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Thread: 500.5 (B) (2) (2)

  1. #1
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    500.5 (B) (2) (2)

    I am moving this to not detract from another post.

    My original question.
    500.5 (B) 2 (2) says something about positive pressure from a vent. Does this mean that it is adding oxygen (positive pressure) to a area instead of removing it? Also does it have to have a shut off for fires and such? Adding oxygen to a fire would be bad in my mind. Unless they are talking cabinets (which makes since there, keep gases out of it) wouldn't they want the fumes removed from the area/room to limit the amount hazardous fumes/gases?

    I was reading this and really was thinking about it more of giving fire a fuel than anything. In a classified area I would think adding fuel to the fire would make it really hard on the guys fighting it.

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    Quote Originally Posted by AKElectrician View Post
    I am moving this to not detract from another post.

    My original question.
    500.5 (B) 2 (2) says something about positive pressure from a vent. Does this mean that it is adding oxygen (positive pressure) to a area instead of removing it? Also does it have to have a shut off for fires and such? Adding oxygen to a fire would be bad in my mind. Unless they are talking cabinets (which makes since there, keep gases out of it) wouldn't they want the fumes removed from the area/room to limit the amount hazardous fumes/gases?

    I was reading this and really was thinking about it more of giving fire a fuel than anything. In a classified area I would think adding fuel to the fire would make it really hard on the guys fighting it.
    This may get a bit long winded. I have mentioned before that, except for locations installed under Articles 511 through 516, a location's electrical hazardous classification cannot be determined from the NEC alone. Section 500.4(B) IN No.2 lists several external standards that may be consulted and IN No.4 discusses ventilation. In addition, Section 500.5(B) INs No 1 and 2 discuss some of the underlying philosophy for Class I, Division 2.

    The two most general classification Standards for Class I locations are NFPA 497 and API RP 500. They both mention in their introductory scopes that they do not address catastrophic releases of flammables/combustibles. Neither does the NEC with regard to classified (hazardous) locations. The significant of this will be discussed later.

    Section 500.5(B)(2) lists the various cases where a location may be classified as Class I, Division 2.
    • Section 500.5(B)(2)(1) is where non-catastrophic releases may occur. Typically this would be an accidental rupture of possibly a leaking process seal.
    • Section 500.5(B)(2)(2) [the OP question] is where mechanical ventilation, as opposed to "natural" ventilation, is used to prevent the buildup of ignitable concentrations. This is usually a case where Protection Technique [NFPA 496] in Section 500.7(D) us used; however NFPA 30 also address means and methods of appropriate mechanical ventilation. Failure of the mechanical system could lead to possible ignitable concentrations.
    • Section 500.5(B)(2)(3) basically says a location adjacent to a Class I, Division 1 location must at least occasionally have a flammable concentration in absence of a positive means to prevent it.

    OOPs hit return to quickly. Catastrophic failures must be addressed by extraordinary means; e.g., NFPA 45, Standard on Fire Protection for Laboratories Using Chemicals.
    Last edited by rbalex; 02-26-18 at 04:09 PM.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  3. #3
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    Quote Originally Posted by rbalex View Post
    This may get a bit long winded. I have mentioned before that, except for locations installed under Articles 511 through 516, a location's electrical hazardous classification cannot be determined from the NEC alone. Section 500.4(B) IN No.2 lists several external standards that may be consulted and IN No.4 discusses ventilation. In addition, Section 500.5(B) INs No 1 and 2 discuss some of the underlying philosophy for Class I, Division 2.

    The two most general classification Standards for Class I locations are NFPA 497 and API RP 500. They both mention in their introductory scopes that they do not address catastrophic releases of flammables/combustibles. Neither does the NEC with regard to classified (hazardous) locations. The significant of this will be discussed later.

    Section 500.5(B)(2) lists the various cases where a location may be classified as Class I, Division 2.
    • Section 500.5(B)(2)(1) is where non-catastrophic releases may occur. Typically this would be an accidental rupture of possibly a leaking process seal.
    • Section 500.5(B)(2)(2) [the OP question] is where mechanical ventilation, as opposed to "natural" ventilation, is used to prevent the buildup of ignitable concentrations. This is usually a case where Protection Technique [NFPA 496] in Section 500.7(D) us used; however NFPA 30 also address means and methods of appropriate mechanical ventilation. Failure of the mechanical system could lead to possible ignitable concentrations.
    • Section 500.5(B)(2)(3) basically says a location adjacent to a Class I, Division 1 location must at least occasionally have a flammable concentration in absence of a positive means to prevent it.


