MORE ABOUT COMPARISON BETWEEN NEC AND IEC CODES
No one should trifle with the safety of personnel and equipment deployed in areas where there are or may be flammable or explosive atmospheres. Standards for dealing with such hazardous areas have been established based on caution and experience. But the 1996 National Electrical Code (NEC) was modified to incorporate International Electrotechnical Commission (IEC) standards in addition to the existing code. Comparison of the two methodologies shows the effects and benefits.

In the United States two methods are now allowed: the traditional method based on Article 500, and the new IEC method allowed for the first time in the 1996 NEC (Article 505).

The IEC methodology was added to the NEC in the 1996 code cycle. This has been and still is controversial. It has taken several code cycles to get this method included into the NEC. The controversy involves business reasons, turf protection, international politics, and safety concerns.

Like traditional area classifications, the IEC method uses three descriptors plus extent and temperature class to classify hazardous areas. But it uses different terms, groupings, descriptors, and temperature ranges. The descriptors used for the IEC method are Class, Group, and Zone.

NEC Article 505, which defines the IEC methodology, defines only Class I areas. Like the traditional method, the definition class is the physical nature of the material—flammable gases, vapors, and flammable or combustible liquids. Under Article 505, no Class II or Class III areas are defined and therefore Article 505 cannot be applied to these areas.

The group designator has the same definition as the traditional method but the groupings are different (Table I). For the IEC method:

Comparison of Groups

The IEC method combines the traditional groups A and B into a single group IIC, making the requirements for acetylene gas the same as for hydrogen and other highly flammable gases.    

GROUP I - Covers underground mines, which in the U.S. are covered by the Mine Safety and Health Administration (MSHA), so
GROUP I is not covered by the NEC and is not applicable;
GROUP IIC - Corresponds to the traditional method's Groups A & B;
GROUP IIB - Corresponds to the traditional method's Group C; and
GROUP IIA - Corresponds to the traditional method's Group D.
The temperature class has the same meaning as in the traditional method but IEC method (NEC Table 505-10(b)) does not have the T1-T6 subclasses of the traditional method (Table II).

Comparison of Temperature Classes

The IEC method has the same temperature classes as the traditional method, but the subclasses are omitted.  

The zone designator (Table III) has the same definition as the traditional method's division designator, but divides the likelihood into three probabilities (Zones 0, 1, and 2) instead of two (Division 1 and 2). Zone 0 is where the hazard is considered to be there all the time or for long periods of time.  

Comparison of Divisions with Zones

The IEC method separates the traditional Division 1 into two zones. By putting the more hazardous conditions of Division 1 into Zone 0, the requirements for Zone 1 are relaxed.  
Zone 1 is where the hazard is considered to be there under normal conditions, including repair and maintenance activities or leakage, or where there are operations or processes which could result in the release of a flammable mixture and could cause a simultaneous failure of electrical equipment. An area that is adjacent to a Zone 0 area where the hazard could be communicated to would also be classified as Zone 1.

Note that this definition of Zone 1 is essentially the same wording as for the traditional Division 1 definition. But the most dangerous likelihood has been separated out. This allows the use of lesser means in Zone 1 than in Division 1, which must cover the most dangerous likelihood (Zone 0) in addition to the less dangerous likelihood (Zone 1).

Zone 2 is equivalent to Division 2. Again, as in the traditional method, by definition any area that is not classified as hazardous is classified as non-hazardous. The extent has the same definition as the traditional method.

The IEC method is illustrated in Figure 2. One qualification regarding its use is that NEC Article 500-3 requires that the area classification, wiring, and equipment selection be under the supervision of a "qualified Registered Professional Engineer." This requirement is a bit unusual for the process industries as most engineers are not registered but rather work under an industry exemption. Also, the term "qualified" is not spelled out.

While a case may be made that this is a new methodology and this would make the application more safe, this was probably an attempt to make the use of this methodology more difficult and thus less attractive.

Also, the NEC Handbook (NFPA-70HB) states that it is the intent that you do not mix and match in the application of these methodologies: these methods are mutually exclusive. A company should not use both methods in the same facility. The exceptions to this are that equipment approved for the traditional area classification may be used in the equivalent IEC area.

We now have two methodologies that we can use to classify hazardous areas, the traditional and the IEC. This provides greater flexibility in choosing how we classify, the equipment, and what type of electrical installation will be provided. 

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