Grounding The Roof Structure

[Crossy]

OK, we're finalising the drawings for our retirement home. Standard Thai construction, concrete with a welded steel roof structure under clay / concrete / ceramic tiles.

Common sense says I should ground the steel, so how, individual stakes (we have three isolated roofs), common bonding with a single stake, do we tie these structures to our mains ground? How big a ground cable should we use blah blah.

Any point having lightning conductors?

Any and all suggestions considered

[Khonwan]

OK, we're finalising the drawings for our retirement home. Standard Thai construction, concrete with a welded steel roof structure under clay / concrete / ceramic tiles.

Common sense says I should ground the steel, so how, individual stakes (we have three isolated roofs), common bonding with a single stake, do we tie these structures to our mains ground? How big a ground cable should we use blah blah.

Any point having lightning conductors?

Any and all suggestions considered

Interesting question. My house is of similar construction (built 11 years ago) but as all steel is already welded to the rebar going to ground, I never considered that it may require further grounding. My power outlets are all earthed using a ground rod.

I haven't installed lightning conductors since (right or wrong) I reasoned risk of lightning is very low in my situation due to proximity of hill forest.

[technocracy]

Now this is something which I have been mulling over. As far as I see the normal tendency as Khonwan says is for the roof to be welded to the steel within the concrete pillars which in turn goes to ground. Also they stick a lightning rod on the top of the house which is part of the roof - this concerns me greatly!

I deal with lightning arrestors (conductors, rod etc whatever you want to call them) quite a bit since I work on networking equipment including wireless. To me the lightning arrestor should be the quickest path to earth AWAY from the device/structure you are trying to protect.

NOT stick a metal rod out the top of the metal frame to attract the lightning to the structure! If it received a direct strike the lightning would most likely destroy most of the roof - we are talking about upto 100kA (and about 1 million volts) of current here. Let alone the possible structural damage it would cause to the concrete structure.

Remember most arrestors are usually useless after receiving a direct strike - so why have one welded to the top of your roof frame! Since the roof frame is also covered in a ceramic insulation unless the frame is visible - sticking out from under the ceramic - the likelihood of it been hit is minimal (unless you stick an attractive piece of metal out the top of the tiles ) If it is hit that you still have the frame as previous mentioned to earth it.

Your aerials and other bits of metal sticking up for the house are the main concern - I have a wireless internet antenna at my wifes aunts house where we are living while our house is built which they stuck up on a huge metal pole with no earth whatsoever - even worse the top half of the house if wood and it's screwed directly to it!! Personally I can't wait take it down and move it before the rainy season starts!

For my house my lightning arrestor will be a direct earthing strip down the side house (that is IF I can get one!) the arrestor rod will be positioned as the highest point of the house. I'd prefer the lightning NOT to go through my house structure!

Edited March 5, 2007 by technocracy

[chownah]

does rebar when installed in a house make a good ground?...has anyone tested it the official way?

[technocracy]

does rebar when installed in a house make a good ground?...has anyone tested it the official way?

In short no the rebar isn't a good ground! The gauge of steel used would depend on the strength but remember with the kind of current that lightning has your rebar would most likely melt particular the small tying steel - so causing major structural weakness/damage to the house.

Also as when such as large current passes through the steel will expand and contract rapidly also furthering structural damage i.e. exploding concrete!

In terms of testing it .. .. the problem with lightning arrestors is that on nature can test them!

Like I said before to have a proper lightning arrestor it needs to take the lightning AWAY from the structure. The arrestor is the piece which if you receive a direct strike you want it strike. This strip can then take the current to earth.

The house is ground via the rebar yes but this isn't what it is designed for - what would exactly happen would be dependant on the strength of the strike. For example you wouldn't expect the earth of your TV to provide the earth for the aerial - the current would have already blown up your TV before reaching the earth! Also the chances of it happening are minimal if you ensure the steel roof frame is totally covered.

You aerials, dishes and other protruding conductors are the main concern these are direct paths into your home!

[Crossy]

does rebar when installed in a house make a good ground?...has anyone tested it the official way?

