Earth rod into solid rock

[Jon Maine]

Does anyone here have experience of installing earth rods for lightening surge protection?

We have a lot of sensitive electronics in the house and I feel I need to protect it all, it would appear using the electricity boards earth which is on the pme is not good enough as I guess it too could be a source of a surge. This means we have to fit an earth rod but we are on solid rock.

Does anyone have experience of such issues?

 

[robgil]

you may have to drill a borehole first.

this may not help your situation much but its an interesting read all the same.

http://www.qc-sites.com/Construction-oct209.shtm

 

[MdB]

What sort of rock? Porous or non porous?

If you are on a porous rock with a high water table then you are in a lot better position than a non porous rock with a low water table.

I would guess that the requirements for a lightning earth would be similar to that for an electrical earth so it would be sensible to fit (somehow) a copper earthing spike into the ground and then test it. If it isn't good enough, fit another and retest. My sparky mate ended up fitting 10 to a house which was on Cornish granite before he got a good reading.

 

[Lime]

Does anyone here have experience of installing earth rods for lightening surge protection?

We have a lot of sensitive electronics in the house and I feel I need to protect it all, it would appear using the electricity boards earth which is on the pme is not good enough as I guess it too could be a source of a surge. This means we have to fit an earth rod but we are on solid rock.

Does anyone have experience of such issues?

Perhaps it is easy to forget that preventing electrical surges affecting equipment is not the same as having a good earth.
What I would concentrate on is that the services and ALL else connected with the sensitive equipment be bonded together.
If you do provide yourself and your equipment with a good earth then the PME will use it too (as well as any passing lightening strike) which may make your position a whole lot worse.

 

[Jon Maine]

Electrical surge protectors appear to work by having a gate which flies open to contact to earth in 25 nano seconds or so. This earth contact then dumps the excess spike to ground through the earth spike. ( thats how it seems to work anyway)

I am going to put this type of device on the incoming for

1/ telecoms

2/ Radio and TV aerial

3/ Mains supply

So if any of these get energised by lightening we should be able to dump before it blows any equipment.

Talking of boreholes you have given me an idea, I have been cursing that the boreholes for the GSHP are all back filled as they would have been an ideal place to drop an earth rod low down where the rock is saturated, however I have just remembered we kept one borehole open for our water supply. Surely I could drop an earth cable with a copper rod on the end down into the water course some 36 metres down to get an earth, surely we couldnt get much better than that.

It would appear that Mendip limestone is not incredibly porous so this could well be a solution, what say all of you?

 

[robgil]

be aware of corrosion in a bore hole from the surrounding rock and water.id say 36 meters might be a tad excessive , isnt 3 meters the norm? over time if the cable were to corrode the rod would be lift way down there doing nothing.

 

[Penners]

This is a question born of complete ignorance of this topic, but if an electrical surge protector simply redirects the surge to earth, why will it not work with a protective multiple earth as well as an earth spike?

 

[Lime]

This is a question born of complete ignorance of this topic, but if an electrical surge protector simply redirects the surge to earth, why will it not work with a protective multiple earth as well as an earth spike?

It probably will.
The surge will discharge through the protectors/suppressors either from live to neutral, live to PME earth or neutral to PME earth.
Introducing a "good" earth will not minimise the problems created by a surge but it may make it much worse.
That is the point I'm trying to make.
If a static sensitive device is connected to well-bonded or isolated equipment supplied by a PME system then the effect of the surge protectors will not be any different even if the "real" earthing is poor.

(BTW the surge protectors work by allowing current to flow when a specific potential difference appears across the protector, usually C280 volts or more.
A small amount of time is taken before this happens (time in which damage can occur) and there is a maximum limit to the amount of current a surge protector can take.
Also PME (protective multiple earthing) means that the house earth wires are directly connected the mains supply neutral and other metalic incoming services, thus keeping them all at the same potential.)


Perhaps it would help to give examples of the danger created by installing/introducing a good earth when using a PME supply.
The purpose of PME is to prevent a large difference of voltage between the neutral and the wire called earth.
It is used where a good earth cannot easily be obtained.
The neutral is rarely at true or real earth potential.
A consequence of that when using PME is that there will nearly always be a voltage difference between the PME earth and real earth.
If the two different "earths" are not connected together a large voltage may at some time be present between them which is dangerous.
If they are connected together then a current will flow between them that may be considerable which is also dangerous.
it is safer not to introduce a good earth in these circumstances unless it is kept completely separate from any other conductor, either directly or through any electrical appliance, in the PME system.

