Lightning Protection

Gents,
A Tad off base but! I am an elec suprvisor on a job site with lots of metal site huts which have all been individually earthed via a seperate earth stake to gound. We are prone to lightning strikes and they have requested a "storm procedure".
Firstly, if the sheds are earthed, are they safe. Should we just stay in a shed? Is there any standards for this? any suggestions of an apropriate site would be appreciated.

Are you talking about safe for personell or safe for electrical equipment? In terms of staff, I guess the shed is safer than being outdoors, but I'm not sure what would happen if you are leaning against a wall if the shed takes a hit. After all, a lightning rod simply intercepts the strike and conducts it to ground. If you are at the same potential as the ground and the shed, you may be OK, but who knows for sure. I know a guy that got a shock that threw him across the room when he was holding onto his aluminum screen door during a thunderstorm.

As far as electrical equipment, you can get reverse current flow from the ground to the equipment. I personally saw this happen when a panel blew up when lightning hit a pole transformer outside a metal shed in Georgia - the current came in on the grounding and literally smoked the entire panel. It did wonders for my heart rate, too!

The Metal shed will send the current around you instead of through you. The problem is that the heat produced by the lightning could burn anybody in side the shed. I would install lightning rods to prevent the bolt from hitting the shed thereby deverting the heat and current into the rod and earth, and not the shed.

Standards

Here in the USA the "STANDARD FOR THE INSTALLATION OF LIGHTNING
PROTECTION SYSTEMS" NFPA 780 (nfpa.org) is the norm. In Europe
you would go by the IEC standard. It is my experience that the US
standards are more demanding. In US you could contact www.harger.com
in Europe I dealt with Indelec in France.
Do some research on the subject or else you may end up with a useless
lightning protection that gives you a false sense of security.
Actually in US it is advisable to obtain a Master UL label for
the installation and only some installation contractors are qualified for it.

This goes way back to high school physics but I think that you would be safe in the shed for the same reason that your safe in your car. A lot of people think your safe in your car because of the rubber tires but your really safe because of Gauss's law, which is something like the electric field inside any conducting shell is zero. In the case of your car that is the metal frame.

Now if you standing on the ground inside the shed I'm not sure what would happen.

Isnt it called the faraday cage effect.

When it is thundering and lightening and you are in one of those sheds, find a wooden chair to stand on

I think everyone is missing something here...We are talking about the intentions delivery of millions of volts, and amps. Even the smallest amount of resistance can cause a voltage drop across a connection. Even if the conductor is undersized a difference in potential will develop across the length of the conductor. In the moment of the strike you can still be the path of least resistance for part of the energy. Considering the very large current flowing in a lightning strike and the very small amount of current required to hurt or kill someone, I don’t believe that you can say that you are totally safe.

Indeed lightning is a mysterious entity. I'm thinking I would not bet my life on it's predictability.

Greg

How long have Quonset huts been around?

I owned an island once in the Keys off the Florida coast. Trust me the ground there isnt great for grounding electric potential unless you go deep. As was mentioned though electricity follows the path of least resistance and if grounding and/or lightning rods are done properly that will be the path of least resistance. In the days when I owned the island, which I had a surplus military all metal Quonset type building on, after installing the ground and lightning rods I never had a problem. Before installing them the hut got hit once, it made a very large burn mark and a small hole. I am not sure if I would have gotten shock from it but being close to it could have caused some severe burns. Once the lightning rods were installed the bldg never got hit again.

I agree with Jiri, there are ways to do this following codes/guidlines that will make the metal buildings safe. The use of metal buildings is not new and many effective methods are well documented.

Are you by any chance near salt water? That makes a great conductor but offers a large path which minimizes current. This is a very common method used in some places to divert lightning via rods from buildings, machinery and generators used on small islands or on off coast installations.

Many THanks

Thank you all for contributing, I had assumed that we are safer in the huts but thanks for explaining why. For the record, Im in Australia. It is comforting to have access to intelligent guys like you lot.