Temperature Code and Class

The temperature class define the maximum operating temperatures on the surface of the equipment  which should not exceed the ignition temperature of the surrounding atmosphere.Ignition temperature is the minimum temperature required, at normal atmospheric pressure in the absence of a spark  or flame, to set afire or cause self sustained combustion independently of the heating or heated element.

                                   Temperature Classification Chart

Temperature  code		Maximum  Surface Temperature
North American	European	°C	°F
T1	T1	450	842
T2	T2	300	572
T2A	---	280	536
T2B	---	260	500
T2C	---	230	446
T2D	---	215	419
T3	T3	200	392
T3A	---	180	356
T3B	---	165	329
T3C	---	160	320
T4	T4	135	275
T4A	---	120	248
T5	T5	100	212
T6	T6	85	185

Intrinsic Safety (IS)

Intrinsically safe device (Exi) defined as "equipment and wiring which is incapable of producing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixtures in its most easily ignited concentration." This is achieved by limiting the amount of power available to the  electric equipment in hazardous area to a level below that which will ignite the gases.As an example hydrogen requires 20- micro joules of energy for ignition, the intrinsically safe equipments are incapable of generating 20- micro joules of energy

In intrinsically safe circuits,a safety barrier is used in the safe area for prevent the high energy level passing into the hazardous area due to any fault.Basically there are  two types of intrinsically safety barriers; Zener barrier (passive devices) and Galvanic isolators  (active devices).Functionally, they achieve the same objective that is prevention of high energy level passing into the hazardous area.


Zener Barriers

Zener barriers are passive devices that contain zener diodes,resistors and fuse to limit excess voltage and current.The zener diodes in the zener barrier are connected in the revers direction.The breakdown voltage of the  diode is not exceeded in normal operation.if this voltage is exceeded due to a fault in non hazardous area,the diode start to conduct, causing the fuse to blow,thus preventing the transfer of unacceptable high energy into the hazardous area.

Galvanic Isolaters

Galvanic isolaters are active devices,these provides galvanic isolation between the hazardous  and safe area using transformers,opto isolatores or relays.

Hazardous Area Classification

The National Electric Code (NEC) defines hazardous locations are those areas "where fire or explosion hazards may exit due to flammable gas or vapors,flammable liquids,combustible dust or ignitable fibers or flyings." The hazardous location are classified as Classes,Divisions,Groups.

Classes

The classes define the type of hazardous in terms of whether it is a gas or vapors,a combustible dust or ignitable fibers or fyings.The type of hazards divided into following three classes.

Class I

Any location in which flammable gases or vapors are may present in the air in sufficient quantities to produce an explosive or ignitable mixture.

Class II

Any area made hazardous by the presents of combustible dust.

Class III

Any area made hazardous by the presents  of easily ignitable fibers or flyings, but in which such   fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitable mixture.

Divisions

Divisions defines the probability of the hazardous material being present during normal or abnormal conditions.

Division 1

Area in which contain hazardous mixtures under normal operating conditions.

Division 2

Area in which atmospheres is normally non- hazards.But may become hazards under abnormal circumstances such as an accidental rapture of a vessels or container or system breakdown.

Groups

Groups classify the exact nature of the hazardous substance.Flammable gas and vapors of class I location are separated into the four Groups A,B,C and D.Flammable dust and debris of class II location are separated into three Groups E,F,and G.

Group A

Atmospheres containing Acetylene.

Group B

Atmospheres containing Hydrogen or gases or vapors of equivalent hazards.

Group C

Atmospheres containing Cyclopropane,Ethylene, Ethyl ether or gases or vapors of equivalent hazards.

Group D

Atmospheres containing Gasoline, Acetone, Ammonia, Benzine, Butane, Ethanol, Hexane, Methane, Naphtha, Natural gas, Propane or Equivalent hazards.

Group E

Atmospheres containing combustible metal dust including Aluminum, Magnesium and other metal dust of similar nature.

Group F 

Atmospheres containing carbonaceous dusts, Carbon black, Coal black, Charcoal, coal or Cock dusts which have more than 8% total volatile material or atmospheres containing these dusts sensitized by other materials so they present an explosion hazards.

Group G

Atmospheres containing Flour, Starch or Grain dust or similar type of materials.