Revision 3.1 - 68 - March 2010
pair will be color-coded; positive and negative being different colors. The pairs should be
sequentially numbered.
The trunk (home run) cable shall have one spare pair. Unused pairs of the trunk cable shall be
connected together with the pair shield and grounded (for noise rejection) at the fieldbus
segment shield ground connection (refer to Figures 7-6, 7-7, 7-8 and 7-9). See Appendix 2 for
Cable Characteristics.
The impedance of the cable shall be 100 Ω ± 20 Ω maximum. For longer (greater than 500 m)
runs, 1.13 mm
2
(#16 AWG) gauge wire should be used (to minimize the cable voltage drop)
and 100 Ω ± 10 Ω specification. Although the larger cable cross-section may result in higher
capacitance, the associated signal attenuation can be kept small and shall not be more than 3
dB/km at 39 kHz (this standard frequency is used to specify cable characteristics and is
sufficiently close to the 31.25 kHz to be applicable).
The trunk cable shall meet the ambient temperature range for the location in addition to the
minimum installation temperature. In addition, installation of the cable shall comply with the
manufacturer’s specified minimum bending radius.
The individual pair shield shall be plastic-coated aluminum tape with a metallic surface inside
in contact with a tinned copper drain wire. All individual pair shields shall be electrically isolated
from each other and from the overall shield.
The overall shield shall be plastic-coated aluminum tape, with the metallic surface in contact
with a tinned copper drain wire and wire braid (if used).
A Factory Acceptance Test (FAT) shall be performed on all supplied FOUNDATION fieldbus
cables. Pending a positive result of the factory test, an on-site insulation integrity test (between
individual wires, individual pairs, and individual wires and shields) before installation may then
be deemed optional.
The conductor resistance, screen resistance, attenuation at 39 kHz and inductance of the
cable shall comply with F
OUNDATION fieldbus standards as stated in FF-844 and IEC 61158-2.
Additional care shall be taken with intrinsically safe wiring techniques to ensure compliance
with all local electrical approval requirements and site practices.
6.7.2 Spur Cables
The spur cables shall meet the F
OUNDATION fieldbus specification as discussed in Section
6.7.1.
Commentary:
An option may be to purchase single-pair FOUNDATION fieldbus cable and use this for both
fieldbus spur wiring and for 4-20 mA device wiring.