NEC 500.5(C)

[erickench]

The above NEC section seem's to be a little confusing. NEC 500.5(C)(1)(2) states the following condition for a Class II Div. I location:

Where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electrical equipment, through operation of protection devices, or from other causes,

And NEC 500.5(C)(2)(3) states the following condition for a Class II Div. II location:

In which combustible dust accumulations on, in, or in the vicinity of the electrical equipment could be sufficient to interfere with the safe dissipation of heat from electrical equipment, or could be ignitable by abnormal operation or failure of electrical equipment.

I do not see the difference between these two conditions. In both cases the explosion would be caused by failure of electrical equipment. The first case mentions protection devices followed by the phrase "or other causes,". What do you guys make of this?

 

[rbalex]

In case 1 (Class II, Division 1) the failure of the electrical equipment also causes the release of ignitable material and becomes a source of ignition. [cf. 500.5(B)(1)(3)]

In case 2 (Class II, Division 2) while the ignitable material is normally present, the equipment, in normal operation, is not a source of ignition. There is no comparable analog with Class I. (If the ignitable mixture were normally present it would be Division 1.)

BTW "Classes" are designated with Roman numerals and "Divisions" are designated with Arabic numerals. I normally don't make an issue of it except I suspect you may be documenting a location. [500.4(A)]

 

[erickench]

But in case 2 it say's "provide a source of ignition through simultaneous failure of electrical equipment, through operation of protection devices, or from other causes,".
Can you perhaps give me an example that describes the differences between these two cases?

 

[rbalex]

You need to reread 500.5(C)(2) again; the word simultaneous doesn't appear in any of the three conditions listed - and it didn't appear in your OP for case 2. It only appears in 500.5(C)(1)(2); Class II, Division 1.

 

[Strathead]

But in case 2 it say's "provide a source of ignition through simultaneous failure of electrical equipment, through operation of protection devices, or from other causes,".
Can you perhaps give me an example that describes the differences between these two cases?

If you really want to know more about classifications and the whys of them, then you should research the NFPA web site for other codes that deal with it. For purposes of your question, see if this helps. The need to treat an area as classified (from an electrical standpoint) comes from the possibility that an electrical spark or equipment heat could cause ignition of material. This takes the proper combination of fuel and oxyegen. This is important to understand, because an area devoid of oxyegen, but filled with ignitable vapor is NOT explosive in that state.

The first section you quote deals with the possibilty, for example, that when or shortly after the ventilation shuts down, but the dust generation activity still continues, the combustible dust concentrations reach the proper mixture level with oxygen that a spark may ignite the mixture.

The second section. Through interlocking or other controls, the dust levels could be prevented from accumulating to ignition levels. Say, for example you have a saw that is interlocked with the ventilation. If the fan is not running, then the saw can not run. Now failure of the fan will not cause ignitable dust concentrations. So the area is not Class II Division I. The saw still produces dust, and over time a film accumulates on all of the equipment in the space. This dust coul be ignited by the sparks of open vented motor brushes, for example. This is a Class II Division II situation, and while I am not currently studying this section, I am guessing that closed motors and dustproof enclosures are a major part of the installation requirements. I suggest you read the equipment requirement differences between the divisions, and you should be able to glean the intent behind the classifications more clearly.

Hope this clear ir up for you.

BTW. I am just a Master Electrician, and have not been specifically educated in this code section, so my information is purely speculation.

 

[rbalex]

Within NFPA terminology, there are no other Codes that deal with the issue; however, there is a Recommended Practice, NFPA 499, that does. It is a reference document in NEC 500.5(B) FPN No.2.

From the OP:

• Case 1 (Class II, Division 1), the equipment failure is both the cause of the presence of sufficient ignitable material AND the source of ignition.
• Case 2 (Class II, Division 2), the " sufficent ignitable material" may, in fact, already be present; however, the electrical equipment, operating normally, is not a source of ignition

Totally enclosed motors [502.125(B)] and dusttight equipment [502.10(B)(4), 502.115(B), etc.] are the generally preferred enclosure types in Class II, Division 2. There are a few exceptions for motors but they are definitely exceptions and take a fair amount of "housekeeping" to be acceptable.

 

[erickench]

Okay, that was a good example.