Fieldbus Non-incendive wiring and hot-disconnect

[mrajakarthi]

For one of the FF project as required by the Project standards the segments are designed for non-incendive application whereas the field devices are ex-proof certified. We are using MTL Relcom FFPS power supply (normal) in the marshalling panel for powering the segments and the segment coupler in the field will be certified for non-incendive application. With this arragnement is it allowed to have the live hot disconnect of the spurs. If this is allowed please give some lights about the segment validation calculations to be followed for this arrangement.
thanks

[jberge]

Yes. Non-incendive permits live working of the spurs. See below link for a presentation. Make sure that instruments are certified non-incendive.

http://www.fieldbus.org.sg/files/20040422%20FF%20and%20hazardous%20area.pdf

[Mike ONeill]

The Hawke TG200 has EEx nA[nL] non-incendive approval and so may have any spur connected/disconnected live in the hazardous area. Visit www.ehawke.com/fieldbus to download a system drawing showing what you can do.

[Phil Saward]

Live-working of the spur connections is certainly allowed, provided the segment protectors are certified as having non-incendive (or EEx nL in IEC/CENELEC terminology) spur connections. This is the case with MTL-Relcom 'Megablock' wiring components that have Spurguard short circuit protection. The required calculations are to match the output parameters of the spur connection with the input parameters of the field instrument, and also to make sure that the sum of the respective values of capacitance and inductance for the spur cable and the field device are compatible with the allowed values for the spur. In practice, there should be no practical limit on spur length due to capacitance or inductance limits. Note that ANSI/ISA standard 12.12.01 includes a relaxation that it is only necessary to match the voltage parameters of spur and field device (and to ignore current and power), on the basis that the device cannot, in normal operation, draw more than its normal operating current and power.

[spragujl]

EEx nL - FF Field Instrument Certification

Question for Honeywell Yokogawa Foxboro Emerson ABB SMAR FF Field Instrument Makers:

Do any of you consistently certify your FF field devices as EEx nL with 32V max voltage?

We have found the most cost effective hazardous area design method for our Zone 2 plants is non-incendive - using current limited wiring blocks with certified EEx nA trunks and EEx nL spurs. We also want the neat added feature of live spur disconnect when we use EEx nL field instruments. We mandate this design now, and I hope other users and EPC designers wake up to the simplicity and cost savings in this wiring method soon.

However, it is VERY difficult to identify which FF instruments are certified EEx nL. We found some EEx n, some nA, others 'non-incendive' - but it appears that specific 'nL' approval is required. Can/Will you instrument makers get consistent ATEX or FM EEx nL certification @ 32V for your all your devices? We'd appreciate it.

Jim Sprague

[Mike ONeill]

One solution may be for the power supply vendors to specifically make a unit with a max. normal working voltage of 24V for use with non-incendive systems. As Phil Saward mentions, the voltage is the more significant parameter.
24V max. working would mean around 23V available in practice, but since there is no I.S. limiting resistor to generate voltdrop, the operational characteristics are limited by the cable resistance; 1000m cable = 50R = 10V @ 200mA (10 devices), 13V at the device coupler, spur drop 0.5V; 12.5V at the device.
This then becomes a very simple solution, without invoking FNICO.
Of course, we still have to convince the approval authorities that the max. voltage on the device cannot be greater than the max. fieldbus power supply voltage being applied - some of them seem fixated on having 32V as the max. allowable fieldbus power supply voltage, even in systems fed with 24V.