Emerson Bristol 2808 Series, Instruction Manual

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2808B Installation / 2-3
Conduit Connection : The Transmitter provides a ½ inch NPT threaded female port for
electrical conduit. This port can mate with threaded conduit or an appropriate threaded
pipe adapter.
Note : The conduit connections must be secured with no less than five threads fully engaged.
In some applications, condensation could form in the conduit, and seep into the Transmit-
ter electronics housing. If allowed to continue, moisture build-up will degrade the Trans-
mitter performance, and eventually cause damage. Installing the Transmitter above the
level of the process connection can prevent this condition. Any moisture forming in the
conduit will then drain away by gravity.
Access to Wiring Terminals : Remove the threaded end cover to access the wiring terminals
(see Figure 3-2). If the cover cannot be loosened by hand, insert a flat metal bar or similar
tool between the cover protrusions and apply moderate counter-clockwise leverage. Before
re-installing the cover, make sure that the threads are clean. Tighten the cover by hand
until all threads are engaged, and the gasket is compressed.
Lead Dress : When feeding wire through the conduit opening of the Transmitter, add about
six inches of slack for terminal connections. Dress the leads in a circular path around the
terminals as seen in Figure 2-1. The additional slack will make the connections more
manageable and prevent mechanical strain on the terminals.
2.4 WIRING OF 4-20mA SIGNAL/POWER LOOP
The 4-20mA signal/power loop can be powered in two ways. Figure 2-2 shows the loop
powered by the receiving device (controller, recorder, etc.), while Figure 2-3 shows the loop
powered by an external supply. In both instances, the 4-20mA current flows through a
250
Ω load resistor and develops a corresponding 1-5V input for the receiving device.
Signal Shielding : Use twisted wire, shielded cable covered by insulating material for the
signal/power wiring. When properly grounded, this cable will minimize pickup of elec-
tromagnetic, and radio frequency interference.
The shield lead of the cable is typically grounded at the input of the receiving device
(computer controller, recorder, etc.) as shown in Figures 2-2 and 2-3. Never connect the
other end of this shield to the transmitter enclosure or attempt to ground the shield at more
than one point along the wire path. Multiple grounds will cause signal errors at the input of
the receiving device.
Although it is recommended to connect the cable’s shield to the power common return of the
receiving device, the actual connection point may differ depending on the design and
application of the device. In some instances, better noise immunity can be had by
connecting the cable shield to the chassis or a designated shield terminal on the device.
Check the instruction manual of the receiving device for the recommended connection
points.