Don,
One of the more subtle differences between the US domestic ?Division? classification system and the IEC ?Zone? system is that the US Divisions are defined in terms of ?possibility? and IEC Zones are defined in terms of ?probability.? I pointed this out to CMP14 several years ago and they redefined US Zones to be consistent with the ?Division? concepts. That wasn?t my intention, but it is what they did.
I can describe a Division 1 location that may never actually be ?hazardous? through its active lifetime. One prime example is the envelope around a vapor relief port. Typically there is a 3 to 5? Division 1 envelope. If the valve never engages (a fairly common case), the area is really not very likely to be flammable. But, since it is possible, it?s Division 1.
As Bob pointed out, many cold locations with enclosed operating areas and poor ventilation are also classified Division 1 ? not because they actually do have flammable mixtures present, but because they could under ?normal? conditions. Remember. ?normal? is not necessarily ?common.?
BTW that is another difference. ?Normal? is undefined in US domestic use but it is a defined term in the IEC system classification.