Lightning protection for metal roofed residential housing in the UK?

Peter Brooks said:

Do metal roofed residential housing have to be grounded? 

If we do that in the UK, and other countries that use EN 62305 or IEC 62305, it should be through the lightning protection system (LPS) conductors, not the electrical installation earthing. However, there is another approach, where air terminators of the LPS can provide a zone of protection for the roof itself - meaning that the roof could be connected to the either the LPS, or the electrical installation earthing system, or both. THe air terminator zone of protection is used for antennas too.

I believe the decision in the UK would not be "residential vs non-residential", but chiefly on height (or adjacency to other tall structures), as we have a very low ceraunic level.

Are we discussing grounding (or earthing) in the context of main equipotential bonding for extraneous conductive parts, or is this primarily related to lightning strikes? My perspective is that the necessity for grounding a metal roof isn’t solely determined by its material composition; rather, it depends on factors such as location and height. Ultimately, the presence of a lightning protection system (LPS) for the entire building is crucial. In my opinion, a metal roof alone doesn’t inherently attract lightning strikes more than a non-metal roof.

The current roof (shingles over plywood) is obviously not electrically connected to the ground in any manner. So the current construction practice is just to remove the defective shingles, replace defective plywood and then add metal sheeting. No electrical (grounding) wire is connected to the new metal sheeting. 

I have also recently discovered that apparently solar panels are not allowed to be mounted on a metal roof, based on home insurance rules.

Peter Brooks

Palm Bay 

The current roof (shingles over plywood) is obviously not electrically connected to the ground in any manner.

Keep in mind that the lightning stroke has passed through several hundred metres of air and is quite happy to discharge through dry masonry and timber structures as well as metallic ones. A conductive roof covering might be thought to do the equivalent of shortening the path by a few mm or at most a few metres.

   - Andy.

To a good first approximation the lightning behaves like a constant current source, where the voltage drop across the object will keep rising towards infinity, until the current does actually flow and discharge the strike-  or at least more accurately the open circuit voltage is several tens of megavolts and will quite happily breakthrough materials often thought of as insulating....

The 50m rolling sphere model for estimating protection zones, is based on an assumption of a maximum voltage of circa 50 megavolts, in turn based on the convenient if a bit flaky rule of thumb that air breaks over at a stess field of about 1 million volts per metre.

In turn this tells us that when the thunder  cloud is 5km up, then the lightning has made a hundred (or more) 50m long (or shorter) zig-zags of successive breakdown before reaching us. - rather like a string of unequal capacitors in series all failing one after the other as each in turn suffers a greater over-voltage.

In turns of your tin can roof, the best you can do apart from erect a higher point near by, is to make it the LPS, and design it to survive the I2C associated with the strike current. This sets a minimum metal thickness that will not get a hole blown in it and requires electrodes and low inductance wiring that tie it firmly to the ground beneath. In effect all the steps in designing a LPS

Mike

Hello Andy:

Here is Florida many houses are protected from lightening strikes (according to our Power company)  by the shielding from the mounted high voltage power line on our wooden poles through which (via a mounted fused transformer) we received our house AC power.

In our case about 100 feet away from the house.

New housing development are using buried power lines to harden the distribution system during hurricanes, so I guess the new houses lose this important lightning protection.

Peter Brooks

Palm Bay 

P.S. One does not have to have visible clouds to get a lightning strike. It happens in the Arizona mountain areas. 

protected from lightening strikes (according to our Power company)  by the shielding from the mounted high voltage power line on our wooden poles

That would not be accepted UK practice. One can make a protection system from wires and metal poles, it is often done for things like temporary fuel or ammo dumps, if they are in an exposed location.  In the simplest form with two scaff poles guyewd by wires that  frame  a shape similar to the outline of a traditional ridge tent

  this sketch really is a tent,  but ignore that. (I'm a scout leader.....)


- the zone of protection is estimated by imaging a sphere 'rolling' against the poles and wires - the radius of the sphere is related to the protection class.

However unless each of your wooden poles has a high current capacity conductor running down it to an electrode, and each wire on the pole top has a MOV and spark gap to effectively ground it during the strike, it is more of a means of moving strike energy from one place to another, rather than dissipating it.

And if the closest approach is 100ft (what's that ? -say  30m or so distance) then the it will not give protection tor anything very tall if at all.

Mike.

Hello Mike:

Most houses in Florida are ranch type (single ground level with no basement) as it has to withstand hurricane winds in excess of 120+ mph and are built on sand, covered with a layer of soil. The water table is only about 10 feet deep.  

If you ever looked at video of the Cape (where they shoot rockets off -like this morning with the Falcon 9 launch) they have 4 big lightning towers around the launch site. They will not launch if there is lightning in the area

If fact earlier this week there was only a 30% chance of launching the "Falcon heavy" due to a major storm (with lightning) projected by radar. However the storm went further south and they launched and also managed to bring back the two side booster to land at the Cape.

Peter Brooks

Palm Bay  

Hello Mark:

Just an update to Florida storms and roofing impacts.

Yesterday, Thursday 27th June at 6 pm, about10 miles from me, an EF-0 Tornado touched down and ripped off a number of roofs. No lightning was observed. About 200 houses lost power.

Peter Brooks

Palm Bay 

Hi Peter

Concerning the prohibition of mounting solar panels on metal roofs due to home insurance regulations, what's the underlying rationale ? Even if a structural engineer assesses the roof and provides a report ? Or is this to do with the actual risk of a lightning direct or indirect strike ?

Andrew