No, replacing a Class II, Group G motor with a Class I, Group D motor is not a safe or allowable action without a thorough assessment and likely additional safety measures. Here's why: Different Hazards

Class II and Class I designations address fundamentally different types of hazardous environments:

Class II

Addresses combustible dust hazards. Group G specifically refers to combustible metal dust (e.g., aluminum, magnesium). The motor is designed to prevent ignition of these dusts, typically by being totally enclosed and surface temperature limited.

Class I

Addresses flammable gas or vapor hazards. Group D specifically refers to gasoline, hexane, naphtha, benzene, butane, propane, alcohol, acetone, benzol, lacquer solvent vapors, or natural gas. The motor is designed to contain an internal explosion or prevent ignition of the surrounding atmosphere.

Different Protection Methods

The construction methods used to achieve explosion-proof/ignition-proof ratings for Class II and Class I are different. Class II motors are typically TENV or TEFC designs with surface temperature limitations. Class I motors are typically explosion-proof (XP) or purged and pressurized.

Dust vs. Gas/Vapor

A Class I, Group D motor is designed to contain an explosion of flammable gas or vapor within its enclosure. It is not necessarily designed to prevent the ignition of combustible dust layers that may accumulate on its surface or within the motor. The Class II Group G motor is.

Temperature Rating

Class II environments have strict surface temperature limits to prevent ignition of dust layers. The T-code of the motor must be less than the ignition temperature of the dust cloud or dust layer that is present in the surrounding atmosphere. Class I motors must also have a T-code, but the T-code is used to determine the maximum surface temperature that the motor will reach and thus will not ignite surrounding vapors. Because dust layers insulate the motors, the surface temperatures that the class II motors operate at is typically much lower.

Why it's dangerous


Dust Ignition

A Class I, Group D motor may run hotter on the surface than a Class II, Group G motor. This could ignite a layer of combustible metal dust that accumulates on the motor's housing.2.

Lack of Dust Exclusion

Class I motors aren't necessarily designed to prevent the ingress of dust. Accumulated dust inside the motor could lead to overheating and ignition.3.

Code Violations

Electrical codes (like the NEC in the US or IEC standards) specifically mandate the correct motor classification for the hazard present. Using the wrong class of motor is a code violation.

What to do


Hazard Assessment

A qualified professional (electrical engineer, certified safety professional, etc.) must conduct a thorough hazard assessment of the area. This assessment will determine the correct classification (Class, Division, Group) and temperature code (T-code) for the environment. The assessment should address the presence and characteristics of any combustible dust.2.

Consult with Experts

Talk to the motor manufacturer, a qualified electrical engineer experienced in hazardous locations, and potentially your local electrical inspector.3.

Compliance

The replacement motor must be suitable for the actual hazardous environment identified in the hazard assessment. This means it must meet the Class, Division/Zone, Group, and T-code requirements for the location.4.

Alternative Solutions

Consider other solutions if a direct replacement isn't feasible:

Purging and Pressurization

Use a purged and pressurized enclosure to keep the hazardous atmosphere out of the motor enclosure.

Remote Location

Move the motor to a non-hazardous location.

In summary, never replace a Class II, Group G motor with a Class I, Group D motor without a proper hazard assessment and confirmation that the replacement is suitable for the actual hazardous environment. Doing so could create a significant explosion and fire risk. Flag for review