No (certainly not as part of an electrical installation), for two reasons :-

1. There is no guaranteed electrical path from the bit of bar your roof is attached to down to the bottom of the support pillar (tack welds and wire twists don't make good conduction paths).
   
2. The re-bar in your foundation should not be in contact with the ground it should be totally encased in concrete, which if not damp, is a pretty good insulator.
   

[chownah]

does rebar when installed in a house make a good ground?...has anyone tested it the official way?

No (certainly not as part of an electrical installation), for two reasons :-

1. There is no guaranteed electrical path from the bit of bar your roof is attached to down to the bottom of the support pillar (tack welds and wire twists don't make good conduction paths).
   
2. The re-bar in your foundation should not be in contact with the ground it should be totally encased in concrete, which if not damp, is a pretty good insulator.
   

Just as I thought. Now that we have (probably) dispensed with the rebar acting as ground I think it would be good to seperate out two different issues here...one being using lightning rods to dissipate ground charges to avoid a lightning strike and the other being using heavy guage conductors to serve as the path for lightning to travel along if a building should be struck by lightning (in the event that the dissipation of ground charge was not effective. Seems like maybe some people are not aware of these two seperate asepects to lightning safety.

Chownah

[Khonwan]

does rebar when installed in a house make a good ground?...has anyone tested it the official way?

No (certainly not as part of an electrical installation), for two reasons :-

1. There is no guaranteed electrical path from the bit of bar your roof is attached to down to the bottom of the support pillar (tack welds and wire twists don't make good conduction paths).
   
2. The re-bar in your foundation should not be in contact with the ground it should be totally encased in concrete, which if not damp, is a pretty good insulator.
   

Just tested - it grounded, but poorly. 220V supply read only 160V to ground whilst in contact with the roof steel.

[chownah]

does rebar when installed in a house make a good ground?...has anyone tested it the official way?

No (certainly not as part of an electrical installation), for two reasons :-

1. There is no guaranteed electrical path from the bit of bar your roof is attached to down to the bottom of the support pillar (tack welds and wire twists don't make good conduction paths).
   
2. The re-bar in your foundation should not be in contact with the ground it should be totally encased in concrete, which if not damp, is a pretty good insulator.
   

Just tested - it grounded, but poorly. 220V supply read only 160V to ground whilst in contact with the roof steel.

Sounds like this is with no current flowing except for the minute current the VOM uses to measure the potential difference.....if the rebar had to act as ground and it had to carry a realistic voltage I think the voltage difference would probably be even less.

[Pink Mist]

get the steel into the ground, bond securely and jobs done

[elkangorito]

Geez Crossy, I wouldn't bother with it.

The only time lightning arresting earthing is used is if the building is deemed to be critical ie has equipment of a critical nature. In such a case, the external parts of the building are literally covered with flat, 'square edged' copper busbar (usually 25mm x 4mm), which must be connected to each other by 316 grade stainless bolts. It is also recommended that the copper busbar be tinned (with tinning solution) to inhibit corrosion. Each building has its own main lightning earth rod, which is usually 25mm diameter & buried very deep. The earth rod can be copper or stainless. Brass bolts/screws etc must not be used anywhere on the installation.

Mind you, these measures are designed to minimise the affect of a primary lightning strike.

If you're still concerned about lightning, protect yourself against secondary strikes by installing a MOV. These things are not cheap but bloody good. They'll also protect against power surges. Companies like Procel, Turcel or Critec make them. In the past when doing projects for the Australian government, I always used Critec but Procel is my second choice.

[Crossy]

As I suspected, lightning conductors would be overkill or even hazardous, not going to bother, there's a massive Wat next door anyway if lightning's going to hit anything 'twill be there. Pukka MOV going on the incoming supply.

I'm just a little concerned at having a large mass of metal that's effectively isolated from deck with the possibility of it building up a dangerous charge (during a storm maybe), maybe a length of 10mm2 down to a standard ground stake for each framework will give me peace of mind.

