FNICO/FISCO ic device number limit

[cavvieira]

Hi

I am desiging a fieldbus segment to operate on Zone 2, and we've opted for the FISCO ic (previously called FNICO) safety strategy. The segment currently holds 10 devices at a total current consumption of 149 mA. The gas group is IIC, so the power supply provides up to 180 mA.

A colleague of mine just told me a FNICO segment should not have more than 9 devices on it, though. He couldn't recall where he's read it, and from my perusing of IEC 60079-27, the only hard limit I've found is 32 devices.

Have any of you read in an international standard about a 9 device limit for FNICO segments?

[Mike ONeill]

There is no formal restriction on the number of devices in a FNICO segment, just the usual practical limitations resulting from Ohms law considerations relating to supply voltage & current and the required load (devices plus cable). I think there is still one manufacturer that supports FNICO power supplies (Cooper MTL), and last time I looked at their datasheets, their power supply for IIC is 12V & 180mA.

The load calculation is fairly straightforward once you allow for the cable length (resistance) and whatever safety margins you select (the absolute minimum of 9V at the device is typically boosted in many specs to at least 10 or 11V at the spur connection). If you are using a device coupler with spur short-circuit protection in the field (recommended), then those units themselves have a trunk-spur voltage drop and some additional segment load to take into account. Generally, most designers add at least 1 spur short-circuit load and a 'guest' device load (hand-held tester) to make sure that the segment is going to be robust and not fall over at the first sign of trouble.

[aagostin]

You could use MTL 9111 FNICO IIC supply, which provides maximum current of 200mA (design current is 180mA, for use by instruments).
Technically, you could have 16 instruments maximum, since this is the limit imposed by the DCS host cards (vs the 32 by the standard), provided the device current is 10mA only. With an average of 15.29mA (see here: http://forums.fieldbus.org/showthread.php?t=2205 ), you can surely have 10 instruments, provided this will give you the distance you need.
As Mike O'Neill described, you have to follow Ohm's law. You can perform calculations easily using freely available tools, e.g. here from MTL:
http://www.mtl-inst.com/news/mtl_fieldbus_calculator_now_includes_redundant_fis co_power_supply/

[Saikat]

Hi Cavviera,

A poster of one wiring block manufacturer three years back showed 9 devices in Gr IIC FNICO. It was simple :) 180 mA devided by 20 mA average current per device. Your friend may be referring to that.
It showed 6 devices in Gr IIC FISCO (120 / 6).

Average current of most fieldbus devices today is much less. You may keep margin for one spur short circuit (add 25mA, depends on the model of wiring block), and some margin for connecting field tester etc.

Regards,
Saikat

[shankar_u45]

Contents of AG-163 and some other technical guides are summrized in attached presentation form. Trust it would be helpful.

[hassan.elsayed]

I am afraid some of the presented slides are not up to date. A few slides are not covering enough the available methods for IS applications and could be misleading. The AG-181which is under revision right now, provides more explanation and methods for segment calculations and covers various IS techniques. There are various literatures in the web which show that Entity concept provide more segment capacities than others, and the high power trunk/ hybrid can not drive more than 16 field devices, not from theory point of view, but from the experimental point of view, due to the effect of the barrier impedance on the communication signal and the limited input supply.

Hassan Elsayed
MooreHawke

[shankar_u45]

Hassan,

You have mentioned that "Entity concept provide more segment capacities ". If you mean number of devices, then needs review.

I am yet to read revised AG-181. To the best of my knowledge, the concepts mentioned in the presentation are proper. Will you please correct wherever wrong?

[Mike ONeill]

shankar_u45,

It's not so much that your slides or their content is wrong, just that you've left out the split-architecture option in the ENTITY design concept. The relaxation in final resistor value that allows FISCO designs additional power in IIB can also be used in ENTITY systems, and Hassan was referring to one system that has 350mA/16 device capability in IIB (you, like many others, automatically assume that ENTITY systems are limited to 80mA/4 devices just because one manufacturers interface unit allowed that for IIC).

It's a moot point since that split-architecture system now has other problems related to the original certification process, and would be a brave purchase by anyone.

[hassan.elsayed]

Dear Shankar_u45

Slide 9 is correct of FISCO compared to standard Entity system (80mA). But if split-architecture is considered, then more devices can be used per segment.

Split-architecture is basically based on locating the safety resistors in the coupler, and another part in the power supply. The output of the IS power supply which is Entity, can deliver up to 500 to 600mA in IIB. When it is connected to its dedicated coupler, then the designed current dropped to 350mA per segment for IIC, using dual trunks. I guess Mike has meant IIC not IIB.

Slide 10, spur cable length should be 60m, not 30m, as shown in slide 27.
Slide 17,
a- it has been mentioned before, that for segment design, a diagnostic tool current consumption should be added, and the additional in access current caused by one spur short circuit (loaded).
b-For IS devices, 9.0V is the lab voltage, this excludes the spur drop voltage and the limitation imposed by the safety resistors integrated in the isolated coupler or IS couplers. This means at least a 10V is required for satisfactory operation of those IS devices.
Slide 21; number of F. devices is no more than 16 as stated in FF 846, phase 2 which is still in preparation.
Slide 23 is correct if FISCO compared to standard Entity figures (80mA). Split architecture would offer more devices for a similar cable length.
Slide 26 FISCO is available in singular module and redundant modules.
Slide 27 , Split Architecture should be inserted under IS column.

Hassan ElSayed

[IanVerhappen]

I would suggest that you read the article on the migration from FNICO to FISCO ic at Control Design magazine at http://www.controldesign.com/articles/2010/FISCO1003.html as it will answer the "next" question you may have as well.

Note that there are a number of ways to increase the energy available on a segment, Hassan is describing a proprietary solution provided by Moore-Hawke, while the FISCO/FNICO solutions are incorporated into and supported by IEC standards.

P.S. The AG-181 Revision 3.1 is also now available for download in the Technical Resources section of the FF web site.