Series 302 FIELDBUS FOUNDATION – Installation, Operation and Maintenance Manual
1.18
It should be considered as intrinsic safety barrier the quantity of devices, the amount of cables and
the capacitance and inductance limit values for the Ex I installation.
Table 1.6 presents a brief comparison between the FISCO and FNICO models and the entity model.
FISCO
Entity Model
FNICO
Cable Length
1000 m - ia (*)
5000 m - ib (*)
1900 m
1000 m
Maximum Spur
Length
30 m(*)
120 m
30 m(*)
Cable and Length
Reactance
Non-considered
Considered
Non considered
(*)Maximum analyzed length. It is possible (the) to use (of) a larger length.
Table 1.6 –FISCO vs. Entity Model
There is a set of rules for applications in hazardous areas that uses intrinsic safety methods. The
fieldbus technology refers to the lengths of the segments, limits of currents in the power supply and
parameters as capacitance and inductance, as well as parameters of equipment failures. The
FISCO method provides an easy implementation for applications intrinsically safe in fieldbus, giving
flexibility, operational safety to the applications and reducing installation costs, since one can handle
up to 10 devices in an Ex network. Besides, the possibility of online handling simplifies
commissioning, startup and maintenance. More power means more devices and less cables, hence
less barriers.
Equipments compliant to FISCO can be directly connected to IS networks based on the entity
model. The reverse condition needs to be evaluated.
Following are described some key points that should be considered during the implementation,
involving classified areas and fieldbus:
•
Which is the hazardous area. Remember that non-incendive is only allowed in Division 2 areas
and intrinsic safety equipment only in Division Div 1 and Division Div 2 areas);
•
Which size and scalability required ;
•
Will there be short circuit protection for the main trunk and the spurs?
•
Which are the acceptable safety level and risks? (Projects involving intrinsic safety take into
account the components failure and allow maintenance while being powered, although without heat.
Conversely, non-incendive equipment doesn't allow maintenance being performed during powering
or even heat exchange.
•
Are there limitations to shutdowns?
•
Do the engineering and the maintenance teams have proven experience with hazardous and
classified areas?
•
Do all the devices have certification compatible with the application?
•
Are the facilities compliant to the area and country safety standards?
For more details consult IEC60079-27, “
Fieldbus Intrinsically Safe Concept
(FISCO)” and “
Fieldbus
Non-Incendive Concept
(FNICO)”.
Increasing the Reliability
There are several ways to increase the fieldbus network reliability. They are the following:
a) Power supply redundancy, according to Figure 1.21.
