Ex-pert Electrical Technologies

Dynamic elaboration on the Art & Science of Hazardous Area Protection

92.Class-Zone-Division

All about Class , Zone ,Division  & Group   (made easy)

three classes

There are four states of matter, as defined in Physics

  1. Gas
  2. Liquid
  3. Solid
  4. Plasma

Common example is WATER, which is liquid at Standard Temperature & Pressure.

It solidifies into ICE at sub-zero temperature and as the temperature increases , it melts into the liquid form (Water as we know it) and converts into vapours and ultimately into a mist as steam.

(The special state of matter known as Plasma will not be discussed here.)

This concept of States of Matter is copied in a modified way, into the concept of CLASSES of Materials (Flammable  Materials, to be exact).

The concept of Classes of Hazardous Area Materials is defined in the following ways by NEC (National Electrical Code of USA) and IEC (International Electrotechnical Commission ).

THREE  Classes in NEC

TWO  Classes in IEC

Classes of Materials

Regardless of the standard, the basic concept is same . 

It is clear that Classes define the TYPE & STATE of the Flammable material in NEC and IEC

Once the Type & State of the Flammable material is defined  as CLASS 1 , 2 or 3 , the next required parameter is HOW MUCH (Quantity ) of the Material.

This parameter is represented by the concept of ZONES  & DIVISIONS.

The quantity of the Hazardous Material present in an area  is measured or quantified in terms of the TIME PERIOD, measured in hours per 365 days , DURING WHICH THAT AREA  REMAINS OCCUPIED  WITH THAT MATERIAL  AND THUS REMAINS PRONE TO EXPLOSIONS.

IN other words:

If an expolsive atmosphere is present in an area for a total of 100 HOURS or MORE per YEAR (365 days) then it is a HIGH risk area because it has MORE QUANTITY of HAZARDOUS TIME (Hours).

Similarly an area where Quantity of HAZARDOUS HOURS is between 1000 & 10 per YEAR then it is a Moderate risk area.

(following the same path we land in an LOW Risk Area) Of course this area has the lowest quantity of HAZARDOUS HOURS …just between 10 & 1 per year.

All other area with ZERO OR LESS THAN 1 ,  HAZARDOUS HOURS are  “SAFE Industrial Areas ” (From our view-point)

Now is the time for giving names to the areas:-  (IEC & NEC 505 )

  • High risk area , with largest quantity of HAZARDOUS HOURS (1000 or more /year) is ZONE 0
  • Moderate risk area , with HH (Hazardous Hours) between 1000 & 10 /year is ZONE 1
  • Low  risk area , with HH (Hazardous Hours) between  10 & 1  /year is ZONE 2
  • Areas with HH less than 1 or 0 are non-Hazardous areas.

As far as DIVISIONS are concerned , the basic concept is same . HAZARDOUS HOURS and their quantity in an area.

The term DIVISION is owned by NEC (500).

From NEC (500) ‘s view the following divisions exists:-

  • High risk areas are DIVISIONS 1 (areas) where the quantity of Hazardous Hours is more than 10 per year
  • Low risk raes are all DIVISION 2 (areas) where the Hazaudous Hours exists in quantities between 10 & 1 per Year.

CLASSES in NECZones in NEC and IEC

Now we already know that:

  • The class give us information about the state of the material
  • The Zone/Division gives us the magnitude of the hazard

The next obvious question is :

What is the identity of the material ?

This information is contained in the “GROUP”

Gas Groups & Dust Groups

Almost all Flammable gases/Vapours used in industrial processes have been tabulated into groups according to their chemical & physical properties/parameters.

The following properties or parameters are most important from “Explosive-Capacity ” ‘s point of view:-

  • Auto Ignition temperature
  • Minimum Ignition Current 
  • Maximum Experimental Safe Gap.

These parameters paly the most important part in safe handling & processing of the flammable gases. The Gas  Group tabulate these parameters and divides the gases into 3  groups :-

(From IEC ‘s view)

  • Group IIA
  • Group IIB
  • Group IIC

Similarly Cumbustable Dust is divided into following Dust Groups

  • Group IIIA
  • Group IIIB
  • Group IIIC

If we follow NEC’s view point then the Gas Groups are as follows:

  • Group A
  • Group B
  • Group C
  • Group D

All about classes-zones-groups

cont….