Also if a good earth is provided to protect a sensitive device or the good earth is connected to the surge protectors then any surge carried by the mains supply or PME earth will channel itself through the good earth.
A lightning strike elsewhere would seek out the good earth to discharge to.
It is better to design the electrical system so that external surges/variations (lightning strikes or the neighbours blowing a fuse) are not "encouraged" to enter the property.

 

[Jon Maine]

This is a question born of complete ignorance of this topic, but if an electrical surge protector simply redirects the surge to earth, why will it not work with a protective multiple earth as well as an earth spike?

It probably will.
The surge will discharge through the protectors/suppressors either from live to neutral, live to PME earth or neutral to PME earth.
Introducing a "good" earth will not minimise the problems created by a surge but it may make it much worse.
That is the point I'm trying to make.
If a static sensitive device is connected to well-bonded or isolated equipment supplied by a PME system then the effect of the surge protectors will not be any different even if the "real" earthing is poor.

(BTW the surge protectors work by allowing current to flow when a specific potential difference appears across the protector, usually C280 volts or more.
A small amount of time is taken before this happens (time in which damage can occur) and there is a maximum limit to the amount of current a surge protector can take.
Also PME (protective multiple earthing) means that the house earth wires are directly connected the mains supply neutral and other metalic incoming services, thus keeping them all at the same potential.)


Perhaps it would help to give examples of the danger created by installing/introducing a good earth when using a PME supply.
The purpose of PME is to prevent a large difference of voltage between the neutral and the wire called earth.
It is used where a good earth cannot easily be obtained.
The neutral is rarely at true or real earth potential.
A consequence of that when using PME is that there will nearly always be a voltage difference between the PME earth and real earth.
If the two different "earths" are not connected together a large voltage may at some time be present between them which is dangerous.
If they are connected together then a current will flow between them that may be considerable which is also dangerous.
it is safer not to introduce a good earth in these circumstances unless it is kept completely separate from any other conductor, either directly or through any electrical appliance, in the PME system.

Also if a good earth is provided to protect a sensitive device or the good earth is connected to the surge protectors then any surge carried by the mains supply or PME earth will channel itself through the good earth.
A lightning strike elsewhere would seek out the good earth to discharge to.
It is better to design the electrical system so that external surges/variations (lightning strikes or the neighbours blowing a fuse) are not "encouraged" to enter the property.

No one is talking about linking the earthing from the surge protectors to the PME, the PME may possibly itself be effected by a surge so that would not be helpful and indeed would be counter productive.

There are gates in the surge protectors which take a few nano seconds to dump a surge to ground. Ground in this instance needs to be exactly that.

I am unsure what can be done outside the building to limit a surge entering the building? The cables as soon as they enter the building will have surge protectors on them. Those will be coupled together with the common earth running to ground in insulated cables through the cellars so even if there is a potential difference there it will not be a danger unless I bare both pme and the grounded earth and held them at the same time. I have so far resisted the temptation to do this with other cables with proven PD so don't consider this is a problem especially when the professional surge protectors stipulate wiring in this way.

My problem is purely how to meet the criteria of the specialist suppliers of surge protectors of providing a good earth seperate from PME, I am sure that seeing all of these professional companies work to the same standard it is industry standard and therefore both safe and produces the required results.

We need to protect

1/ telecoms

2/ radio and tv aerials

3/ incoming power supplies

So any useful suggestions of my questions asked will be gratefully received.

 

[Flyfisher]

Er, what are the actual electrical ratings of these surge suppressors? I thought they were typically for protecting mains supplies from spikes of a few hundred volts. Will a bolt of lightning of a few million volts, capable of jumping a few kilometers through free space, really be deflected by a fancy switch the size of a pea?

 

[biffvernon]

Two things, Jon. First, if you are looking for a good earth connection why not just drop it down your borehole into the water - much better than connecting to a piece of sometimes dry limestone.

But, secondly and significantly, it won't do any good if you suffer a direct hit.

About 10 years ago my house was struck by lightning. A chimney stack and large part of masonry gable end wall was scattered over a 30 yard radius. The electricity then wended it's way along electric cables - exploding them and any overlying plaster, destroying the consumer unit, capacitors on central heating pumps disintegrated, a telephone exploded with small plastic shards scattered about, my computer died a violent death (though the hard disk survived intact). The electricity then strolled down the lane where, 200 yards away, a big flash emerging from my neighbour's woodworking machinery was reported. Computer modems 300 yards away were destroyed. The telephone cables along the road had to be replaced.

I'm not saying that all attempts at protection are futile, but it's worth remembering that electricity that can arc across a few thousand feet of air, is not always going to worry about our puny surge protectors. As Flyfisher says, these are designed to protect against the vagueries of man made electricity, not the natural stuff.

