Micromod RetroPAK: SLC Installation Manual - Manual (Page 49)

SLC RetroPAK Controller
POWER, GROUNDING, AND BUILT-IN I/O CONNECTIONS
25
3.4 GROUND CONNECTIONS
A protective ground terminal (green metal stud) is provided at the bottom of the terminal
blocks near the power connections, Figure 3-1. 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
The green metal protective ground stud connects directly to the metal instrument chassis,
and to the power input filter in the instrument power supply. The two terminals identified as
chassis in Figure 3-1 are also internally connected to the protective ground stud. 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 of the built-in analog output circuits.
Common is available on terminals 16 and 41 for convenient connection of the outputs (see
Figure 3-1 and Section 3-7).
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 Figure 3-1. 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
• electrostatic discharges