Tuesday, July 3, 2012
Foundation Fieldbus Concepts-2
EPC School 2012 Series
I for Instrumentation
I for Instrumentation
Foundation Fieldbus Loop
FIELDBUS WIRING METHODS:
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 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 L enable online work.
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. Traditional Fieldbus IS applications designed as per entity concept had several limitations. The power supplies supplied enough power for barely 3-4
instruments per segment and it was very cumbersome to match the entity parameters of the devices and the power source. Due to the limited DC power to be shared by a number of field devices, long cable runs and terminators storing capacitive energy, the traditional intrinsic safety installation and
interconnection rules restrict the application of entity based IS systems. FISCO and FNICO were developed in order to find a solution to provide more power over a Fieldbus into a hazardous location while still keeping the energy level below that which could cause an explosion.
Foundation Fieldbus JB terminations in field
FISCO was developed by PTB in Germany and is based on experiments performed by PTB. In 2002, the IEC 60079-27 standard was published, which describes the FISCO model. FISCO is applicable to Class1, Div 1 locations. Power supplies, terminators, cable and field instruments for use with FISCO
systems must comply with the requirements defined in IEC Technical Specification 60079-27. Where the individual components have been designed and certified according to these requirements and carry FISCO marking where appropriate, they may be assembled without further approval of the overall
combination. The criteria for such interconnection is that the voltage(Ui), current(Ii) and the power(Pi) which the intrinsically safe apparatus can receive and still remain intrinsically safe, considering faults, must be equal or greater than the voltage (Uo), current (Io) and the power (Po) which can be delivered
by the associated apparatus (power supply unit). In addition, the maximum unprotected residual capacitance (Ci) and inductance (Li) of each apparatus (other than the terminators) connected to the fieldbus must be less than or equal to 5 nF and 10 microH respectively.
In each IS fieldbus segment, only one active device, normally the associated apparatus (power supply), is allowed to provide the necessary power to the fieldbus system. The allowed voltage Uo of the associated apparatus used to supply the bus is limited to the range of 14VDC to 17.5 VDC. All other
equipment connected to the bus cable have to be passive, meaning that except for a leakage current of 50 microA for each connected device, the apparatus is not allowed to provide energy to the system.
FISCO offers the ability to connect and disconnect the field wiring in the hazardous area while under power and without gas clearance certificates.
FISCO is based on the following conditions:
1. The Fieldbus must be based on the Machester Bus Powered physical layer in accordance with
IEC 61158-2
2. Only one active power source (ie., power conditioner) is permitted per segment. All other components
act as passive current sinks (instruments)
3. The basic current consumption of a field device is atleast 10 mA.
4. The electrical parameters for various devices shall be as follows
5. Total cable length is limited to 1000m for gas group IIC and 1900 m for gas group IIB (trunk plus all
spurs)
6. Maximum spur length is 60 m for all gas groups
7. The Fieldbus cable must conform with the following parameters
Typical FISCO power conditioners can supply 12,8 V and 100 mA for Class 1, Div 1, groups A-D. This practically results in 4~8 devices per segment. This is still far below the capabilities of a general purpose Fieldbus installation. If in a given application, all devices are certified in accordance with FISCO and the above mentioned conditions are fulfilled, intrinsic safety check in the loop can be carried out by the following comparison.
FNICO (Fieldbus NonIncendive COncept)
Continued....
Labels: control valve segment surge FF cable FF spur fieldbus Rahul Kapoor HART cable Foundation trunk, fieldbus segment, FISCO, FNICO, loop
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