1 December 2003
No lightning in these skies
Foundation fieldbus manufacturers have developed several options for wiring instruments in hazard ous areas.
By Jonas Berge
Foundation fieldbus (FF) uses wiring according to the IEC 61158-2 standard. Over the past few years this bus technology has become phenomenally popular, and this has prompted manufacturers to develop several solutions for using FF in hazardous areas.
Most of the development is toward maximizing the number of devices per wire. In fact, Profibus PA uses the same wiring standard. Here we cover both European and North American terminology. North American terms are in parentheses.
Flameproof Ex d is one way of installing in hazardous areas, but this method is at a disadvantage in fieldbus, because it is necessary to power down the entire network to service one device. Ex ia and Ex nL enable online work.
Explosive gas is the mix
Intrinsic safety is a technique to ensure the safety of a plant when instruments are located in hazardous areas where explosive gases may be present.
This happens by limiting the voltage and current supplied to the device in the first place. Also, there are restrictions on the stored energy in the system, which could release through a fault of the system.
For industrial process applications, gas group IIC is the most easily ignited, IIB the next, and IIA the least easily ignited. Group IIC contains hydrogen and acetylene.
Group IIB contains gases such as ethylene. Group IIA contains gases like propane. The energy limit, which must be applied to an instrumentation system, depends on which groups of gases are or could be present.
Most users specify gas group IIC certified instruments, to cover all requirements with a single range of products, and most instrument manufacturers design to that requirement.
For traditional analogue instruments, this was not a problem. Even for this most stringent gas group, the power available under IS rules is quite adequate to power a single 4–20 mA device.
However, for a multidrop fieldbus, the restriction of power is a serious limitation. For gas group IIC applications, only about 80 mA is available from a 19-volt power supply.
Unfortunately, designers of loop-powered instruments have generally been unable to reduce the power consumption of their devices to the recommended 10 mA. As a result, it has been possible to connect only three or four devices to a single bus segment.
A main advantage of intrinsic safety is that it is possible to disconnect and connect instruments, spurs, and the trunk while the power is still on. This simplifies commissioning, maintenance, and expansion, as the bus and the associated loops can be running while work takes place. It is possible to do this work without having to obtain a gas clearance certificate. Note that for intrinsic safety there exists both Ex ia, tolerating two faults, and Ex ib, tolerating only one fault. Ex ia works in Zone 0, while Ex ib only works in Zone 1.
The basic concept of intrinsic safety is the same for fieldbus as for a conventional installation. The main difference is that in fieldbus several devices connect to one barrier.
For IEC 61158-2 all devices operate on 9-32 volts direct current (VDC), being electrically almost identical even for input and output devices and analog as well as discrete; therefore only a single barrier type is required. The main difference between field devices from different manufacturers is their power consumption; some are as low as 12 mA. Because the power available to an intrinsically safe bus has limits, it is important to select devices with low power consumption to enable as many devices as possible to connect to each barrier. The power consumption is the main limiting factor to the number of devices on the intrinsically safe segment, pushing the limit well below 32 devices. However, barriers may be multidropped, still resulting in 16 devices per interface port. The barrier may be a zener barrier or an intrinsically safe galvanic isolator. Intrinsically safe fieldbus devices are current sinking and do not provide power to the network.
Typically the barriers are in the safe area. If installed in the hazardous area, a flameproof enclosure with a flameproof seal needs to be a part. There are two schemes for supplying intrinsically safe power: the traditional entity concept and the newer fieldbus intrinsically safe concept (FISCO) model. FISCO provides more power, thereby enabling more devices and longer cable.
The main advantage of intrinsic safety is that it permits devices to disconnect and connect while under power. This is important for fieldbus, because many devices get their power from the bus, making it very disruptive to switch off the power. The drawback of intrinsic safety is that very little power is available, which puts limitations on some devices. For example, intrinsically safe fieldbus solenoid valves are not as robust as their regular counterparts. The limited power also means that active current limiting electronics for short-circuit protection are not suitable for intrinsic safety, because during short-circuit current the draw is as high as 55–60 mA, which kills the bus anyway. Moreover, they also have slight power consumption during normal operation for which there usually is no margin.
