NEC 800.90 - Evaluating "Lightning Exposure"

[RickHoegberg]

I'm finding the language for determining whether there is lightning exposure in 800.90 FPN No. 2 (2) to be problematic. At stake is whether protectors are needed for a comm run.

See the FPN for the NEC text.

Using standard parsing based on the punctuation to derive clauses, I get 3 clauses, any one of which allow you to omit protectors: (1) Interbuilding cable runs <= 42m (2) directly buried or in underground conduit (3) continuous metallic conduit bonded to the building GES at each end.

However, this only makes logical sense if (1) AND (2). However (3) then seems to just hang out there. I'll spare everyone from translating the clauses into formal logical statements.

What I suspect is some copy editor mistake and misplaced commas.

A tiny bit of editing gives us something much more logically coherent:

Interbuilding runs of 42 m (140 ft) or less directly buried, or in underground conduit where a continuous metallic shield or a continuous metallic conduit containing the cable is bonded to each building GES.

In this revision, lightning exposure is avoided if: (1) Directly buried cable is limited to 42 m (2) Cable in conduit of any length is bonded at both ends to the building GES.

The current NEC code is ambiguous about whether cable in conduit is limited to 42 m - unless - it is bonded.

My contention is that buried metal conduit of 42 m is a greater lightning attractor than 42 m of direct buried so it doesn't make sense to argue
that the 42 m limit applies equally to directly buried cable and unbonded metal conduit. That's why I think the 42 m limit applies only to the direct buried cable and lengths for metal conduit is irrelevant. What is relevant for metal conduit is whether it is bonded at both ends.

Comments? Other interpretations?

 

[don_resqcapt19]

First the FPN is just information and not an enforceable code rule. In this case it tries to give some guidance as to when you are not required to have protectors at each end.

In my opinion all three apply...1) the length is less than 42 meters, 2) the run is underground, either directly buried or in a conduit, and 3) the cable has a metallic shield that is bonded to the grounding system at each end, or the conduit is metallic and bonded to the grounding system at each end.

If you don't have those conditions then you need protectors at each end.

 

[RickHoegberg]

Lighting protection - again

Lighting protection - again

Hi Don.

Thanks for your reply - Especially since it raises an issue I have also fretted about but didn't include, namely that the FPN is a guide not a requirement.

What I can't imagine going well is the following: I'm at a design review and the client asks me "why didn't put protectors on?" I reply: "well the FPN indicated I should but that's just a guide, not mandatory".

In practice, what is theoretically just a guide ends up being mandatory if I'm to have a leg to stand on.

As for the FPN, I'm having trouble seeing it as a 3-part clause, where all must be true i.e. [(1) AND (2) AND (3)], per your interpretation because then (2) is redundant, even contradictory. It would suffice to simply say (1) Lightning exposure is avoided if the cable is < 42 m AND (3) enclosed in metallic conduit bonded at both ends.

Consider: Adding (2) into the mix really muddies things up: Can cable be direct buried AND in conduit? Of course not, these are mutually exclusive so it must be that it is direct buried OR in metallic conduit (as written) but then how can we talk about continuous metallic conduit when talking about direct buried cable? We can't do that either. So this demands that (3) is not linked to (1) by an AND but instead is separated by an OR.

That's why I keep coming back to the interpretation that (2) has to be split into 2 parts and the first half applies only to (1) and the second half only to (3) and that in the end we end up with just 2 possibilities for no lightning exposure:

Direct buried < 42 m
OR
In metallic conduit (length irrelevant) provided bonded at both ends

Can I get an opinion from an NEC committee member? Where would I go for that?

 

[don_resqcapt19]

It doesn't say directly buried and in a raceway. It say or. It is talking about a continuous metal conduit if the cable is in a raceway and it is talking about a cable with metallic shield if it is directly buried. The FPN has 3 clauses, but two of the three have and "or" in the clause. The first one in the "less than 42 meters" and that applies to both of the following clauses, the second is the "cable is directly buried or in a raceway", the third is the "directly buried cable has a metallic sheath that is bonded at both ends, or the conduit is metallic and bonded at both ends".

If you are a NFPA member you can get an opinion from a staff member. There is also a process for a formal interpretation, but getting an answer takes months and your question must be written so that it can be answered yes or no.