MV Trestle Lightning Protection

[Fady saco]

Gentlemen,
I'm working on reviewing a MV Trestle design, the design engineering team not showing any Air terminals on the top of the Trestle and the claim is they are using the Trestle heavy steel structure and intense grounding system in leu of the Air Terminals. the argument is there's a cable tray with MV Cables sitting on the top of the Trestle and it is higher that the Steel structure minimum 6", the Steel structure will be painted and I do think that will make the cable tray on the top is the first point of the lightning strike.

please advice if Air Terminals will be required by NFPA 780 or NEC and if you can help me with finding any article to support

Regards

 

[tom baker]

This is not an area that the NEC covers, and little experience here. From previous lightning posts, I would recommend finding a UL certified protection specialist.

 

[Phil Timmons]

Closest model that comes to mind is a typical HV to MV substation. We usually put up independent poles (or masts) to about 60 feet and run a common ground connected wire above the whole site. This link below has some decent design notes. Lightning protection is a bit "black art." You may want to dump this problem on some specialist(s), if you have any budget for that. Does the site have anybody that does that? https://eepower.com/technical-artic...g-methods-for-lightning-strikes-part-3-of-4/#

 

[Fady saco]

Thanks for Reply

 

[ron]

The code doesn't require lightning protection.

However if lightning protection is electively being installed, then it should be installed per NFPA 780, which depending on the material of the tray and or the surrounding materials, either it gets bonded or gets and air terminal.

 

[paulengr]

MV shielded cables are highly susceptible to transients. Better have surge arresters at terminations at a bare minimum.

NFPA 780 doesn’t mandate anything, just recommends how to do it.

I wouldn’t bother with the whole air terminal and wiring. Just attach angles sticking up
Over the tray or string a single static line bonded to the trestle. Use the rolling sphere or similar models in NFPA 780 but since the whole structure is a ground you don’t need to run wiring like you do with wood structures.