep the ground wire as short as possible and
use the largest practical wire gage.
3.4.1 Chassis and Shield Grounds
Model C controllers have a chassis terminal on the upper right of the termination (Figure 3-
1a). Model A and B controllers have chassis terminals on the upper and lower right of the
termination (Figure 3-1b). The protective ground connects directly to the metal instrument
chassis, and to the power input filter in the instrument power supply. Terminals identified as
chassis in Figure 3-1a and Figure 3-1b are also internally connected to the protective ground.
The chassis terminals can be used for shield connections.
3.4.2 Circuit Common Connections
The instrument circuit common is isolated from the protective ground. This makes it easier to
avoid dc ground loops, and helps isolate the instrument from noise which may be present on
the protective ground.
Instrument common is the negative return for both built-in analog output circuits. Common is
available on terminals 18 and 39 for Model C, and 16 and 41 for Models A & B (see Figures
3-1a and 3-1b and Section 3-6).
Circuit common is also available at terminal 25 for connection to an instrument system
ground. If the installation does not include an instrument system ground, then connect circuit
common to one of the terminals identified as chassis in Figures 3-1a and 3-1b. Never leave
circuit common completely floating. Circuit common must always have some dc path to
ground to prevent the possible build up of static charges, and to reduce noise pickup.
3.4.3 Electrical Noise
Electrical disturbances can be caused by lightning, motors and motor driven devices, relays,
solenoids, and communication equipment. These disturbances often introduce electrical
noise in power lines, transmission lines, and site grounds. The successful operation of any
microprocessor-based device depends, in part, on the precautions taken to minimize the
effect of these disturbances. Often called "transients" or "voltage spikes", this form of noise
is infinitely variable in terms of amplitude, frequency, and duration.
Common sources of this type of noise are:
 loose or poor quality connections (especially power connections)
 arc welding equipment
 switches operating inductive loads
 relays, solenoids and other coil operated devices
 high current conductors – electric heater circuits
 fluorescent or neon lamps
 motors and motor driven devices
 switch mode devices – SCRs, thyristors
 lightning or electrostatic discharges
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