By Ian Verhappen
The Fieldbus Intrinsically Safe Concept (FISCO) provides the live working benefits of Intrinsic Safety (IS) without many of the limitations associated with the intrinsic safety (IS) entity techniques. These burdens usually include extensive calculations and restrictive power limitations, which are two main reasons why FISCO often is selected for installations in classified areas.
In addition, because fieldbus applications need more complex multipoint connections (fieldbus power supply and multiple devices) than analog 'point-to-point' signals, traditional IS entity calculations were very cumbersome.
As a result, manufacturers approached the German Physikalisch Technische Bundesanstalt (PTB) to develop the FISCO model. Through a series of tests, the resulting PTB-W-53e report determined that using only one active power supply (load-sharing redundant power supplies are not supported) on the bus under the test conditions of trunk and spur length of cable with a range of characteristics, the cable parameters and internal capacitances and inductances of the devices have no negative influence on the intrinsic safety of the system, and therefore can be neglected.
In 2002, the results of this work became a technical specification (TS IEC 60079-27) and, in 2005, a full IEC standard (IEC 60079-27). Fieldbus Non-Incendive Concept (FNICO) also was published as part of the first Edition of IEC 60079-27. Table 1 summarizes the limiting conditions of a FISCO compliant system.
The IS standard described the requirements for 'Ex ia' for zones 0, 1 and 2 and 'Ex ib' for zones 1 and 2. The fifth Edition of IEC 60079-11 was published in July 2006, and added the classification explosion protection 'Ex ic' suitable for use in Zone 2 only. The newest editions of the IEC standards specify that FISCO 'ia' is suitable for Zones 0, 1 and 2; FISCO 'ib' can be used in Zones 1 and 2; and FNICO can be applied in Zone 2. As the name implies, FNICO was based on non-incendive principles, and therefore in the second edition IEC 60079-11 of is to be replaced by FISCO 'ic.' Apparatus designed and approved to the FNICO requirements of the first edition of IEC 60079-11 may be used in an 'ic' FISCO system, which provides backwards compatibility through grandfathering because existing FNICO installations will continue to be allowed to use FNICO-approved components in applications today and also after the new edition of IEC 60079-15 is published, likely in 2011.
The FISCO approach is inherently a system concept as there is only a single source of possible energy (the fieldbus power supply) into the flammable atmosphere, and all other devices and equipment must not put energy on to the bus—hence, the low capacitance and inductance limits for the field devices. The T4 temperature rating also is not a significant constraint since this temperature is well below the T6 rating for most hydrocarbon industry gases.
Despite the above statement regarding a single source of energy, it is possible to connect separately powered devices to a FISCO network provided the bus terminals remain passive and do not present more than the internal inductance and capacitance limits defined in Table 1 to the network.
FISCO power supplies must comply with the Uo, Io, Po, Ci, and Li parameters of Table 1. However, there is no specification relative to the maximum permitted external capacitance Co or Inductance Io. All FISCO power supplies can be either linear or trapezoidal output for category 'Ex ia' installations (Gas Group A through D in North America), while for gas group 'Ex ib' (Gas Group C/D in North America) a rectangular output power supply can be used to make higher energy levels possible. Most FISCO units are trapezoidal as that allows for more energy than linear, which is what must be used for IS barriers and isolators.
It should also be noted that FISCO power supplies can, in many cases, also be used as repeaters to boost a signal, and so get an additional 1000 or 1900 meters extension of the network. Alternately, and as demonstrated the AG-163, they can also be wired in parallel to make more effective use of a single H1 port by increasing the number of devices to which it is connected.
However to determine if you need to use a repeater, you need to determine the maximum trunk length as a function of system load. The following formula combined with the mandatory FISCO documentation, as shown in Table 2, can do so with minimal effort on your part.
where R = Ω/km x Length of cable and
Vd = voltage at "lumped load" device
Vp = voltage at power supply terminals
Id = current requirement for device d
Idmin = network device with minimum current draw
ISC = spur short-circuit current draw of field device coupler
IHH = current budget for handheld communicator (typically 10 mA)
IF = current draw of field device coupler
These values can be obtained from the manufacturer's data sheets and recorded as part of the process of completing Table 2.
Therefore, the calculation will consist of the following steps:
- Verifying that the sum of the device currents is less than the available current available from the FISCO Power Supply
- Confirm that the lengths of the trunk cable and spurs is less than the maximum in Table 1
- If you suspect that you may be approaching the lower limit of 9 volt operating voltage due to voltage drop in the trunk (i.e. trunk is greater than approximately 400 meters) substitute the necessary values in the formula above. Also, if the formula results in Vd ≤ 9 volts, it may be necessary to perform a detail calculation to confirm the actual estimated voltage at each device. Fieldbus power supply companies all offer free downloadable calculators to do these calculations.
It should be noted that the IEC standard only stipulates that the columns highlighted in gray (at the top of Table 2) need to be documented. I believe you can now see how FISCO really is fortified IS—more power means more devices can be connected to a single H1 port. This is especially true now that FNICO is part of this standard, is easier to use because it supports "live working," and is easy to design because, if you remain within the design parameters, then the calculation consists of simply completing a table and perhaps for insurance purposes performing a simple worst case 'lump sum' calculation.
Ian Verhappen is an ISA Fellow and Certified Automation Professional. He blogs for ControlGlobal.com as "The Great Kanduski" and operates Industrial Automation Networks Inc., a consultancy specializing in industrial communications, process analytics and heavy oil/oil sands automation. He can be reached at firstname.lastname@example.org.
A commonly used alternative to FISCO and FNICO is the "Fieldbus Barrier or High-Energy Trunk" concept which uses an 'Ex e' trunk. This means it is not "live workable" to provide the option of higher voltage and current levels from the fieldbus power supply (typically 24 - 28 volts and 350 - 500 mA). This enables both more devices to be connected to the network, and longer trunk lengths because of the higher starting voltage. The resulting High-Energy Trunk powers the Fieldbus Barrier, which contains the necessary electronics, such as isolation and IS circuitry to make each of the spurs IS/FISCO compliant. High-Energy Trunk effectively separates the FISCO power supply into two parts, with the power conditioning circuitry in the power supply and the IS circuitry located in the fieldbus barrier in the field. Rather than the H1 trunk connecting these two sets of circuits being on a circuit board, it is replaced with the H1 Trunk cable.
Another new technology in development by PTB and others, as an alternative to FISCO is Dynamic Arc Recognition and Termination (DART), which uses active electronics at both ends of the trunk cable to monitor for the potential creation of a spark, and then isolate the circuit from the power before it reaches a level that could cause a spark and hence ignition. More on DART is at www.technology-dart.com/en/index.php.