Earth Rod install

[Buzz1980]

Hi all
Having to install a earth rod In for a hot tub, hot tub just installed outside the house. I am planning on installing earth rod 100mm from edge of house what are the chances that there will be services below this line. The gas pipe and water pipe are further along the wall about 2M away I wouldn’t think the pipes will be running right along side the house would they ?

 

[marcuswareham]

Hi all
Having to install a earth rod In for a hot tub, hot tub just installed outside the house. I am planning on installing earth rod 100mm from edge of house what are the chances that there will be services below this line. The gas pipe and water pipe are further along the wall about 2M away I wouldn’t think the pipes will be running right along side the house would they ?

Are the gas and water pipes metal?, if so they will be connected to the TN-C-S earth

2m is too close too buried TN-C-S potential metal work, meaning making it TT will not get rid of the risk due to PEN fault voltage appearing on the TT earth rod.

The TT rod wants to be a decent distance from anything TN-C-S (most DNOs recommend around 10m)

 

[timhoward]

I believe most have metallic heating element housing which are earthed meaning there is a chance the water can rise in potential should a PEN fault occur, although depends on the conductivity of the water and distance to the heating element to dictate how bad the concicences are

I appreciate this discussion, as there seem to be many views out there about this topic. Good to know about the earthed housing.

Yes earth mat under and around the hot tub i think is the best way connected to the TN-C-S earth , if i was to put one at my house I would go that way,

Has this actually created a new problem in broken PEN situation that you step out with wet feet and now have a directly connected potential of 230v underneath where you are standing? In normal circumstances this certainly adds value, in worst case it probably makes things even worse?

The TT rod wants to be a decent distance from anything TN-C-S (most DNOs recommend around 10m)

And there lies the problem - many properties don't have that much space.

My internal conclusion (that I welcome constructive debate on) is that if enough space TT wins, but TNCS with no rod/mat is 2nd prize subject to a careful assessment.

 

[Buzz1980]

So looking at most of the posts around my post. Everyone will just keep it on the TNC-S system earthing ? Which has gone away from how I have thought hot tubs should be installed. On my thinking it’s not that the even the PEN having a broken neutral and causing the system earth to become live. But as on a TNCS system the neutral and earth combined and ure standing with wet feet on earth then u can get tingles from the water and could potentially rise to a dangerous level. With installing a earth rod, u are taking away that potential danger. I can’t think how being 1m 2m or 10m away from house would make any difference as if broken PEN then most of the time in your house u are insulated from any danger. Or has hot tub installs changed as I don’t do a lot of these installs now ?

 

[pc1966]

Ultimately the risk is the open PEN fault and somehow anything on the house CPC being elevated in potential compared to the Earth.

But what matters is potential difference, what someone in contact with the pool can observe compared to the ground around it. So a buried set of wires/mesh connected to the CPC/earth circuit around the pool area would bring that difference down to low levels. It need not be very deep, just enough not to get damaged/brought up be any gardening activities, so 30-50cm probably OK.

The connection from the house to any external earthing arrangement should be in 10mm^2 cable due to the risk of high diverted neutral currents in the event of a fault, but the actual grid/loops/fingers of copper could be a little smaller, something like "1kg 3.25mm Bare Copper Wire" from here:

wires.co.uk : Bare Copper Wire

BARE COPPER WIRE

www.wires.co.uk

Better still would be 16mm copper as fully meeting BS regs for burial such as this whicxh is sold per meter:

Superlec 1X16 BARE SOFT DRAWN CU STRAND BS6360/81

Non insulated, non sheathed, flexible stranded bare copper cable for use in earthing electrical systems and power distribution networks.

www.superlecdirect.com

With a decent MET style terminal block in a plastic box to link the insulated 10mm wire to a set of 4 or so lengths arranged to cover the area of the tub and a meter or two beyond in all directions.

 

[timhoward]

So a buried set of wires/mesh connected to the CPC/earth circuit around the pool area would bring that difference down to low levels. It need not be very deep, just enough not to get damaged/brought up be any gardening activities, so 30-50cm probably OK.

Out of interest, would this arrangement typically achieve a lower impedance than a rod?
I noticed section 702 mentioned 20 ohms earlier.
(Appreciating this thread btw)

 

[pc1966]

Out of interest, would this arrangement typically achieve a lower impedance than a rod?
I noticed section 702 mentioned 20 ohms earlier.
(Appreciating this thread btw)

It is hard to say.

Low impedance (i.e. the ability to pull down a CPC with significant diverted neutral current) needs a combination of lots of contact with conductive soil. Generally to get reliable conductive soil you need to go deep where the moisture level is there all-year and it is not likely to frost and go high-impedance.

So it is quite possible to have a mesh a modest depth under dry-ish soil that is medium impedance but great at avoiding a voltage difference, but not as good in terms of earthing as, say, a 2.4m rod driven down to where the soil is damp.

