Motor $ense
Hazardous Areas
Hazardous Areas are locations where the potential for fire or explosion exists because of gases, dust, or easily ignitable fibers or flyings in the atmosphere.
In North America, hazardous areas are separated by classes, divisions, and groups to define the level of safety required for equipment installed in these locations. Classes define the general form of the flammable materials in the atmosphere. Divisions define the probability of the presence of flammable materials. Groups classify the exact flammable nature of the material.
Explosion-Proof Motor Classifications
Explosion-Proof-Hazardous Locations
DIVISION I - Location in which ignitable concentrations of flammable or
combustible material exist and come in contact with the motor.
DIVISION II - Locations in which ignitable concentrations of flammable or
combustible material exist but are contained within closed systems or containers
and normally would not come in contact with the motor.
EXPLOSION-PROOF-U.L. CLASSIFICATIONS CLASS I -Those in which flammable
gasses or vapors are or may be present in the air in quantities sufficient to
produce explosive or ignitable mixtures.
Group A - Atmospheres containing acetylene
Group B -
Atmospheres containing hydrogen, gases or vapors of equivalent hazard such as
manufactured gas
Group C - Atmospheres containing ethyl or ether vapors.
Group D - Atmospheres containing gasoline, hexane, benzene, butane,
propane, alcohols, acetone, benzol, lacquer solvent vapors, natural gas, etc.
EXPLOSION-PROOF-U.L. CLASSIFICATIONS Class 11-Those which are hazardous
because of the presence of combustible dust.
Group E - Atmospheres containing metal dust, including aluminum,
magnesium, or their commercial alloys.
Group F - Atmospheres containing carbon black, charcoal, coal or coke
dust.
Group G - Atmospheres containing flour, starch, grain or combustible
plastics or chemical dusts.
Ignition Temperature
In addition to having physical properties that make them different from each other (Class) and different explosive characteristics once they are ignited (Group), hazardous substances also have one other characteristic that must be dealt with. This characteristic is the ignition temperature. The minimum ignition temperature is the minimum temperature at which the substance will start to burn or explode.
Example: Virtually every car is equipped with and electric cigarette lighter. Obviously this lighter will not light a cigarette when the lighter is cold. Similarly when it is heated to the point where the element glows, it will light a cigarette. At some point in the range between cold and glowing hot, there is a temperature where it would not light the cigarette, but an increase of only one degree would make it hot enough to ignite the tobacco. The temperature at which ignition is possible would be the ignition temperature of the cigarette.
Materials can have the same physical properties (Class) and similar explosive properties (Group), but substantially different ignition temperatures. There is no consistent relationship between explosion properties and ignition temperature.
For example: A common Class I, Group D solvent, acetone, has an approximate ignition temperature of 465°C. Gasoline, also Class I, Group D, has an approximate ignition temperature of 280°C. Therefore, Class I, Group D equipment with a maximum surface temperature at 350°C could be used in an acetone environment, but the same equipment operated in gasoline vapors could cause ignition or explosion due to the 280°C ignition temperature of gasoline.
Some Group C gasses have higher ignition temperature than certain Group D gasses, and some have lower. The same can be said for any two Groups.
NEC Temperature Codes
|
Temperature Identification Numbers |
|||
|
Maximum Temperature (For All Conditions) |
T-Code |
||
|
Celsius |
Fahrenheit |
||
|
280* |
536 |
T2A*** |
|
|
260* |
500 |
T2B*** |
|
|
230** |
446 |
T2C*** |
|
|
215** |
419 |
T2D*** |
|
|
200** |
392 |
T3 |
|
|
180** |
356 |
T3A |
|
|
165** |
329 |
T3B |
|
|
160** |
320 |
T3C |
|
|
135** |
275 |
T4 |
|
|
120** |
248 |
T4A |
|
|
100** |
212 |
T5 |
|
*Class I, Group D only, requires caution statement
** Requires thermostats
***Not applicable to motors for use in Class II locations