More Fieldbus Power for Hazardous Location ApplicationsOctober 1, 2005 By: Bernd Schuessler, Pepperl+Fuchs Sensors
Process industry engineers who use fieldbus in their hazardous location applications (Class I, Division 1 and Class I, Division 2) are looking for additional power that will let them connect more instruments to their fieldbus segments. Three emerging methods promise to help meet this need: FISCO (Fieldbus Intrinsically Safe COncept), FNICO (Fieldbus NonIncendive COncept) and the High Power Trunk concept.
Entity, FISCO and FNICO
When fieldbus technology was first introduced, the entity concept with cabinet-mounted barriers and power supplies was standard for hazardous areas. But it supplied enough power for only three or four instruments per segment, and matching entity parameters of devices and power source was quite cumbersome. Because the entity model was based on the cable's concentrated inductance and capacitance, less electrical power could be transmitted into the hazardous area than became possible with subsequent models. Furthermore, for each configuration or addition of a fieldbus device, intrinsic safety usually had to be rechecked.
The FISCO model was therefore developed for Class I, Division 1 applications to verify intrinsic safety without requiring a parameter evaluation. FNICO was created for Class I, Division 2 applications to simplify the certification of a nonincendive fieldbus segment. The two models have certain features in common:
- 1. The fieldbus is based on the physical layer per IEC 61158-2 (ISA S50.02).
- 2. There is only one active source (power supply with trapezoidal output characteristic), which is certified in accordance with FISCO/FNICO per system. All other devices act as passive current sinks.
- 3. The basic current consumption of a device is ≥10 mA.
All devices must meet the following requirements:
Ui / Vmax ≥o / Voc of power source
Ii / Imax ≥ / Isc of power source
Pi / Pmax ≥o of power source (FISCO only)
Nodes have negligible internal inductance and capacitance:
Ci ≤5 nF, Li ≤10 μH
- 1. Maximum cable length is 1000 m
- 2. Maximum spur length is 60 m
- 3. The fieldbus cable must have the following characteristics:
15 Ω/km < R' <150 Ω/km
24.1 Ω/mi. <R' <241 Ω /mi.
0.4 mH/km ≤L' ≤1 mH/km
0.64 mH/mi. ≤L' ≤1.61 mH/mi.
80 nF/km ≤C' ≤200 nF/km
129 nF/mi. ≤C' ≤322 nF/mi.
- 1. The bus segment must be terminated on both ends.
- 2. The type of devices to be connected to a FISCO- or FNICO-approved barrier are shown in Figure 1.
Note that most FISCO devices can also be used with the entity concept, but neither FISCO nor FNICO models consider the cable capacitance and inductance to be concentrated, as in the entity model. More devices can therefore be connected to the bus in a FISCO or a FNICO system than in an entity system.
Figure 1. Devices Connectable to FISCO- and FNICO-Approved Barriers.
Not a Complete Solution
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