Table of Contents
SIGNALPOINT PRO OPERATING MANUAL
6.2 Terminal connections
4
3
2
1
6.3 Maximum cable length calculation
The limiting factors in calculating maximum cable lengths when using barriers and
isolators are the total capacitance and inductance. Barriers and isolators have a fixed
amount of capacitance and inductance that can be connected to their outputs. The
cable between the field device and barrier/isolator will have a value for capacitance
and inductance per metre or kilometre that will be available from the manufacturer or
supplier.
To calculate the maximum cable lengths, calculate the total capacitance and
inductance for the length of cable, add any capacitance or inductance due to the field
device (Signalpoint Pro capacitance and inductance = 0). The resulting totals should
not be greater than the value shown for the barrier or isolator.
Example using
MTL7787+ dual channel zener barrier:
Capacitance permitted by the barrier
Inductance permitted by the barrier
Internal capacitance of the field device
Internal inductance of the field device
Capacitance of the cable per metre
Inductance of the cable per metre
Total allowable capacitance for the cable = Ca
Total allowable inductance for the cable
Terminal 1
Terminal 2
Terminal 3
Terminal 4
Note: The maximum cable length is determined by
the capacitance and inductance of the cable, but
must be equal to, or less than, the capacitance (Ca)
and inductance (La) values indicated on the barrier
used. If the cable capacitance and inductance are
not known, 60 pico-farads and 0.20 micro-henry per
foot should be used for calculation.
Diagram 5: Terminal connections
= Cb
= Ib
= Cf
= If
= Cc
= Ic
= Ia
11
MAN0853 Issue 3 10/08
Connection
Sensor Wire Colour
+VE
Signal
Not used
Screen
Red
White
Black
Braid
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