I have zero experience of this type of structure (UK roof structures are usually wood), always smart to interrogate the collective knowledge

[Pink Mist]

Geez Crossy, I wouldn't bother with it.

The only time lightning arresting earthing is used is if the building is deemed to be critical ie has equipment of a critical nature. In such a case, the external parts of the building are literally covered with flat, 'square edged' copper busbar (usually 25mm x 4mm), which must be connected to each other by 316 grade stainless bolts. It is also recommended that the copper busbar be tinned (with tinning solution) to inhibit corrosion. Each building has its own main lightning earth rod, which is usually 25mm diameter & buried very deep. The earth rod can be copper or stainless. Brass bolts/screws etc must not be used anywhere on the installation.

Mind you, these measures are designed to minimise the affect of a primary lightning strike.

If you're still concerned about lightning, protect yourself against secondary strikes by installing a MOV. These things are not cheap but bloody good. They'll also protect against power surges. Companies like Procel, Turcel or Critec make them. In the past when doing projects for the Australian government, I always used Critec but Procel is my second choice.

I have used galv water pipe, cut at an angle, to assist driving into ground, all legal at the time, but it does rust

[elkangorito]

As I suspected, lightning conductors would be overkill or even hazardous, not going to bother, there's a massive Wat next door anyway if lightning's going to hit anything 'twill be there. Pukka MOV going on the incoming supply.

I'm just a little concerned at having a large mass of metal that's effectively isolated from deck with the possibility of it building up a dangerous charge (during a storm maybe), maybe a length of 10mm2 down to a standard ground stake for each framework will give me peace of mind.

I have zero experience of this type of structure (UK roof structures are usually wood), always smart to interrogate the collective knowledge

I'd be inclined to up the cable size to 35mm - a bit expensive but even a close & large size secondary strike will melt 10mm like butter.

Edit: don't forget to use a 'spark gap' device on your main protective earth conductor. For this reason, the lighting main earth rod should be well away from the protective main earth rod.

Edited March 5, 2007 by elkangorito

[Pink Mist]

so lets get into the lightning strike theory, not my area of expertise, but 35mm is that gonna do it???

[elkangorito]

so lets get into the lightning strike theory, not my area of expertise, but 35mm is that gonna do it???

Neither will 200mm on some occassions.

[undercover]

Make a good seperate earth to wet ground. the thickest wire you can find and make sure the conections are clean, you can also earth the roof but as was said tack welds do not make a garenteed contact, also one more thing to concider is if you ground the roof and something like your washing machine is grounded to it and has a problem then your whole roof will be live for what ever time it takes for your trip to cut out, should be instant but what if its not ?

Edited March 5, 2007 by undercover

[Totster]

I've been reading with interest.

Would putting a lightning conductor up on the roof actually be inviting a strike rather than the lightning finding another source to the ground, like a high tree nearby ?

Just a question, I have no knowledge of this, so excuse the question if it seems stupid.

One of the reasons for asking is that a German guy in the village was saying the always ground the roof structure in Germany

totster

[elkangorito]

The following is an extract from Oz standards. What do IEC/IEE standards say Crossy?

5.10 OTHER EARTHING ARRANGEMENTS

In general, the earthing arrangements of the following systems should be independent systems; however, if they are connected to the electrical installation earthing system they should not reduce the integrity of the electrical installation earthing system:

[a] Lightning protection.

Static electricity protection.

[c] RFI screened installations.

[d] Information technology installations.

[e] Electromedical installations.

[f] Explosion protection systems.

[g] Cathodic protection systems.

NOTES:

1 Requirements for the installation of protective earthing and bonding conductors for the above systems might be contained in other Standards.

[elkangorito]

I've been reading with interest.

Would putting a lightning conductor up on the roof actually be inviting a strike rather than the lightning finding another source to the ground, like a high tree nearby ?

Just a question, I have no knowledge of this, so excuse the question if it seems stupid.

One of the reasons for asking is that a German guy in the village was saying the always ground the roof structure in Germany

totster

There are 'lightning rods' & 'lightning rods'. Rods that have a sharp point at the end will attract static charges easier than rods with rounded ends. Most static discharge rods are round balls, which are designed to remove high static charges from the atmosphere before enough potential (voltage) is created to initiate a lightning strike. Well, that's the theory anyway.

