wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Foundation Fieldbus 35-7
35.9 Installation of Segments in Classified Areas
Because FF requires that the device be connected to a network to perform maintenance, the majority
of fieldbus installations with equipment in classified areas are using the IS technology. IS technology
allows “live working,” meaning all field work can be done “hot,” with the network energized and communications
with the host at all times. “Live working” means you can work without continuous gas
testing and is therefore the safest way to protect your plant. An alternative to IS, or its equivalents as
described below, is to use explosion-proof technology.
One advantage of fieldbus technology and digital communications, however, is that a significant portion
of the work that in the past had to be done at the device for analog systems (i.e., range change) can
now be completed over the network without actually having to go to the physical device in the field.
IS technology is based in the reduction of the energy to the field to levels below the ignition curves
shown in Figure 35.4, plus a safety margin. For analog circuits, this is typically done using IS barriers;
however, in the case of FF, this circuitry is either incorporated in the fieldbus power conditioner or in
the fieldbus barrier mounted in the field junction box.
A typical FFPS delivers approximately 24–28.V DC and currents of up to 360.mA power to the field
segment. This is well above all the explosion limit curves shown on Figure 35.4. FF offers two additional
technologies other than an IS power supply that is limited to approximately 80–110.mA at around 12.V
to address this problem. One solution is with IS-equivalent power supplies and the other is to reduce the
available energy level in the field using the above-mentioned fieldbus barrier technology.
35.9.1 FISCO—Fieldbus Intrinsically Safe COncept
Unlike the IS entity concept, which is derived from theoretical calculations, the FISCO is based on
actual field trials and experiments by the Physikalisch-Technische Bundesaanstalt (PTB) research center
in Germany. Because this IEC-60079-15 standard is based on experimentation, all installations must
operate within the maximum limits within which the experiments were conducted. These limits are that
the total cable length of the system is limited to a maximum of 1000.m in IIC/Groups A, B gases and
1900.m in IIB/Groups C, D (limited by FF-831 and therefore identical to any fieldbus system). The
maximum spur length for any FISCO installation is 60.m per spur.
Because the FISCO power supply must operate at voltages below the ignition curve, FISCO systems
are not able to connect to trunk and cable combinations that are as long as non-IS systems. Despite this,
FISCO is able to support trunk lengths greater than 500.m with full system current loads. It should be
500
Power supply characteristics
Short circuit current (mA)
400
300
200
100
12 14 16
IIB non-incendive
IIB intrinsically safe
IIC non-incendive
IIC intrinsically safe
18
Open circuit voltage (V)
FIGURE 35.4
Power supply characteristics.
© 2011 by Taylor and Francis Group, LLC