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The answer here is “why not” choose Intrinsic Safety since a fieldbus power supply is required for Exd, whilst a Fieldbus Non Incendive Concept (FNICO) power supply is required for FNICO (Zone 2 only) and a Fieldbus Intrinsically Safe Concept (FISCO) power supply for Zone 1.
Zone 0 applications can be accommodated by additional hardware and use of the Entity concept. The Entity concept limits the number of devices on a segment to about 4.
FISCO (Zone 1 and 2) , power supplies for different gas groups are available, these support the following number of instruments (based on MTL data).
(1) Gas group IIC Power Supply - you can connect approximately 8 instruments in a segment, this is based on a current draw of 15mA per instrument (this must be checked) and total current availability of 120mA and if bandwidth is not an issue.
(2) Gas group IIB Power Supply - 265mA available which can support up to 16 instruments
There is a trunk and spur length restriction on FISCO network which you need to be aware of, for MTL power supplies this is 1.0 km for a IIC gas group and 1.9km for a IIB gas group per standard FF816 and AG163 and 60m per IEC 60079-27 respectively. For more information the FF Intrinsic Safety Application Guide AG163 can be obtained from the Fieldbus Foundation.
The maximum permitted trunk length is then determined from the requirement to have a minimum of 9 volts available at the field device terminals. The trunk length results after applying Ohm's law to the combination of cable resistance (usually 50 ohms per kilometer), current flow, and the minimum power supply voltage.
There are many permutations of the possible combinations, but if an average field device current is about 15 mA, then a typical IIC power supply can feed eight devices at the end of 500 meters of trunk. Typical worked examples can be found in the Fieldbus Foundation document AG 163.
FNICO (Zone 2 only) - provides more power on the trunk, this again is dependant on the relevant gas group.
(1) Gas group IIC Power Supply - you can connect approximately 12 instruments in a segment, this is based on a current draw of 15mA per instrument (this must be checked) and total current availability of 180mA and if bandwidth is not an issue.
(2) Gas group IIB Power Supply - 320mA available which can support up to 20 instruments
Of course with the Exd and FNICO view of the world one can get more devices on a segment, however realistically more than 12 should not be installed anyway (even then the design has to be carefully scrutinised) so there is very little difference.
for IS there is minimal change in cabinet space requirements, hardware and
benefits are really cost and HSE related as follows:
Non armoured cables can be used (in cable racks), however armoured cable should be used in high traffic areas where damage may occur, however as the instruments are likely to be close coupled this would be a rarity.
in some installations a ”cold” work permit is required for working on IS, it
is not really necessary as the concept means that it is quite safe to work on
the instrument live if necessary.
requires that the loop is switched off to access/maintain the instrument which
actually will mean the segment, so not only one instrument is isolated.
The only other alternative is to work under a hot work permit.
This is again a cost adder and involves risk.
There is always the figment of imagination by people not really HA cognisant that Exd is “Easy” and requires less work than Exi both in design and maintenance. Actually this is not true, Exd is not forgiving at all whereas IS is in that as long as you have selected the correct equipment in the first place and a barrier is in place even live working is permitted. How often do you see bolts missing, incorrect gaskets, poor weatherproofing, wrong and poor installed glanding etc on an Exd installation. This is not to say that there is no need to inspect and maintain IS installations, of course you do, however if a gland is poorly installed it will not mean that there is a potential for an explosion.
the amount of live working will be minimised with FF diagnostics and “remote
access” to transmitter data there will be the odd occasion when it will be
necessary, also adding an instrument to a segment means that live access is also
Live working is allowed on FNICO circuits however you must make sure that the equipment is certified as Exnl (Energy Limited) or Exi . Exn equipment must be disconnected. This concept is suitable for Zone 2 applications only.
If Exi transmitters are used in a FNICO environment it is
necessary to set up strict rules. What happens if the zoning changes?….as
there is an Exia certified transmitter installed this could lead to confusion.
Thus the FISCO solution has advantages in that;
There are no mixed protection techniques
It is suitable for both Zone 1 and 2 applications
Simple design rules are in place
are readily available.
There are methods for integrating with the Entity concept.
Up to 8 devices in a IIC gas group, 16 devices in a IIB gas group
Elimination of cable parameter calculations
Simplification of the safety documentation - just a list of devices
Addition of new devices without reviewing the safety case
As a minimum 12 devices can be installed on a FNICO segment (based on 15mA current draw per instrument), rather than the 8 devices for FISCO, there are some cost savings with this approach, however each site has to weigh up the advantages and disadvantages of the increased housekeeping associated with mixed systems for Zone 1 and 2 applications.
More useful information can be found at the following links
Fieldbus technology: Cut through the confusion- Ian Verhappen - ICEPros Inc thanks to chemicalprocessing.com
Economic Fieldbus Solutions for Hazardous Area installations - Jonas Berge
Fieldbus Non-Incendive Concept takes FISCO into Zone 2 and Division 2 Hazardous Areas - Phil Saward MTL Instruments
FISCO- what is it?- From MTL Instruments
FNICO - What is it? - From MTL Instruments
Managing and Technical Editor- ICEweb