The protection of electrical apparatus for use in flammable atmospheres is embracing a new technique: Ex ic. As a logical extension of the existing intrinsic safety concept, it formalises the application of intrinsic safety in Zone 2 hazardous areas, but users and manufacturers alike are wrestling with the details. The discussion centres as much on the withdrawal of the Ex nL technique, as on the application of Ex ic which replaces it. The paper deals primarily with the impact on countries that follow IEC and ATEX practice, but there are parallels with the ‘nonincendive’ technique that is adopted in North America.
What is Ex ic?
Ex ic is a form of intrinsic safety protection for electrical apparatus in flammable atmospheres. It is a logical extension of the long-established Ex ia and Ex ib protection levels, and is suitable for use in, or connection into, Zone 2 hazardous areas where there is a low probability of a flammable risk. Ex ic retains the unique characteristic of all intrinsic safety systems in which the wiring and field apparatus may be ‘live-worked’ in the presence of a flammable atmosphere.
The safety factors and equipment construction requirements are however more relaxed than for the Ex ia and ib protection levels, to take account of the lower risks associated with Zone 2. In common with the other protection levels, Ex ic is best suited to instrumentation circuits, where the levels of voltage and current are consistent with those permitted by intrinsic safety, although Ex ic can take advantage of the Zone 2 relaxations to drive somewhat larger loads.
The IEC area classification system
Zone 0: In which an explosive gas-air mixture is continuously present or present for long periods. (Level of protection Ex ia.)
Zone 1: In which an explosive gas-air mixture is likely to occur in normal operation. (Level of protection Ex ib.)
Zone 2: In which an explosive gas-air mixture is not likely occur in normal operation, and if it occurs it will exist only for a short time. (Level of protection Ex ic.)
The concept of a third level of protection for intrinsic safety has existed for several decades, but only recently has it been formally adopted into the major standards systems in common use worldwide. Meanwhile, other methods of protection had filled the gap. In particular, Ex nL, a subset of the ‘safe in normal operation’ standard, Ex n, had provided a workable but somewhat loosely defined alternative. Ex nL (where the L referred to its energy limited provenance), borrowed heavily from the published intrinsic safety ignition data to provide a method of protection in which instrumentation circuits could be live-worked in Zone 2. In North America, the ‘nonincendive’ method also delivers a roughly equivalent approach as part of its wide portfolio: the ‘nonincendive with field wiring parameters’ technique allows limited live-working in Division 2 hazardous areas.
So why have things changed at last? One major factor is that Ex nL had for a long time been an uncomfortable bed-fellow with the other protection concepts that were part of the overall Ex n concept. Whereas Ex nL conformed to the relatively low voltage and current levels of energy-limitation, there were no such restrictions for the Ex nA (non-arcing), Ex nC (containment) and Ex nR (restricted breathing) techniques, which were therefore appropriate for heavy-current switchgear and rotating machines.
Readers interested in the exact details of how the safety standards have been modified can find the full white paper at: http://tinyurl.com/q43f6p2
For more information contact Gary Friend, Extech Safety Systems, +27 (0)11 791 6000, [email protected], www.extech.co.za
| Tel: | +27 10 055 7300/1 |
| Email: | [email protected] |
| www: | www.extech.co.za |
| Articles: | More information and articles about Extech Safety Systems |
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version