Edited March 5, 2007 by elkangorito

[Pink Mist]

I've been reading with interest.

Would putting a lightning conductor up on the roof actually be inviting a strike rather than the lightning finding another source to the ground, like a high tree nearby ?

Just a question, I have no knowledge of this, so excuse the question if it seems stupid.

One of the reasons for asking is that a German guy in the village was saying the always ground the roof structure in Germany

totster

thats the whole idea of a lighning rod, to attract the strike

[Totster]

I've been reading with interest.

Would putting a lightning conductor up on the roof actually be inviting a strike rather than the lightning finding another source to the ground, like a high tree nearby ?

Just a question, I have no knowledge of this, so excuse the question if it seems stupid.

One of the reasons for asking is that a German guy in the village was saying the always ground the roof structure in Germany

totster

thats the whole idea of a lighning rod, to attract the strike

I understand that part.. but would it attract a strike that was not really coming in the first place ?

totster

[elkangorito]

I've been reading with interest.

Would putting a lightning conductor up on the roof actually be inviting a strike rather than the lightning finding another source to the ground, like a high tree nearby ?

Just a question, I have no knowledge of this, so excuse the question if it seems stupid.

One of the reasons for asking is that a German guy in the village was saying the always ground the roof structure in Germany

totster

thats the whole idea of a lighning rod, to attract the strike

I understand that part.. but would it attract a strike that was not really coming in the first place ?

totster

The next time you drive past a high voltage switchyard (sub-station), you will notice that there are usually many long & high poles erected. These poles are designed to take the lightning strike away from the equipment in the yard. With these sub-stations, it is assumed that it is a lightning magnet, unlike an average building, so instead of trying to de-energise static charge before it can reach a 'striking' potential, it attracts a strike. But I don't think you'd want to have 10 metre high, pointy poles in your back yard every 5-10 square metres.

On the other hand, many critical establishments who are in areas that endure a high number of lightning strikes, tend to use large round stainless steel balls on the top of poles to keep the immediate atmosphere at a low potential, thus preventing any lightning strikes.

[elkangorito]

Here's a link that proves interesting. The round domes are used to create 'streamers' (a small arc) in order to prevent the production of a 'leader' (a full strike).

http://www.lightningman.com.au/lightning_protection.html

Also, this link to a PDF doc about earthing is pretty good too.

http://www.lightningman.com.au/Earthing.pdf

Edited March 5, 2007 by elkangorito

[Pink Mist]

I've been reading with interest.

Would putting a lightning conductor up on the roof actually be inviting a strike rather than the lightning finding another source to the ground, like a high tree nearby ?

Just a question, I have no knowledge of this, so excuse the question if it seems stupid.

One of the reasons for asking is that a German guy in the village was saying the always ground the roof structure in Germany

totster

thats the whole idea of a lighning rod, to attract the strike

I understand that part.. but would it attract a strike that was not really coming in the first place ?

totster

tots you are funny

[Naam]

"a German guy in the village was saying the always ground the roof structure in Germany"

*****

hmmm... how does one ground a WOODEN roof structure?

[elkangorito]

"a German guy in the village was saying the always ground the roof structure in Germany"

*****

hmmm... how does one ground a WOODEN roof structure?

Easy!!!!!! Install an 'in roof' sprinkler system to keep the wood wet at all times, then attach an earth cable to the wet wood.

[chownah]

I saw old houses up in the German Alps that surely had wooden roofs and they put big rocks on top of them....isn't this their way of grounding the roof?

Chownah

Edited March 5, 2007 by chownah

[GuestHouse]

It is generaly good practice to ground all metal work in a structure. So, go ahead and ground your roof.

[Naam]

I saw old houses up in the German Alps that surely had wooden roofs and they put big rocks on top of them....isn't this their way of grounding the roof?

Chownah

sure! if you stand strategically positioned and one of the rocks falls off and on your head you are "grounded".