Approval Standard 3640, Land Mobile Radios for Use in Class I, Division 1 Hazardous (Classified) Locations, was introduced in 2013 and provides manufacturers with increased flexibility, through multiple protection methods, to achieve equivalent protection for their products for Class I, Division 1, hazardous (classified) locations.
Standard 3640 or, FM 3640, is based, in part, on the U.S. National Electrical Code/Section 500.7, which identifies nine protection techniques that can be used. One of the techniques, known as "Other Protection," allows flexibility by permitting approaches such as combining various Class I, Division 2, techniques to provide protection for Class I, Division 1, hazardous (classified) locations.
Under FM 3640, combined protection concepts should depend on dissimilar physical protection principles in order to avoid protection defeat by a single type of failure. For example, the combination of "d" (flameproof) and "q" (powder filled) both depend on the avoidance of flame propagation and may not be useful in combination. An example of an acceptable combination of two independent protection concepts might include: intrinsic safety "ib" further enclosed by encapsulation "mb," or intrinsic safety "ib" further enclosed by a flameproof enclosure "d." These are just two examples; many others are possible under FM 3640. Compliance to FM 3640 can also be achieved by compliance to FM 3610.
To learn more about FM 3640, please see this related story from Approved Products News, VOLUME 29, NUMBER 2, 2013.