Suggested Barriers And Isolators - Honeywell Signalpoint Pro Technical Manual

SIGNALPOINT PRO OPERATING MANUAL
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
Cb = 0.083 microfarads
Ib = 3.05 millihenries
Total allowable capacitance Ca= Cb-Cf, Ca = 0.083-0 = 0.083
Total allowable inductance Ia = Ib-If, Ia = 3.05-0 = 3.05
If the cable type is known, then the parameters from the manufacturer should be
used otherwise refer to the Signalpoint Pro control drawing P-1446, page 2 of 2
which suggests values of:-
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 = Ca/Cc = 0.083/0.0002 = 415 metres
Maximum length of cable due to inductance = Ia/Ic = 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.

6.4 Suggested barriers and isolators

Listed below are some suggested barriers and isolators for use with Signalpoint Pro.
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.
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 (see
section 6.1.1)
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MAN0853 Issue 3 10/08