Lightweight Faraday cage and lightning

[Reid]

Hi all,

I am curious if a screen tent would provide any protection from a lightning strike. The screen would have to be metal, and would have to continue through the roof. I realize this is not how most screen houses are built.

 

One reason I ask is because I am planning on building a camper trailer. My current trailer has metal siding which gives me some peace of mind during a thunderstorm.

I am thinking about using fiberglass on my next build but don't want to have to worry about being struck. I thought about putting a layer of metal screen in the fibreglass. What do you think?

Sorry if this is the wrong place to ask.

[swansont]

Lightning strikes can briefly carry tens of kiloAmps of current and hundreds of MJ of energy. I suspect a metal mesh would simply be vaporized. Lightning rods on a building I worked on were more than a cm thick.

[Enthalpy]

Lightning protection doesn't have to be a complete Faraday cage. It's usually a few conductors: typically 2-3 masts, connected together through the tip of the roof, and to Earth through a single conductor. The conductors I see are like 6 to 8 mm in diameter.

 

If no mast is possible, you won't know in advance where the bolt may strike, and then a metal sheet is better, BUT this sheet would need to evacuate the big current density from any point, hence be thick.

 

A common rule is that a mast protects a cone of 2*45° under it, so it must be tall, or you need several ones.

 

Antennas are bolt catchers, generally unwanted ones. Prefer antenna designs that are in short-circuit for DC. Beware the antenna cable will definitely carry the bolt inside unless you provide a path to Earth, be it the antenna cable itself (less than perfect) or a separate cable.

 

The Earth contact must be excellent. This is difficult at a house, more so at a mobile home, impossible when moving. In a car, where this Earth contact is bad, the Faraday cage protects better.

[Reid]

Thanks for the replies.

How about a metal frame that is connected to one or two ground rods driven a couple feet into the ground?

Would this be similar to how a house provides some lightning protection?

[Enthalpy]

Yes. The frame would have to be fine-meshed enough so the current in and out jumps to the mesh, but the conductors must be thick enough.

 

A good earth is difficult to achieve. It demands wet soil, and normally has wires spreading from the rod. Not easy with a mobile home... With an imperfect earth, I'd prefer a good Faraday cage, especially at the bottom of the mobile home.

[Reid]

Does a Faraday cage carry the current through it, or does it cause the current to flow around it?

Is there any standard or formula to determine how heavy the mesh needs to be?

 

I have been reading a lot of lightning safety articles lately and the common theme is lightning is unpredictable. I just want to improve my chances.

[Enthalpy]

"Unpredictable, improve my chances": this looks like everyone's situaton...

Though, academic researchers could trigger lightning and guide it to the ground, using a powerful pulsed laser that creates an ionized path for the current. I suppose a similar weapon already exists for a few years, possibly shot from above but not necessarily.

 

The Faraday cage carries the current in its conductors. It avoid the current to flow through the persons within the cage, and (more important for other uses) also reduces the fields resulting from the curent flowing through the conductors. Hence if the Earth contact isn't perfect, it's important that the floor as well conducts electricity, otherwise the current might choose a different path to a better Earth point.

 

The are probably standards which I ignore; for customers, you have to comply with the standards, but if designing for myself, I'd just look around and copy existing designs. I've seen plain and braided copper, zinc; lighter aluminium brings some difficulties.