FNICO Takes FISCO Into Hazardous Areas

For fieldbus installations in flammable atmospheres, each of the different explosion protection methods has its own cost and ease-of-maintenance merits. While intrinsic safety (IS) is not preferred in North American hazardous locations, the newest advances in IS practice can benefit fieldbus non-incendive concept (FNICO) installations in Class I, Division 2 hazardous areas.

11/01/2003


For fieldbus installations in flammable atmospheres, each of the different explosion protection methods has its own cost and ease-of-maintenance merits. While intrinsic safety (IS) is not preferred in North American hazardous locations, the newest advances in IS practice can benefit fieldbus non-incendive concept (FNICO) installations in Class I, Division 2 hazardous areas.

The springboard for these developments is the recent introduction of the Fieldbus Intrinsic Safety Concept (FISCO), which is described in IEC Technical Specification 60079-27. When compared with IS fieldbus systems installed using the conventional, "Entity" concept, FISCO simplifies the rules governing energy storage in field cables, and makes more power available to the fieldbus trunk. This arrangement means more fieldbus instruments can be connected in the hazardous area, and allows users to spend less time justifying and documenting an installation's safety.

FISCO was developed by the German approvals organization, PTB, which showed that, provided inductance and capacitance per unit length of field cables are within defined limits, the risk of spark ignition does not increase with the total length. This makes safety of the installation independent of cable lengths, and eliminates the need to calculate cable parameters or match them to maximum allowable figures for the power source.


Applying FISCO rules to non-incendive fieldbus circuits in Division 2 hazardous areas, in which the explosion hazard is expected only in abnormal circumstances, allows a relaxation of safety factors on voltage and current, while using similarities between IS and non-incendive. The resulting FNICO rules benefit fieldbus installations in Division 2 by allowing:

  • Freedom from explosion-proof junction boxes and conduit wiring systems;

  • Live-working throughout, on trunk and spurs, without gas clearance;

  • Simple safety documentation, such as a list of devices;

  • Freedom from cable parameter calculations, without flaunting the National Electrical Code; and

  • More field devices per trunk than with equivalent FISCO installations.

In addition, traditional area classification practice is presently being reappraised to focus on realistic zoning, instead of overcautious zoning. This is encouraging plant operators to seek ways to relax to Zone 2. So, while intrinsic safety remains the safest technique for any zone or division, non-incendive apparatus promises reduced capital and maintenance costs, as well as adequate safety in Division 2.

Non-incendive fieldbus protection

Because fieldbuses must be able to add or disconnect instruments without losing data or cutting power, certifying them in a hazardous area needs to allow some activity on powered wiring.

Though the term "non-incendive" is commonly applied to general wiring techniques for Division 2, using it in FNICO specifically means "energy-limited" in the same way as for intrinsic safety. This is different from Division 2's "non-arcing" circuits, which permit no arcing or sparking. Energy-limited circuits may be live-worked while under power and in the presence of a flammable atmosphere. This is justified because normal operation includes opening, shorting and grounding of the field wiring. Non-incendive differs from intrinsic safety in the factor of safety applied and consideration of faults (see Table 1):

Consequently, FNICO fieldbus circuits may be opened, shorted or grounded without risk of ignition, and without the need for gas monitoring. Moreover, the same rule applies to the trunk and spurs, so that it's not necessary to apply different, and potentially confusing, maintenance procedures to different parts of the field circuit.

FNICO system assembly

The key physical-layer components of a typical FNICO installation (see illustration) are its power supply, cables, termination components and its fieldbus instruments.

In simple cases, the FNICO power supply is located at the interface between the safe and hazardous areas. Connections to the host control system are made at its "safe area" terminals, and those to the field trunk occur at its "hazardous area" terminals. It combines functions required for reliable fieldbus communications, which are defined by the appropriate FOUNDATION fieldbus standards, with those of an energy-limited interface. This is similar to building an IS barrier into the output of a fieldbus power supply, though in this case the barrier complies only with requirements for Division 2 defined in Table 1. Voltage, current and power available at the supply's output, and the number of field instruments it can support, are determined by the Gas Group for which it's designed.

