Lightning Protection

[bkludecke]

OK Floridians; I'm adding surge protection at panelboards in an observatory dome. The dome is fiberglass but has alot of steel structure just inside. There is quite a bit of branch circuit and control wiring run in pvc conduit inside the dome. One of the scientists working there thinks the pvc should be changed to ferrous metal conduit to protect the wiring from the EMF present in a lightning strike. Is there any validity to his concern? There is a grounding (bonding) conductor run inside the conduit as you would expect. The dome has been struck by lightning before and it wiped out quite a bit of equipment fed from the circuits in question. We are also adding ground rods etc. to agment the existing uffer grounding system.

 

[petersonra]

how would metal conduit provide any additional protection to the branch circuits?

how does adding ground rods provide any additional lightning protection over an existing ufer?

if you have serious lightning problems on an installation IMO it is appropriate to have an engineer with extensive experience in lightning protection design a comprehensive system. I do not believe the design of a lightning protection system is a DIY type project.

 

[bkludecke]

The integrity of the uffer ground/gec is in question so the ground rods are just to insure a code compliant grounding electrode system. Doing a fall of potential test of the uffer ground is out of the question because the observatory dome is built in th middle of a lake.

 

[petersonra]

The integrity of the uffer ground/gec is in question so the ground rods are just to insure a code compliant grounding electrode system. Doing a fall of potential test of the uffer ground is out of the question because the observatory dome is built in th middle of a lake.

IMO if the integrity of the Ufer GEC connection is in doubt, the proper response is to inspect it and make corrections as necessary. I do not see how adding rods brings the installation back up to code if the Ufer is there and not properly connected. In any case code would require the rods to be bonded to the Ufer. How do you do that if the existing Ufer connection is in doubt?

 

[dereckbc]

The use of ferrous metal conduit is valid as it will aid in sheilding EMP.

 

[bkludecke]

IMO if the integrity of the Ufer GEC connection is in doubt, the proper response is to inspect it and make corrections as necessary. I do not see how adding rods brings the installation back up to code if the Ufer is there and not properly connected. In any case code would require the rods to be bonded to the Ufer. How do you do that if the existing Ufer connection is in doubt?

I wish it were that easy. The uffer and connections are not accessable or inspectable; they are either cast in concrete or under car size boulders under water. This is avery unique installation. The GECs going to the uffers are bonded at the transformer disconnect at the dome and that is where the GECs for the ground rods will connect. The uffers may very well be OK. Keep pounding on me though; I can take it and am learning every day.

 

[RayS]

I agree with Dereck's post about the conduit being a shielding benefit. You may want to consider ridgid instead of emt.

If your near a lake, im thinking a big anchor would make an excellent ground...
But seriously, I think if you can put a rod actually in the lake with a bare GEC it'd be great.

And don't forget to keep the lightning pro. downleads away from the instrumentation.

PS: the building steel is bonded and well grounded?

 

[haskindm]

Use a clamp type grounding resistance meter instead of fall of potential to check the existing ground. This does not require that the building be disconnected from the ground to test or access to the Ufer ground. See Mike Holt's Ground resistance video.

 

[bkludecke]

Thanks all. So the metal conduit is a benefit for the EMF effect from lightning? I learn more every day.

 

[RayS]

Thanks all. So the metal conduit is a benefit for the EMF effect from lightning? I learn more every day.

Yep. Even a thin non-ferrous layer works for small fields, like general noise (cable shielding) But for the large magnetic fields during a stroke, I think you need a heavy ferrous shield. If there is a relatively small area that needed additional protection there are special shielding materials (mu-metals) but the cost will go up.

ps there was an engineer that replied to some other lightning posts that really seemed to know this stuff. If you could look up his posts he may be a great source.

 

[jimioy]

I would advise you to get with a Lightning Protection engineer. I would think a ground loop of bare copper 1/0, air terminals properly spaced per the "100' Ball" method around the dome and any connected structures, connected with properly sized cross connect conductors ultimately attached to main conductors, which in turn are connected by down conductors to ground rods attached to ground ring. NFPA 780 is the reg. and Robbins is a good vendor for parts.

 

[bphgravity]

I would install high-quality TVSS at the service equipment or structure panelboard and hope for the best. It is unlikely any installations beyond that would be as effective.