 

[Feltwell]

Hmmm, this post has made me think......

Whilst my driveway is currently dug up and resembles Glastonbury Festival on a wet year, I'm taking the opportunity to replace the lead water pipe. Being an older electrical installation (renewed in the 80's I believe, although consumer unit has been done since then) I am presuming that the earthing arrangement for the house is via the copper plumbing, which in turn relies on the lead pipe going into the ground??

What should I do now this is to be replaced with plastic?

 

[biffvernon]

Jon, you might find this thread on Green Building Forum interesting:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4988&page=1#Item_0

 

[Lime]

Hmmm, this post has made me think......
Whilst my driveway is currently dug up and resembles Glastonbury Festival on a wet year, I'm taking the opportunity to replace the lead water pipe. Being an older electrical installation (renewed in the 80's I believe, although consumer unit has been done since then) I am presuming that the earthing arrangement for the house is via the copper plumbing, which in turn relies on the lead pipe going into the ground??
What should I do now this is to be replaced with plastic?

If the wiring was installed compliant with the regulations (then current) it should already have an alternative earth fitted.
As an aside, leaving the old lead pipe in the soil albeit disconnected would give a reasonable earth if so required.

 

[Flyfisher]

About 10 years ago my house was struck by lightning.

An interesting description of what can happen with a direct hit. Out of interest Biff, was the damage covered by insurance?

Lightning can be funny stuff though. The closest we've ever come to a direct hit was about 5 years ago when our son said he saw one of our apple trees take a direct hit during a storm. I'm sorry to say we didn't take much notice of him at the time because the house is much higher than the fairly close apple tree and there's also a scots pine within 10m that towers over both, so I didn't believe that lightning would go for the apple tree over the house or the pine.

However, the following morning I could clearly see burn marks on a broken-apart bough of the apple tree. Very odd.

 

[Jon Maine]

Er, what are the actual electrical ratings of these surge suppressors? I thought they were typically for protecting mains supplies from spikes of a few hundred volts. Will a bolt of lightning of a few million volts, capable of jumping a few kilometers through free space, really be deflected by a fancy switch the size of a pea?

The ratings are as suggested not going to deal with a direct hit, however if we happen to be in the ripple zone of another direct hit they may be enough to save several thousand pounds worth of electrical kit.

Two things, Jon. First, if you are looking for a good earth connection why not just drop it down your borehole into the water - much better than connecting to a piece of sometimes dry limestone.

But, secondly and significantly, it won't do any good if you suffer a direct hit.

Yes borehole seems to be a sensible option to me, wanted to check I wasn't being daft and had overlooked anything. I was thinking about using a section of copper pipe dangling on a 10mm cable ( brazed or soldered ). What do you think?

be aware of corrosion in a bore hole from the surrounding rock and water.id say 36 meters might be a tad excessive , isnt 3 meters the norm? over time if the cable were to corrode the rod would be lift way down there doing nothing.

I am only talking that distance as that is where the aquifer is located, there is (was last time measured) 12m of head above but that may fluctuate so to be sure to be in the water 36m would be the number. I dont think that corrosion of copper in water surrounded by rock should be more significant than a copper earth rod in solid ground. It will hopefully outlast the lifetime of the electronics in question.

So all in all I think I know which way I will go, earth in the borehole and surge protectors which hopefully act quick enough to ground any spike large enough for them to deal with. We have two substially larger buildings next door,so hopefully will not suffer a direct hit and certainly one of them has lightening protection

 

[Lime]

No one is talking about linking the earthing from the surge protectors to the PME, the PME may possibly itself be effected by a surge so that would not be helpful and indeed would be counter productive.

Then how do you propose to stop the surge travelling via the PME earth to the equipment?
Fitting surge protectors across the PME earth and your better earth will allow a very high voltage to be present until the dump, as you refer to it, takes place.

There are gates in the surge protectors which take a few nano seconds to dump a surge to ground. Ground in this instance needs to be exactly that.

What type are they?
It is usual to use several different types for good protection.
For instance, gas discharge tubes are robust, take a very high current but are slow.

I am unsure what can be done outside the building to limit a surge entering the building? The cables as soon as they enter the building will have surge protectors on them. Those will be coupled together with the common earth running to ground in insulated cables through the cellars so even if there is a potential difference there it will not be a danger unless I bare both pme and the grounded earth and held them at the same time. I have so far resisted the temptation to do this with other cables with proven PD so don't consider this is a problem especially when the professional surge protectors stipulate wiring in this way.
My problem is purely how to meet the criteria of the specialist suppliers of surge protectors of providing a good earth seperate from PME, I am sure that seeing all of these professional companies work to the same standard it is industry standard and therefore both safe and produces the required results.

