23
FISCO Rules
The FISCO Concept allows the interconnection of intrinsically safe apparatus to associated
apparatus not specifically examined in such combination. The criterion for such interconnection is
that the voltage (Vmax), the current (Imax) and the power (Pmax) which intrinsically safe
apparatus can receive and remain intrinsically safe, considering faults, must be equal or greater
than the voltage (Uo, Voc or Vt), the current (Io, Isc or It) and the power (Po) which can be
provided by the associated apparatus (supply unit). In addition the maximum unprotected residual
capacitance (Ci) and inductance (Li) of each apparatus (other than terminators) connected to the
Fieldbus must be less than or equal to 5nF and 10uH respectively.
In each I.S. Fieldbus segment only one active source, normally the associated apparatus, is
allowed to provide the necessary power for the Fieldbus system. The allowed voltage (Uo, Voc or
Vt) of the associated apparatus used to supply the bus cable must be limited to the range 14Vdc
to 24Vdc. All other equipment connected to the bus cable has to be passive, meaning that the
apparatus is not allowed to provide energy to the system, except a leakage current of 50µA for
each connected device. Separately powered equipment needs galvanic isolation to ensure the
intrinsically safety Fieldbus circuit remains passive.
The cable used to interconnect the devices needs to comply with the following parameters:
Loop resistance R': 15....150
Ω
/km
Inductance per unit length L':0.4....1mH/km
Capacitance per unit length C': 80....200nF/km
C' = C' line/line+0.5 C' line/screen, if both lines are floating
or
C' = C' line/line + C'line/screen, if the screen is connected to one line.
Length of spur cable: max. 30m
Length of trunk cable: max. 1km
Length of splice: max = 1m
Terminators
At the end of each trunk cable an FM Approved line terminator with the following parameters is
suitable:
R= 90....100
Ω
C = 0 ....2.2µF
System evaluation
The number of passive devices like transmitters, actuators, connected to a single bus segment is
not limited due to I.S. reasons. Furthermore, if the above rules are respected, the inductance and
the capacitance of the cable need not be considered and will not impair the intrinsic safety of the
installation.
Notes.
1. The intrinsic safety FISCO concept allows the interconnection of FM Approved Intrinsically
Safe devices with FISCO parameters not specifically examined in combination as a system when:
Uo or Voc or Vt
≤
Vmax, Io, Isc or It
≤
Imax, Po
≤
Pi."
