Dust-IgnitionProof Requirement For Resistors in Class II Div 2, 502.120(B)(3)

cowski

Member
Location
New York
Occupation
Engineer
We were planning to mount a small VFD braking reistor in a dusttight enclosure. We're in a Class II Div 2 environment.

It seems like we would have to increase our protection level to dust-ignitionproof per 502.120(B)(3). Am I reading this correctly? Dust-ignitionproof would require a NEMA 9 enclosure, right? I looked at the definition for dust-ignitionproof in Article 100 expecting to see NEMA 9 but it references ANSI/UL 1202-2013 instead. Does that point to NEMA 9? 502.120(B)(3) seems a little restrictive in that dust-ignitionproof is the only mitigation technique. 500.7 doesn't seem to apply. Does purging an enclosure make it dust-ignitionproof per ANSI/UL 1202-2013?

Are there any other parts of the code that modify the requirement in 502.120(B)(3)? 501.120(B)(3) is less restrictive. I'm suprised the same surfance temperature expemptions conditions do not apply to Class II. Or is there a more general section that would allow them to apply? Something like 500.8(A)(3)?

Thanks in advance,
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Purge/pressurization techniques reduce the classification inside the enclosure to typically non-hazardous so no special protection required inside the purged/pressurized enclosure.

NEMA types are no longer used. UL types now.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
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Professional Electrical Engineer
Have you considered 502(B)(4)? Essentially, dusttight is all that is necessary in Class II, Division 2. See Table 110.28 Notes for enclosures that are considered to be dusttight.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
502.120 Control Transformers and Resistors.
(B) Class II, Division 2. In Class II, Division 2 locations, transformers
and resistors shall comply with 502.120(B)(1) through
(B)(3).
(3) Resistors. Resistors and resistance devices shall have dustignitionproof
enclosures that are dusttight or otherwise identified
for the location.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Have you considered 502(B)(4)? Essentially, dusttight is all that is necessary in Class II, Division 2. See Table 110.28 Notes for enclosures that are considered to be dusttight.
I meant to reference Section 502.10 (B)(4).
 

cowski

Member
Location
New York
Occupation
Engineer
Thank you Bob(s)!!!

(3) Resistors. Resistors and resistance devices shall have dust-ignitionproof enclosures that are dusttight or....
I interpreted this as "resistors must be placed in dust-ignitionproof enclosures that are also dusttight or...". So both requirements need to be met.

Are you saying dusttight is the only requirement? Dusttight is easy, but dust-ignitionproof seems to require UL Type 9 (the 'new' NEMA 9, right?) or a purge.
 

cowski

Member
Location
New York
Occupation
Engineer
Or, would the enclosures provided with some resistors be considered dust-ignitionproof if they are completely sealed?
So this would be dust-ignitionproof:​
1656599332076.png
But certainly not this:​
1656599339010.png
And probably not this:​
1656599350018.png
I'm not sure. I obviously don't see any reference to "dust-ignitionproof" in product literature. But the higher quality resistors do seem to be comepletely (hermitcally?) sealed.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Dust-Ignitionproof [as applied to Hazardous (Classified) Locations].
Equipment enclosed in a manner that excludes dusts
and does not permit arcs, sparks, or heat otherwise generated
or liberated inside of the enclosure to cause ignition of exterior
accumulations or atmospheric suspensions of a specified dust
on or in the vicinity of the enclosure.
From article 100.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Or, would the enclosures provided with some resistors be considered dust-ignitionproof if they are completely sealed?
So this would be dust-ignitionproof:​
But certainly not this:​
And probably not this:​
I'm not sure. I obviously don't see any reference to "dust-ignitionproof" in product literature. But the higher quality resistors do seem to be comepletely (hermitcally?) sealed.
I think that is just a coating for better heat transfer.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Philosophically, a major difference between Class I and Class II protection techniques is that, unless they are essentially hermetically sealed, gases will get inside an enclosure no matter how the sealing is otherwise accomplished. That is not so with Class II; dusts, can in fact, be effectively excluded from the interior of an enclosure if properly sealed per Section 502.15. That is why 502.10(B)(4) only requires dusttight enclosures for Division 2. Also see also the opening paragraph of Section 500.8 (B)(1) that emphasizes it is the surface of enclosures need to have various temperature restrictions.
 
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