USING THE ENTITY CONCEPT
The entity parameters for voltage, current, power, capacitance, and inductance stated in the approval certificate for intrinsically safe devices and barriers make it easy to select matching devices and barriers. Because several devices are multidropped off a single barrier, it is necessary to compile the entity parameters of all devices to match against the barrier. In the traditional entity concept the cable capacitance and inductance are concentrated and therefore must count when considering the total capacitance and inductance for the hazardous side segment of the network. For Ex ia IIC (groups A and B) the output power is approximately 1.2 watts or some 60 mA at 11 VDC. Due to the current limit only a few devices can connect to each linear barrier. Similarly, the low voltage output limits the cable length, as only a small voltage drop can occur.
It is necessary to select a barrier that has voltage, current, and power output lower than the field device with the lowest corresponding entity parameter. The barrier must be able to handle the total external capacitance and inductance of all the devices connected to the safe side plus the network cable. Normally it is the cable capacitance that is the limiting factor for distance in intrinsically safe installations based on the entity concept. An easy way to evaluate the network is to make a table of the entity parameters for all network components.
Using the entity concept the maximum number of devices on a barrier designed for gas group IIC (groups A and B) is about four devices. Gas group IIB includes gases less easy to ignite than those in group IIC, and therefore has less stringent limits on energy. From a barrier designed for gas group IIB (group C) more power is available, making it possible to add many more devices on the bus. If acetylene or hydrogen may be present in the area, it is necessary to choose a barrier for group IIC (groups A and B); otherwise a barrier for group IIB (group C) is possible.
The presence of hydrogen is not uncommon in many plants, and hence a IIB (group C) barrier is not possible in many installations. Most installations indeed have areas where hydrogen and acetylene are present and other areas where they are not. Therefore it is possible to designate some areas in the plant as IIC and others as IIB. However, users are traditionally reluctant to do so, because it is extra work to identify which area is which, and it means two different sets of barriers and equipment.
Most field instruments only handle IIC power levels and therefore cannot connect directly to a IIB barrier. The reason for this is again that IIC is more popular. However, barrier manufacturers have come up with innovative schemes to bring down the power level at the device end. In this scheme, IIB level of power on the trunk, as much as 350 mA at 18 volts, is provided through the barrier in the safe area, permitting a long trunk wire run into the field and many devices on the same network without repeaters. This amount of current is easily sufficient for 16 devices. The junction box contains a resistor that brings the power on the spurs down to a level suitable for IIC devices. In this two-step solution, standard devices can use group IIC in greater numbers with a longer trunk.
The system designer needs to identify which area is IIB, i.e., where the trunk is permitted to run and where the IIB junction box can be mounted. However, many plants assume the entire installation is IIC simply to avoid taking the trouble to find out what it really is. However, there are benefits in taking the trouble to figure out where IIB is sufficient. Moreover, because there is so much power available, it is often unnecessary to make any voltage drop calculations.
CHECK POWER LIMIT OF DEVICES
In the FISCO model, the cable capacitance and inductance are not concentrated nor unprotected as long as the cable parameters are within given limits. For the same reason, FISCO-type barriers have no specified permitted capacitance or inductance. FISCO barriers have a trapezoidal output characteristic providing 1.8 watts of output power for Ex ia IIC (groups A and B), which in turn enables more devices to connect than a traditional entity barrier does. Most FISCO devices can also be for entity concept; so check the device’s various certifications.
Not all FISCO-approved devices can handle 1.8 watts. There are FISCO model barriers available that provide only 1.2 watts of output power, suitable for devices with a low power rating. Be careful to check the power limit of devices. Low-power FISCO equipment may go under the name small FISCO or fisco. Low power fisco barriers power fewer devices but still have the benefit of longer cable. They also eliminate the need to calculate inductance and capacitance.
FISCO-certified devices have low capacitance, and inductance is considered negligible—less than 5 nanofarads and 10 microhenries, respectively. Cables with parameters within the ranges specified can work in FISCO installations for lengths up to 1 kilometer (3,300 feet) with a maximum spur length of 30 meters (100 feet). Therefore there is no need to make the analysis for the cable and devices in the network for these parameters.
The maximum cable length is calculated based on voltage drop as it is for a safe-area installation, because there is no capacitance or inductance limitation up to 1 kilometer as long as one heeds specified cable parameters.
It is important that the barrier and field devices are all FISCO certified. Non-FISCO barriers and devices cannot be used in FISCO-style installations. FISCO field devices must be able to handle the high power output of a FISCO barrier. To be compatible with a typical FISCO barrier the Pi (Pmax) of the device should be larger than the typical 1.8 watts provided by the barrier.
