Earthing with solid rock below surface - burying tape for lightning earth

[Simon47]

A quick search for earth tape didn't come up with much, though this thread was of interest.

I'll start by mentioning that I've led a sheltered life and only ever dealt with PME (TN-C-S) so never had to deal with earth rods.
At our church, we've had an inspection of the lightning protection system (LPS), and as I expected the report's come back saying it doesn't comply with LPS standards but "There are hundreds of churches with lightning protection fitted to just the bell towers & spires only, they do not comply with LP standards, but the limited zones of protection provided by them is accepted as the norm, as long as the earth resistance on the system is 10.0 ohms or below."
I suspect most of you are ahead of me here, needless to say our earth rod doesn't give 10 ohms. It's reading 230 ohms ?
Talking to one of the church wardens, he recalls when the current one was fitted. It was to replace another that he described as "about 4 foot long, and when the guy had hammered it in, there was still two foot sticking up out of the ground", adding that the guy said "that's about as good as you'll get" as he'd just hit rock.
I can't see much point trying to get any decent rods in the ground, so tape seems a better bet. I'm thinking if we hire a mini digger over a weekend, we should be able to get a decent length buried - and put some service ducts in while we're at it for other things we've been thinking about.
The good news is that this is on the north side of the church, never gets any sunlight, and as far as I know, never dries out ?

As an aside, I asked them for their guidance regarding bonding of the electrical earth to the LPS earth, their suggestion was that given the high impedance of the current earth, it would be rather pointless. Assuming we get a decent earth, it's my intention to bond as per BS7671.

So for those with experience of such things ...
Anyone got any thought what sort of length we'd need to bury ?
Pointers to guidance on how to calculate (or at least, guesstimate) in advance how much we might need ?
And assuming I get a decent earth, what's the correct way to connect a bonding cable to the LPS conductor (in the earth rod pit, to the tape down the building (below the test/isolation joint)) ?
If necessary I can probably borrow an earth rod test rig - I'm thinking I'll need one to test it afterwards and see what we've got, and also it might be an idea to measure the soil resistivity before we start.

Obviously if funds were plentiful we could just get someone in with a remit of "make it so". But funds, in common with most rural churches, are very much not plentiful ?

Thanks for any insight anyone can give.

 

[DPG]

Always surprises me that the Lord decides to strike churches with his burning thunderbolts. You would think of all the buildings in the country, the most safest would be the churches.

Though shalt fit a ruddy earth spike or I will smite thee with fire, Daz 4:13.

 

[UNG]

I would suggest you install an earth mat or mats rather than just strips of tape, another option is multiple rods properly spaced and linked together.
There is a bit more to lightning protection than just burying pieces of copper in the ground as you need to consider possible step potential issues if a strike occurs and people are around, it would be worthwhile contacting ABB Furse, A N Wallis or one of the other lightning protection manufacturers for some technical advice and possible solutions

 

[marcuswareham]

might be worth having a chat with www.earthingservices.com

They sell other alternatives to earth rods, for use in almost any location, including conductive concrete or foam which will shape around any rocks and give good surface area contact.

 

[Simon47]

Thanks.
One thing that I think you may have overlooked is the finance side of things. It's not that we aren't prepared to spend what's needed, it's a case of there isn't money to spend. To put things in perspective, we had a good collection today - it reached 3 digits (just). The last few weeks it's not made 3 digits. OK, there is some other income, but as I think you can imagine, the bills for keeping a big old shed of a building in good order (not to mention the diocese wanting their contribution) quickly eats up what income we have.
Hence, in reality, getting in specialists is pretty much a last resort.

So at this point, I'm looking at whether there are reasonably easy ways to get down to 10 ohms.
Tape is definitely the easiest as it's a matter of dig a trench, drop it in, backfill and compact. It'll take a while to settle and get full contact, but it's the minimum of excavation.
Mat (yes I already know about the principle) would simply need massive amounts of excavation - though I suppose we could DIY a grid on a lareg grid (not that we have mush distance between the building and boundary wall on that side).
And rods are a non starter in our ground.

Hence asking if people have any direct experience - e.g. how much tape might we need to bury to get down to 10 ohms in damp soil ? Even a rough idea will give me something to go on.