A typical FNICO power supply incorporates a repeater function, connections for 24 V dc supply input, and a switchable terminator. If installed in a suitable enclosure, it can be located in a Division 2 hazardous area, which adds flexibility and allows for more complex topologies.

Likewise, a FNICO system's cables are defined by the normal operational requirements for FOUNDATION fieldbus H1, and by the energy-storage considerations required for safety. However, neither imposes an onerous burden. Operationally, cables acceptable for safe area installations can be used. These can include special types for fieldbus and, with some length limitations, existing cable from legacy analog installations. Safety constraints for FNICO cables are the same as those for FISCO, and are met by most conventional cable types (see Table 2):

Meanwhile, the growth of fieldbus has led to the introduction of specially designed wiring hubs. These are usually modules with pre-assigned trunk and spur connections, possibly built into a field-mountable junction box. These save on installation time and prevent the complexity of assembling an equivalent unit from DIN rail-mounted terminals. In a FNICO system, requirements are simple: the wiring hub and its enclosure must be suitable for Division 2, and appropriate for its installed environment. This is not difficult because approved wiring components are available for installation in a junction box of the user's choice. Notably, explosion-proof enclosures are not necessary in Division 2.

Finally, FNICO benefits from relaxed regulations for hazardous areas of abnormal or infrequent hazard by using IS-certified fieldbus instruments. Basically, IS instruments can be used in a FNICO system. The safety case is easily made because an IS instrument is designed to be safe in Zone 1 or 0, and so it exceeds any constructional requirements for Division 2. It has maximum values of input voltage, current and power that must not be exceeded, while the FNICO power supply has equivalent values assigned to its output. Compatibility is established by comparing the two sets of figures, but it's only the voltage parameters that need to be satisfied because maximum current and power are never delivered in normal operation. FNICO systems can therefore be assembled using IS certified devices, and devices with both "Entity" and FISCO approval can be used.

Documenting FNICO's safety

The rules for describing FNICO system safety closely follow those for FISCO. For example, there's no need to calculate maximum permitted cable lengths, provided the cable complies with Table 2. The steps for documenting FNICO's safety include:

  1. Confirm that the cable conforms to the FISCO/FNICO requirement;

  2. Check that all field instruments carry non-incendive or intrinsic safety approval;

  3. Check that field wiring components and terminators carry suitable approval;

  4. If the field instruments are not FISCO or FNICO certified, check that the maximum safety input voltage (V max) of each instrument is greater than the output safety voltage (Voc) of the FNICO power supply.

  5. List components included in each trunk, noting manufacturer, model number, method of protection, ambient temperature and temperature classification.

In addition, the "simple apparatus" concept is recognized for use in non-incendive circuits by the National Electrical Code, which uses the same definition for IS circuits. This means that apparatus, such as passive wiring components and enclosures, don't contribute energy to the non-incendive circuit, and so they can be included without modifying the safety analysis.

In summary, as use of fieldbus accelerates in the process industries, there will be a growing emphasis on finding workable solutions for hazardous areas. Those delivering the most flexibility and lowest life-cycle costs will emerge as favorites.

Non-incendive wiring, embodied by FNICO, is the intelligent, forward-thinking approach to Division 2 wiring, and not based on mechanical protection or a flaunting of the National Electrical Code. Though it's recognized that common Division 2 practice is to work live anyway, or maintain constant gas monitoring, FNICO will be favored by users seeking a sure, documented approach to hazardous area safety.

The speed with which FNICO becomes established will depend on the extent to which the FNICO Technical Specification is absorbed into national standards, and then recognized by field instrument manufacturers. As of May 2003, the draft specification was out for informal review by national certifying authorities in Europe and the U.S.

Comparison of requirements for intrinsic safety and non-incendive

Protection method

Safe with how many faults?

Factor of safety applied to current

Division of use

Intrinsic safety

1

1.5

1 and 2

Non-incendive

none

1.1

2


FNICO cable requirements

Parameter

Value

Loop resistance

15 V/km to 150V/km

Loop inductance

0.4 mH/km to 1 mH/km

Capacitance

80 nF/km to 200 nF/km

Maximum length of each spur cable

30 m in A,B and C,D

Maximum length of each trunk cable

1 km in A,B and 5 km in C,D





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