Another advantage of FISCO is that device replacement is simpler, because matching is easier. Devices that only have entity approval cannot connect to a FISCO bus. However, barriers exist that connect between a FISCO bus and an entity device, enabling use of entity devices in a FISCO system.
Using the FISCO concept the maximum number of devices on a barrier designed for gas group IIC (groups A and B) is about eight devices. From a barrier designed for gas group IIB (group C) more power is available, making it possible to add many more devices on the bus. FISCO devices designed for use with IIB (group C) barriers must be able to handle 5.32 watts.
Because each safety barrier, in particular the linear barrier, only connects a few devices, it would be rather uneconomical to connect only one barrier per communication port. Some devices have a current consumption so high that only two can connect to a linear barrier. One may use barriers with built-in repeaters. This makes it possible to run one safe-area segment from the communication port to several barriers, each one with a hazardous-area segment, thus forming a larger network allowing a full 16 devices per network. Built-in repeaters eliminate the need for external repeaters.
ENERGY LIMITED, NONSPARKING
Live connection and disconnection is necessary for easy maintenance of multidropped fieldbus instruments. Like intrinsic safety, the nonincendive concept also permits live work on the instruments but is restricted to use in Zone 2 (Division 2). However, Zone 2 is sufficient for many installations, making nonincendive a possible option. In fact, if there are Division 1 areas in your plant you may actually be breaking environmental emission laws, and therefore such areas are becoming less common. Unlike Ex ib and Ex ia intrinsic safety that consider one and two faults, respectively, energy limited does not consider faults.
A regular power supply and power supply impedance can be used to provide as much as 1,000 mA on the trunk. This permits many devices on the same trunk without repeaters. The higher voltage permits longer wires. Unlike intrinsic safety, it is possible to use redundant power, making the fault tolerance of nonincendive better than it is for intrinsic safety.
The junction boxes in the field contain short-circuit energy limits for each spur, limiting the current through the spur to 30–60 mA. The high-power trunk connections need mechanical protection to prevent involuntary separation due to vibration, as well as warning labels and guards requiring special removal tools to avoid unintentional live disconnection by the maintenance technician. No protection is required for the spurs. In other words, the trunk is nonsparking (nonarcing) Ex nA, while the spur is energy limited (nonincendive) Ex nL.
Live work is permissible only on the current-limited or fused spurs. Live work on the trunk is not permissible without gas clearance. For most practical purposes this is OK, because work on the trunk is very rarely required.
When fieldwork is required, it is typically on the current-limited spurs to disconnect a device for maintenance or calibration, or to reconnect it after service.
As to FF nonincendive concept (FNICO), it builds on the same concept as FISCO, but is based on nonincendive technology rather than intrinsic safety. Therefore it is only applicable to Zone 2 (Division 2). FNICO puts the current limitation on the trunk without any further limitations on the spur. The current on the trunk is therefore lower than the scheme of limited spurs and unlimited trunk, but higher than it is for FISCO. FNICO uses a safety factor of 1.1 as compared to the more stringent 1.5 for FISCO. Technicians can work FNICO spurs and trunks while they are live.
For Ex nL it is necessary to match capacitance, inductance, current, voltage, and power just as it is for intrinsic safety, but like FISCO, FNICO makes this much easier by eliminating the need to match the capacitance and inductance within the stated limits for cables and devices. The cable and device limits are the same as they are for FISCO. One can add or change devices out of the network without recalculation.
A drawback of intrinsic safety, particularly the entity concept, is that the length of the trunk is limited. One way to overcome this problem is to combine intrinsic safety with other technologies. For example, several suppliers make use of an increased safety Ex e trunk that brings lots of power into junction boxes in the field and that contains intrinsic safety barriers. One can install increased safety junction boxes with barriers in Zone 1.
A regular power supply and power supply impedance can provide as much as 1,000 mA on the trunk. This permits many devices on the same trunk without repeaters. The higher voltage permits longer wires. The high-power trunk connections need mechanical protection to prevent involuntary separation due to vibration, as well as warning labels and guards requiring special removal tools to avoid unintentional live disconnection by the maintenance technician.
Junction box solutions available in the market have either a single barrier shared by four connections, or four independent barriers. The scheme with the four individual barriers has built-in terminators, which limits the spur cable to 120 meters. IT
Behind the byline
Jonas Berge is the author of Fieldbuses for Process Control Engineering, Operation and Maintenance, ISA Press. Write him at firstname.lastname@example.org.