Micromod MOD: 30ML and Modcell Maintenance for 2001N, 2002N, and 1800R - Manual (Page 142)

Technical Notice
• If the installation includes an AC power neutral, test for voltage between it and this ground. If there is
more than 2 volts, it is quite likely that there is a defective ground or neutral connection.
Shields
• Usually, shield wires are connected to ground only at one end.
• In noisy environments, it may be necessary to connect the shield to ground at both ends. In this
case, it is imperative that a high quality ground is available at both ends, to prevent ground loop
currents.
• All shield grounding is to be made on the Chassis terminals. There are two of these terminals, one
above terminal 49 and one below L2/N. They connect directly to the instrument housing, as does the
ground lug.
Instrument, I/O and Communication Circuit Commons
• The instrument circuit common, built-in analog inputs, modular analog I/O and communication circuit
commons are 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.
• If the installation does not include a dedicated instrument system ground, then connect circuit
commons to one of the terminals identified as Chassis.
• Never leave circuit commons completely floating.
• Circuit commons must always have some dc path to ground to prevent the possible build up of static
charges, to reduce noise pickup, and to comply with EU EMC requirements.
• If a separate instrument ground is available, connect commons to it, instead of the power or safety
ground.
• Instrument circuit common (terminal 25) and the negative or return of all isolated analog signal inputs
and outputs are to be connected to ground. Note that the built-in analog outputs are not isolated and
require no additional grounding, when the instrument common is tied to ground.
• When using the built-in communications, connect terminal 1 of each instrument on the bus together.
This circuit common must also be connected to ground at one point.
• The common of ICN and RS-232 modules is to be connected in the same manner as the built-in
communications.
• RS-485 modules provide no connection to circuit common and usually do not require grounding. For
installations where high levels of noise or static charges are present, it may be necessary to ground
the circuit. If the circuit is grounded at the PC interface or another device, adding a ground could
cause adverse affects. If the communication circuit is not grounded elsewhere, it may be grounded at
the terminations. To ground at the terminations, it is necessary to use two 60 ohm resistors in series,
instead of one 120 ohm resistor. The junction between the two resistors may then be connected to
ground.
Topic: MOD 30ML/Modcell Technote: TNML1001-1
MicroMod Automation, Inc. Rev. 3, Feb-2005
GroundingWiring.doc Page 2 of 4