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6
5 .6
THC Burnout Detection
The MTL838C/MTL831C/MTL831C-PS system provides for thermocouple burnout
detection. This is implemented by periodically measuring the resistance of the
thermocouple and comparing it to the initial measurement. If the resistance
increases by more than 50 ohms, a warning is available in the sensor status (readings
are still available). For more information, see the instructions for the MTL838C
Receiver. The following are events that lock in the initial resistance measurement:
•
The MTL831C/MTL831C-PSis powered up (initial resistance measurement is
locked in after 30 seconds).
•
A Channel configured as a THC goes open for at least 1.3 second and then
closed (this is intended to reset the initial value when the THC is replaced).
•
A Channel that was not configured as a THC is changed to a THC. This
configuration change would come from the PC software or Modbus.
5 .7
Mixed Inputs
It is possible to mix any combination of different input sensor types on the 16
input channels.
Connect the different sensors appropriately, as shown in the connection diagram on
the unit overlay.
5 .8
Data highway
The data highway connection is also the source of DC power for the MTL831C/
MTL831C-PS. It is important that the polarity of the wiring on the "+" and "-"
terminals is correct. A shielded twisted-pair wire is recommended. The "S" terminal
is wired to the cable shield. It is not internally connected in the MTL831C/MTL831C-
PS.
5 .9
Transmission distances
The maximum data highway length will depend upon two key factors: the type
and quality of the cable used, and the level of electrical interference present in the
environment.
Typically a user might expect: IS applications - 1km
Non-IS applications - 2km.
The reduced distance for IS applications is due to the voltage drop from the current
limiting resistor in the MTL5553 IS isolator and may also be limited by the inductance
and capacitance of the data highway cable that is required for the IS cable parameter
calculations. (See IEC/EN 60079-14 for further details).
We recommend using 16 gauge Foundation Fieldbus cable that has been
checkmarked as meeting the FF standard for data highway cables. This will help
ensure that the maximum lengths can be achieved with successful communication
in the presence of noise.
Also, do not run the data highway cable near or in parallel with high voltage lines,
VFDs, or motors driven with a VFD as these can create noise that would interfere
with communication.
INM MTL831C Rev 7
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