Below we have a picture of a return duct for an air handling unit in a high-rise office building. Because the duct work is wider than 36 inches, the sampling tube is penetrating the duct work. As per NFPA 72 17.7.5.5.7, the electrician failed to properly seal the penetration; this can lead to the smoke within the duct to escape the duct work and not reach the duct detector itself.
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