Protection against lightning is much more than just protectors and a good earth.
Design is paramount.
You have not answered MdB's question about what type of rock you are situated upon.

Lightning can gain access to your equipment in three distinct ways.
..Via incoming services and data lines as a voltage surge.
..Through the earth/soil by means of a high voltage gradient.
..By electromagnetic radiation causing some form of induced voltage perhaps within the semiconductors themselves.
The usual method is to create a faraday cage in the form of a mesh in the superstructure of the building.
Incoming services and data lines enter the premises at one point through an earth shield connected to a conductive mesh to earth.
It is here the protection is fitted too.
However, as you obviously realise, the problem of location (i.e. on rock) is not ideal and requires some inginuity to accomplish if at all.
Whatever earthing system is eventually chosen, multiple spikes - a buried mesh - deep drilling or any other method, a local lightning strike will disrupt the earthing because of the poor soil conduction.

 

[Jon Maine]

No one is talking about linking the earthing from the surge protectors to the PME, the PME may possibly itself be effected by a surge so that would not be helpful and indeed would be counter productive.

Then how do you propose to stop the surge travelling via the PME earth to the equipment?
Fitting surge protectors across the PME earth and your better earth will allow a very high voltage to be present until the dump, as you refer to it, takes place.

There are gates in the surge protectors which take a few nano seconds to dump a surge to ground. Ground in this instance needs to be exactly that.

What type are they?
It is usual to use several different types for good protection.
For instance, gas discharge tubes are robust, take a very high current but are slow.

I am unsure what can be done outside the building to limit a surge entering the building? The cables as soon as they enter the building will have surge protectors on them. Those will be coupled together with the common earth running to ground in insulated cables through the cellars so even if there is a potential difference there it will not be a danger unless I bare both pme and the grounded earth and held them at the same time. I have so far resisted the temptation to do this with other cables with proven PD so don't consider this is a problem especially when the professional surge protectors stipulate wiring in this way.
My problem is purely how to meet the criteria of the specialist suppliers of surge protectors of providing a good earth seperate from PME, I am sure that seeing all of these professional companies work to the same standard it is industry standard and therefore both safe and produces the required results.

Protection against lightning is much more than just protectors and a good earth.
Design is paramount.
You have not answered MdB's question about what type of rock you are situated upon.

Lightning can gain access to your equipment in three distinct ways.
..Via incoming services and data lines as a voltage surge.
..Through the earth/soil by means of a high voltage gradient.
..By electromagnetic radiation causing some form of induced voltage perhaps within the semiconductors themselves.
The usual method is to create a faraday cage in the form of a mesh in the superstructure of the building.
Incoming services and data lines enter the premises at one point through an earth shield connected to a conductive mesh to earth.
It is here the protection is fitted too.
However, as you obviously realise, the problem of location (i.e. on rock) is not ideal and requires some inginuity to accomplish if at all.
Whatever earthing system is eventually chosen, multiple spikes - a buried mesh - deep drilling or any other method, a local lightning strike will disrupt the earthing because of the poor soil conduction.

I propose to fit a unit from the PME earth surge gated to our earth to solve that issue. So what if there is a high voltage present? As long as I can provide a good ground in the aquifer 100 feet plus down it will disappear surely. If its to big to then I loose some kit in the house but I know I have made an effort for protection against surges the kit can cope with. The arrestors are not gas but am unsure of their construction although they will handle 40Ka.

Stated earlier in the post

It would appear that Mendip limestone is not incredibly porous

so I thought I had answered Mdb.

I cant fit a faraday cage in a grade 2* merchants House really can I? Although you are taking this far further than is needed, all i need to achieve is protection incase of surges which enter via cabling damage the electronics system in the house or we could be facing a £25,000+ bill for the sake of a hundred quids worth of kit that could protect it from an indirect hit.

Direct hits we have already established with this old type of property are going to cause a problem whatever you do. Fair enough if you are building a new steel frame building put in your faraday cage but not really suited to period properties unless you invent a time machine and go back to suggest it to the original builders

 

[Flyfisher]

Not much use for Jon's house, but using wire mesh under a rendered finish would be a way of constructing a faraday cage.

But it's all a bit excessive if lightning strikes are covered by insurance. Did we discount that yet?

 

[Jon Maine]

Not much use for Jon's house, but using wire mesh under a rendered finish would be a way of constructing a faraday cage.

But it's all a bit excessive if lightning strikes are covered by insurance. Did we discount that yet?

I am not sure about insurance, just want to avoid an future problem for a little money and effort if at all possible.