Suggested Barriers And Isolators - Honeywell Series 3000 MkIII Technical Manual

SERIES 3000 MKIII TECHNICAL MANUAL
Internal inductance of the field device = Li
Capacitance of the cable per metre = Cc
Inductance of the cable per metre = Lc
Total allowable capacitance for the cable = Ccmax
Total allowable inductance for the cable = Lcmax
All capacitance measurements are in microfarads, all inductance measurements are in millihenries.
Using the Safety Description of an MTL7787+ for a IIC gas as an example:
Safety Description: 28v, 93mA 0.651W
Co = 0.083 microfarads
Lo = 3.05 millihenries
Total allowable capacitance for the cable Ccmax = Co-Ci, Ccmax = 0.083-0 = 0.083
Total allowable inductance for the cable Lcmax = Lo-Li, Lcmax = 3.05-0 = 3.05
If the cable type is known, then the parameters from the manufacturer should be used otherwise refer
to the values suggested below:-
In North American Installations:
Cc = 60pF/foot (0.00006 microfarads) and Lc = 0.2 microhenries/foot (0.0002 millihenries)
In European Installations:
Cc = 200pF/m (0.0002 microfarads) and Lc = 0.66 microhenries/m (0.00066 millihenries)
Using the values per metre for European Installations:
Maximum length of cable due to capacitance = Ccmax/Cc = 0.083/0.0002 = 415 metres
Maximum length of cable due to inductance = Lcmax/Lc = 3.05/0.00066 = 4621.21 metres
As is often the case, capacitance is the most limiting figure and so the maximum cable length will
be 415 metres.
Note: Due to circuit limitations, do not run cable in excess of 1219metres (4,000ft) even if the above
formulas allow a longer length.

6.3 Suggested barriers and isolators

Listed below are some suggested barriers and isolators for use with Series 3000 MkIII.
MTL7728+ (single channel zener barrier)
MTL7787+ (2-channel zener barrier)
MTL5042 (Galvanic Isolator)
Pepperl+Fuchs KFD2-STC4-EX1 (Galvanic Isolator)
Note: It is up to the user to ensure that the barrier or isolator used is suitable for their application ,
and carries a listing (certification) appropriate to the authority having jurisdiction.
Note: In case the user installs the detector in safe area without any barriers and any isolators, an
appropriate load resistor (33~250Ω) must be connected to the detector
Caution: A single channel barrier solution is only suitable when used with a controller that
provides the load resistor in the source or positive supply line where the negative of the
barrier input is tied to earth ground.
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