10
3-3-2. Maximum overall cable length
The maximum network expansion depends on the type of protection and the cable
specifications. The overall cable length combines the length of the main cable and the length
of all spurs (>1 m/3.28 ft). Note the following points:
The maximum permissible overall cable length depends on the cable type used.
If repeaters are used, the maximum permissible cable length is doubled. A maximum of
three repeaters are permitted between user and master.
3-3-3. Maximum spur length
The line between the distribution box and field device is described as a spur.
In the case of non-Ex applications, the max. length of a spur depends on the number of spurs
(>1m/3.28 ft):
Number of spurs
1 to 12
13 to 14
15 to 18
19 to 24
25 to 32
Max. length per spur
120 m (393 ft)
90 m (295 ft)
60 m (196 ft)
30 m (98 ft)
1 m (3.28 ft)
3-3-4. Number of field devices
In accordance with IEC 61158-2 (MBP), a maximum of 32 field devices can be connected
per fieldbus segment. However, this number is restricted under certain conditions (explosion
protection, bus power option, field device current consumption). A maximum of four field
devices can be connected to a spur.
3-3-5. Shielding and grounding
Optimum electromagnetic compatibility (EMC) of the fieldbus system can only be guaranteed
if the system components and, in particular, the lines are shielded and the shield forms as
complete a cover as possible. A shield coverage of 90% is ideal.
To ensure an EMC protective effect, connect the shield as often as possible to the reference
ground.
For reasons of explosion protection, you should refrain from grounding however.
To comply with both requirements, the Foundation Fieldbus™ basically allows three different
types of shielding:
Shielding at both ends
Shielding at one end on the feed side with capacitance connection to the field device
Shielding at one end on the feed side
Experience shows that the best results with regard to EMC are achieved in most cases in
installations with one-sided shielding. Appropriate measures with regard to input wiring must
be taken to allow unrestricted operation when EMC interference is present. These measures
have been taken into account for this device. Operation in the event of disturbance variables as
per NAMUR NE21 is possible with one-sided shielding.
Where applicable, national installation regulations and guidelines must be observed during
the installation!
Where there are large differences in potential between the individual grounding points, only
one point of the shielding is connected directly with the reference ground. In systems without
potential equalization, therefore, cable shielding of fieldbus systems should only be grounded
on one side, for example at the fieldbus supply unit or at safety barriers, Fig. 3-2.