There are various approximations that can be used to compute the impedance of an earthing rod or wire, but they are directly dependent on soil resistivity, and that parameter can vary from 10 ohm.m to 1000 ohm.m or more (i.e. 100:1 range).

 

[pc1966]

Using the formulae for the "cruciform" arrangement of 4 legs of L/4 (i.e. L = total buried cable length) covered here:

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

If I plug in the following values:

• L = 6m (so 2 * 3m joined in the middle)
• h = 0.4 (40cm deep)
• d = 0.00451m (buried 16mm^2 cable)
• rho = 150 ohm.m soil as "typical"

I get a earth impedance of 19.74 ohms. But that is directly related to the soil value, and if dry could be 10 times or more higher. So treat any calculated value as very much "ball park" accuracy.

Also for a hot-tub I would, ideally, have a buried ring around the outer region of the tub as that is the bit you would stand on.

 

[pc1966]

For comparison, a 1.2m rod of 0.016m diameter in the same soil 'rho' comes out at 35.38 ohms.

 

[Buzz1980]

Hi all

I have looked at another Avenue. How would it be if I kept it on the PME system, then used a PEN monitoring system and installed hot tub on hot tub mats so when touching water in hot tub your not in contact with earth and if loss of PEN then monitoring system will kick in ? Will this take away the need for a earth rod ? Thanks in advance

 

[marcuswareham]

Hi all

I have looked at another Avenue. How would it be if I kept it on the PME system, then used a PEN monitoring system and installed hot tub on hot tub mats so when touching water in hot tub your not in contact with earth and if loss of PEN then monitoring system will kick in ? Will this take away the need for a earth rod ? Thanks in advance

Which PEN monitoring system are you talking about?

 

[marcuswareham]

Has this actually created a new problem in broken PEN situation that you step out with wet feet and now have a directly connected potential of 230v underneath where you are standing? In normal circumstances this certainly adds value, in worst case it probably makes things even worse?

Although the water in the tube will be connected to the same earth conductor as the earth mat, so you are in contact with the water and the ground with the earth mat under so the difference in potential should be much reduced

 

[Buzz1980]

Hi

just looked up this seems it can’t work. Matt-E have stated not to be used with hot tubs ? So the hot tub is 2meters away from house plan is to run 4mm T&E from consumer unit to isolator, then isolate the T&E earth, at the isolator. Install a earth rod, about 1m away from house. Run 6mm earth cable from rod to isolator. Then run 4mm flex to hot tub in conduit surface from the isolator. So the hot tub will be on a TT system.
There are no extraneous conductive parts near by only a gas pipe which is about 3 meters away from hot tub which is exposed and connected to TNCS earthing system.
Would there be any issues with this install or would anyone recommend differently ? Thanks in advance

 

[marcuswareham]

But as on a TNCS system the neutral and earth combined and ure standing with wet feet on earth then u can get tingles from the water

I believe this is due to resistance in the supply cables adding volt drop, thus causing a small difference between the PME earth and true earth, it would depend on how far the property is from the local transformer. but as you say problem is gone with TT

 

[marcuswareham]

Hi

just looked up this seems it can’t work. Matt-E have stated not to be used with hot tubs ? So the hot tub is 2meters away from house plan is to run 4mm T&E from consumer unit to isolator, then isolate the T&E earth, at the isolator. Install a earth rod, about 1m away from house. Run 6mm earth cable from rod to isolator. Then run 4mm flex to hot tub in conduit surface from the isolator. So the hot tub will be on a TT system.
There are no extraneous conductive parts near by only a gas pipe which is about 3 meters away from hot tub which is exposed and connected to TNCS earthing system.
Would there be any issues with this install or would anyone recommend differently ? Thanks in advance

Just as well as the single-phase version of the matt-e doesn't really offer complete protection against a PEN fault, and also does nothing about the tingles you mentioned

Worth checking there is not any class 1 outside lights or anything like that within reach of the hot tub also, but you have probably sussed this already

3m still seems close, although others on here with more knowledge and experience than me may correct me

I cannot find it now but I remember reading a Weston power document saying a minimum of 8m separation

However this UKPN document says 2m is enough

http://library.ukpowernetworks.co.uk/library/asset/f6acd931-9c79-411f-a98f-f801bce6e63L/EDS+06-0017+Customer+LV+Installation+Earthing+Design.pdf

 

[Buzz1980]

Much appreciated for the response. Yeah there are lights but step ladders to touch them. I can get the rod about 3/4 meters away from the gas pipe and about 1m away from house is the best I can do. I will drive rod in after digging inspection pit, then do as say above. Reason I try to avoid putting hot tubs on TNCS is that I had issued before when went to a job customer was getting tingles from the water when standing outside of tub and that was on TNCS.