 

[pc1966]

If you take the formula for the cruciform shaped buried conductor (appropriately enough) plug in a typical 150 ohm.m earth resistance, it needs about 36m of buried 16mm^2 conductor (40cm deep, not very senitive to that parameter) to get to 10 ohms.

Measurements and Calculations Of Earth Electrode Systems (BS 7430) | EEP

System earthing involves the provision of a connection to the general mass of earth. This connection should have a resistance not greater than that required to operate safety mechanisms to isolate the electricity supply from a fault situation.

electrical-engineering-portal.com

I think typically they put in a buried ring around the building so conductors come down at various places to join it and a few radial out of that. Will look for some resources I remember reading...

 

[timhoward]

Interesting to read this thread - I used to live in Cumbria and was a former churchwarden and former deanery treasurer so I completely understand the financial side of it as well as some of the challenges of a rather rocky area!
I'm aware of one church having to solve this issue but they managed with two or three linked rods.

CEF sell bare copper tape with a csa of 75 sq mm, which should reduce the distance, I'll run the numbers for this later tonight if no-one beats me to it.
Out of interest - which area?

 

[pc1966]

I was going to add that I'm not sure 16mm copper is acceptable for a LPS, as pointed out above typical tape is 75mm^2.

However, if you attempt the adiabatic for a 100kA 10us/350us lightning surge it is about 16mm! For radials off a higher current ring or similar attachment to the down conductor they may be OK. The earth resistance is not so impacted by CSA, more total length.

 

[timhoward]

I've now used formula d from the link that @pc1966 provided which is a straight run of a conductor.
I think I've got this right, I've included the formulas in case you want to recreate it (or so someone else can point out any mistakes!).
It seems to suggest that again with soil resistivity of 150 ohm-meters, 21 metres of 75 sq mm csa tape buried 50cm down would get you to 9.9 ohms.
(If you install all 25m in the pack it comes out as 8.69 ohms)

The main variable is the resistivity of the soil and as you say it's worth checking that in advance.

 

[Robson689]

You would be looking at around £700+ for a 900x900x3mm lattice mat to be installed in soil, connected to the existing electrode and certified. This is normally half a day labour for 2 men. It would also be backfilled with marconite usually 4 bags.

This would be the standard approach for a single earth sat on rock.

 

[pc1966]

I've now used formula d from the link that @pc1966 provided which is a straight run of a conductor.
I think I've got this right, I've included the formulas in case you want to recreate it (or so someone else can point out any mistakes!).
It seems to suggest that again with soil resistivity of 150 ohm-meters, 21 metres of 75 sq mm csa tape buried 50cm down would get you to 9.9 ohms.

Yes, much as I do for linear tape.

(If you install all 25m in the pack it comes out as 8.69 ohms)

The main variable is the resistivity of the soil and as you say it's worth checking that in advance.

The soil is always the dominant unknown!

 

[Mike Johnson]

It always amaze's me that the richest land owner in the UK can't afford to do the job itself.

 

[timhoward]

It always amaze's me that the richest land owner in the UK can't afford to do the job itself.

You'd think, but in fact the land owner has absolutely nothing to do with it in practise.
The short version of a very complicated arrangement is that a local C of E church usually gets a vicar/priest in charge (often shared between several churches these days) and a house for them to live in provided. They are actually the only things a local church doesn't have to fund itself.
All other costs, utility bills, insurances, repairs etc. have to be paid for out of local funds. The only income is what a congregation give and gift aid on it. It also has to pay a fee to C of E based on a complex formula which is typically at least £50k+ a year as a contribution towards housing and employment costs and pension costs.
So locally many churches are pretty skint, it all depends on the disposable income of the people who attend them.
I can totally see why it would appear differently at face value though.

 

[Mike Johnson]

The Church of England has it all ways it would seem, get everyone else to pay for everything, I think if they sold off a few assets, like the City of London, they could pay for their own upkeep and not keep sponging off the congregation.

 

[nicebutdim]

The Church of England has it all ways it would seem, get everyone else to pay for everything, I think if they sold off a few assets, like the City of London, they could pay for their own upkeep and not keep sponging off the congregation.

I believe people give to churches of their own free will.

Long time since I've been through the door of any, but it always seemed to me that donations were optional.