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Thread: Fire Alarm NFPA 72 21.2.4 3' rule

  1. #1
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    Apr 2008
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    Fire Alarm NFPA 72 21.2.4 3' rule

    When controlling magnetic door hold opens I've often used a relay built into the FACP to break a 120V feed to the magnets to have a single relay operate multiple doors, or using a few addressable relays at convenient locations to break the feed to multiple doors.
    Does this violate NFPA 72 21.2.4? Should I have an individual relay within 3' of each device?
    This ties into my previous thread in that, if this is OK for Door Holders, wouldn't this also be OK for my project where I have 3 elevator shaft dampers in a classified space that are powered closed. Can't I just break the feed at a convenient location outside of the classified space? Or do I need a relay within 3' of each damper? Made it a separate thread as I though if it applies to door holders, it will have a more general interest to the forum.

  2. #2
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    Quote Originally Posted by toddelec2 View Post
    When controlling magnetic door hold opens I've often used a relay built into the FACP to break a 120V feed to the magnets to have a single relay operate multiple doors, or using a few addressable relays at convenient locations to break the feed to multiple doors.
    Does this violate NFPA 72 21.2.4? Should I have an individual relay within 3' of each device?
    This ties into my previous thread in that, if this is OK for Door Holders, wouldn't this also be OK for my project where I have 3 elevator shaft dampers in a classified space that are powered closed. Can't I just break the feed at a convenient location outside of the classified space? Or do I need a relay within 3' of each damper? Made it a separate thread as I though if it applies to door holders, it will have a more general interest to the forum.
    The 3 foot rule can be waived IF a failure in the loop from the relay to the controlled device puts the device in a fail-safe condition. So, if you have a fire door or fire/smoke door in a corridor, the closed position is the safe condition. If an electro-magnet normally holds the door open and the power circuit fails, the door closes, going to the safe condition. No need for the 3 foot rule to be used.

    In the case of the shaft dampers, what is their purpose?

  3. #3
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    Apr 2008
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    Quote Originally Posted by gadfly56 View Post
    The 3 foot rule can be waived IF a failure in the loop from the relay to the controlled device puts the device in a fail-safe condition. So, if you have a fire door or fire/smoke door in a corridor, the closed position is the safe condition. If an electro-magnet normally holds the door open and the power circuit fails, the door closes, going to the safe condition. No need for the 3 foot rule to be used.

    In the case of the shaft dampers, what is their purpose?
    The dampers come out of our elevator code. They are powered closed and are required to open on temperature rise, power failure, any fire alarm, so damage to the wiring between the fire alarm relay normally closed contacts and the damper would have them fail safe. Your answer sent me back to the code book. I see 21.2.6 allows Class D wiring, and 12.3.4 and A12.3.4 even directly mentions door holders, so I would agree that a common sense reading of these articles would be that the 3' rule can be waived for fail safe conditions, however I think that this is as usually poorly written. Though it says Class D is acceptable, it does not state that the 3' rule can be waived. Of course if questioned, I'll be arguing that this is perfectly clear that the 3' rule obviously doesn't apply. Thanks.

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