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NFPA 780 Annex L

Question asked by kbourassa on Sep 17, 2019
  1. ADJ is defined by equation L. as the equivalent collection area of the adjacent structure which equals 40xLL. However, this is the same methodology in the determination of AL. Seems that the methodology used should be similar to that of AD. Why two different variables with the same formula if they’re the supposed to represent different things?
  2. Refer to equation L. for the definition of AM. Confirm if using the equation for AD (for rectangular and complex structures) with an added 500 meters to the length, height and width would be sufficient to determine this variable (i.e. for rectangular the proposed formula would be AD = (L+500) x (W+500) + 6 x (H+500) x (L + W + 1000) + 9 x pi x (H+500)2.
  3. AI is defined as the collection area of flashes to ground near the service. Refer to equation L. and figure L.  Confirm formula, Length of service (meters) x (4000 – 40), is accurate to determine the area.
  4. Refer to section L. in the determination of PZ. It says “…Where SPDs are not installed, the probability of failure of internal systems due to a flash near the connected service (PZ) can be taken from Table L.6.7.8. Where SPDs are installed, the value of PZ can be taken to be the lower value of PC or PZ.” There is no prior calculation method for the determination of PZ.  In the case of an SPD being installed am I to evaluate PZ from the aforementioned table and then choose the lower of PC and PZ?
  5. There are two different formulas for the same variable, LA. Refer to equation L.6.6.3 and L. for these formulas.  The risk component RA takes into account this variable but it is not clear which LA should be used.
  6. The variable, PW appears to be no different than PV.  Refer to sections L. and L. for description. I want to make sure there isn’t something I’m missing. Please confirm if this is the case.
  7. There were many errant table references in NFPA 780, i.e. section L. defines the requirements for evaluating PA and then references table L.6.7.1; which is the table for the service environmental coefficient, CE. The correct table for PA is L.6.7.2.