    OOPs hit return to quickly. Catastrophic failures must be addressed by extraordinary means; e.g., NFPA 45-20 II, Standard on Fire Protection for Laboratories Using Chemicals.
    My question is about the "positive mechanical ventilation" part of 500.5(B)(2)(2). To me this means push air into the area not pull air out of it. To me this is the part that seems strange. Why positive and not negative pressure.

  4. #4
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    Quote Originally Posted by AKElectrician View Post
    My question is about the "positive mechanical ventilation" part of 500.5(B)(2)(2). To me this means push air into the area not pull air out of it. To me this is the part that seems strange. Why positive and not negative pressure.
    It is simply about moving air through the location.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

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    Quote Originally Posted by rbalex View Post
    It is simply about moving air through the location.
    I should have said moving the air mechanically as opposed to naturally. (I did say that in my original response)
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

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    Quote Originally Posted by rbalex View Post
    It is simply about moving air through the location.
    I guess I am looking at it like a fart fan, why push the stink out the bathroom under the door than outside through a pipe. Either way the stink is leaving the room though.

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    Quote Originally Posted by AKElectrician View Post
    I guess I am looking at it like a fart fan, why push the stink out the bathroom under the door than outside through a pipe. Either way the stink is leaving the room though.
    To insure sufficient flow, as well as discharge which will not create a classified area at the outlet or in adjacent rooms, the ventilation scheme has to include both an inflow and an outflow.
    Given that, you can either push air in through the inlet or pull it out through the outlet. The biggest practical difference between the two is that pushing air in may create a risk of movement of potentially contaminated air through incidental openings or air leaks into an adjacent area. This is where establishing a positive pressure in an area adjacent to Div 1 may keep it from being classified as Div 2.

    If there is a concern about simply storing combustible liquids in a room, it might be simplest to keep the vent fan of the fume hood running at all times (maybe at a lower flow rate than during active use of the hood space?)

    And, once again, given a particular CFM of airflow, the concentration of oxygen will not be significantly different within the space. There might be some issues with blowing air directly at a flame, but proper diffusers on the inlet can make that in significant too.

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    "The biggest practical difference between the two is that pushing air in may create a risk of movement of potentially contaminated air through incidental openings or air leaks into an adjacent area."

    My whole point.

    Edit
    I guess why is it positive mechanical ventilation, instead of just mechanical ventilation? They want air moving around is what your both saying, but why with positive pressure? Wouldn't negative pressure work just as well if not better?

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    Quote Originally Posted by GoldDigger View Post
    To insure sufficient flow, as well as discharge which will not create a classified area at the outlet or in adjacent rooms, the ventilation scheme has to include both an inflow and an outflow.
    Given that, you can either push air in through the inlet or pull it out through the outlet. The biggest practical difference between the two is that pushing air in may create a risk of movement of potentially contaminated air through incidental openings or air leaks into an adjacent area. This is where establishing a positive pressure in an area adjacent to Div 1 may keep it from being classified as Div 2.

    If there is a concern about simply storing combustible liquids in a room, it might be simplest to keep the vent fan of the fume hood running at all times (maybe at a lower flow rate than during active use of the hood space?)

    And, once again, given a particular CFM of airflow, the concentration of oxygen will not be significantly different within the space. There might be some issues with blowing air directly at a flame, but proper diffusers on the inlet can make that in significant too.
    NFPA 496 gives a pretty thorough description of what needs to be done relative to vents and exhausts. Depending on the size and geometry of a location, it still may take an HVAC consult to determine the existence of any "dead air". I was fairly (only fairly) good at identifying them for simple locations, but still wanted my HVAC buddies to confirm.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  10. #10
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    Quote Originally Posted by AKElectrician View Post
    "The biggest practical difference between the two is that pushing air in may create a risk of movement of potentially contaminated air through incidental openings or air leaks into an adjacent area."

    My whole point.

    Edit
    I guess why is it positive mechanical ventilation, instead of just mechanical ventilation? They want air moving around is what your both saying, but why with positive pressure? Wouldn't negative pressure work just as well if not better?
    "Maintained" may have been a better choice of words, but "positive" has been the word for MANY Code cycles. Here's your opportunity for a Public Input submittal next time around (2023).
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

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