Explosion Proof Electric Motors: Class, Groups & Divisions

Explosion Proof Motors

To understand what explosion-proof really means for electric motors, you need to be familiar with the two basic scenarios related to motor explosions. The first involves the surface temperature of the motor as it heats up. When the surface reaches the autoignition temperature (AIT) of a flammable material that has accumulated in the environment, a spontaneous explosion can result.

The second scenario starts with an anomalous spark from the electric motor that results in an explosion within the motor housing. That, in turn, ignites incendiary liquid or dust in the environment and a bigger explosion results.

UL Classifications, Groups and Divisions

Let’s talk about how UL (Underwriter’s Laboratory) determines what type of motor is suitable for different explosive (or potentially explosive) environments.  There are three parts to this: Classifications, Groups, and Divisions.

Classifications

Classifications are based on the atmosphere that the motor will be exposed to under its normal operating conditions. There are three classes:

• Class I for gases, vapors flammable liquids
• Class II for combustible dusts
• Class III for ignitable fibers, flyings

Groups

These classifications are broken down into groups which are determined by how volatile or explosive the materials are and how concentrated the material is going to be in the motor’s operating environment. The groups are represented by letters with A being the most volatile. The groups become progressively less volatile as the list continues.

Here are a few examples of typical materials found in each group:

• Group A: acetylene
• Group B: butadiene, hydrogen,
• Group C: ethylene, cyclopropane
• Group D: gasoline, acetone, alcohol, natural gas, propane
• Group E: metal dusts such as aluminum and magnesium
• Group F: dusts such as charcoal, carbon black,
• Group G: dusts such as grain, flour, combustible plastics, chemical dusts

You’ll notice that Groups A – D apply to Class I while Groups E – G apply to Classes II and III.

Divisions

There are two types of Divisions in the UL system. Here’s a breakdown of what would be classified as Division 1:

• A hazardous concentration of flammable gases, vapors, or suspended combustible dust.
• They are present continuously, intermittently, or periodically.
• This is under normal operating conditions.

Basically, Division 1 means the hazards are present under everyday operating conditions.

Here’s a breakdown of what would be classified as Division 2:

• Volatile, flammable liquids or flammable gases are present.
• They are normally confined within closed containers or systems.
• They only escape under abnormal operating or fault conditions.

For Division 2, the hazard is only present during abnormal conditions — it’s not an everyday thing. Here’s something else important to note: combustible dusts are not normally in suspension and they are highly likely to be thrown into suspension, which is why they aren’t considered Division 2.

It’s very important to recognize the difference between Division 1 and Division 2: far too many people purchase a Division 1 motor for operating conditions that would actually be Division 2.

T-Codes

There is one more issue related to explosion proof motors: T-codes, or temperature codes, that represent the maximum surface temperature of a motor under all conditions, including locked rotors and burnout. The T-code for a motor has to be less than the AIT of the gas or mixture that will be present in the environment.

Here is a summary of the T-codes and the maximum surface temperature they represent:

• T1: 842°F
• T2: 572°F (T2A: 536°F, T2B: 500°F, T2C: 446°F, T2D: 419°F)
• T3: 392°F (T3A: 356°F, T3B: 329°F, T3C: 320°F)
• T4: 275°F (T4A: 248°F)
• T5: 212°F
• T6: 185°F

Selecting the Right Explosion Proof Motor

With all that said, here’s what you need to select the correct motor:

• Correct classification (I, II, or III)
• Correct group (Groups A – D apply to Class I, Groups E – G apply to Classes II and III)
• Whether the environment is Division 1 or Division 2
• The AITs of the media involved
• The T-code for the area

First off, keep in mind that Class 1, Division 1 motors are typically labeled as explosion proof while Class II, Division 1 motors are dust ignition proof. However, this isn’t true of Division 2 motors.

One of the characteristics here is the Division: remember that Division 1 applies to normal operating conditions while Division 2 applies to abnormal or fault conditions. If the environment involves hazardous conditions only when there is a fault condition or abnormal circumstances, you only need a Division 2 motor, not a much more expensive Division 1 motor.

Let’s talk for a moment about TEFC, Totally Enclosed, Fan-Cooled, motors for a moment.

TEFC motors have a closed-off chamber which prevents outside air and air inside the frame from being freely exchanged. An external fan attached to the shaft blows air over the frame to help keep things cool.

TEFC motors can be used in Class I, Division 2 hazardous locations as long as they don’t have an ignition source (e.g., switching mechanisms, arc-producing brushes) and the internal, external, and maximum surface temperature of the motor are less than the T-code for the area. Furthermore, the majority of standard TEFC motors are actually suitable for general Division 2 Explosion Proof locations–and the cost of a Division 2 motor is going to be significantly less than that of a Division 1 motor.

Conclusion

Navigating the classifications, groups, divisions, and T-codes for a hazardous location motor can be very tricky but certainly not impossible. One of the most common errors we’ve witnessed is choosing a Division 1 motor for a Division 2 environments, which results in an over-engineered, more expensive motor.

Here at Hi-Speed Industrial Service, our experts will gladly work with you to find the right motor for a hazardous location, and we won’t sell you a more expensive explosion-proof or dust ignition proof motor when a Division 2 TEFC motor would be suitable. Our electric motor sales team takes pride in finding the most optimum motor for your application. Contact us today to learn more about our electric motor sales and service.