Stephen Mitschke
August 12th, 2003, 12:56 PM
As requested, here is some more information on FNICO, the FieldbusNon-Incendive COncept. Firstly we need to define what we mean by anon-incendive fieldbus installation. In North America, the Factory MutualDivision 2 standard is commonly referred to as the non-incendive standard,but it actually includes two separate hazardous area protection techniques,as well as a number of other 'minority' techniques for specialapplications. The two main approaches are:1. Normally non sparking (known as non arcing in Europe) which cannot belive worked in a hazardous area without a gas clearance certificate, and2. Non-incendive field wiring (known as energy limited in Europe) which canbe live worked in a hazardous area. This is like Intrinsic Safety, butwithout consideration of faults and with a lower factor of safety.Non-incendive field wiring can be used in Zone 2 and Division 2 hazardousareas, and will support more devices from a non-incendive power supply thanan Intrinsically Safe solution, because more power is available in thehazardous area.FNICO uses the non-incendive technique throughout. With the MTL FNICOpower supplies, the fieldbus trunk, spurs and field devices can all be live-worked. FNICO also takes advantage of the key benefits of FISCO (FieldbusIntrinsically Safe COncept), which are:* eliminate the need to calculate cable parameters* reduce safety documentation to a list of devices* allow addition of additional devices simply by adding them to thedevice listThis is much simpler than conventional installations, where theadministrative work involved in complying with end users procedures isusually considerable. For example, simply adding a new field device to annon-incendive fieldbus segment would require recalculation of cablecapacitance and inductances and all the documentation to be updated.FNICO is therefore aimed at Division 2 and Zone 2 applications where theadvantages of FISCO are required, but with more current available to thefieldbus trunk than with intrinsic safety. Field instruments may either becertified as IS, or Division 2 with non-incendive field wiring parameters.Further details are available on our web site:http://www.process-io.com/Download.nsf/854270a29370dec5002568ac0041c57b/0c1d9ee34fdab71f80256b51005c4774?OpenDocumentAn alternative approach for fieldbus in Division 2/Zone 2 is as follows:Some short circuit protected wiring components, such as the RelcomMegablock SpurGuardTM range, are certified so that with a normallynon-arcing trunk, the spurs are certified as non-incendive field wiring.This may be used with any general purpose fieldbus power supply, includingredundant isolated power conditioners. This approach does not allow liveconnection/disconnection of the trunk, but this is rarely required. Itdoes allow live connection/disconnection of the spurs or field devices.Further details are available on our web site:http://www.process-io.com/Download.nsf/854270a29370dec5002568ac0041c57b/af513b3592de6bf180256b6800434f4f?OpenDocumentMTL are currently writing a detailed Application Note on fieldbusnon-incendive systems, AN9027. I will send a copy of this to FUN list whenit is available in the next few weeks.Live working is often required on a fieldbus system in commissioning,maintenance or plant extensions. The practical advantages of live workingwithout a gas clearance certificate, will be dependant on site practices.Some sites see this as a major advantage where their is considerable timeand effort required in getting a gas clearance certificate and/or the gasmonitoring has to be done by operations personnel, while the instrumenttechnician works on the fieldbus. However, as non-incendive field wiringfor Division 2 or Zone 2 installations requires no additional designrestrictions, whilst offering the flexibility of liveconnection/disconnection without the need for a gas clearance certificate,it is certainly worth considering when designing a hazardous area fieldbussegment.Roger Highton Dana LommenMTL Instruments Relcom Inc