Earth Rod install

[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.

 

[pirate]

By coincidence, I was watching John Ward's video on this the other day, and it made for interesting watching,

 

[Buzz1980]

That has totally thrown me that video now. Had some info stating that hot tub cannot be connected to a IT or TNC earthing arrangement but I don’t know if that means you are able to connect to a TNCS earthing arrangement as I know TNC and TNCS are slightly different. Or do I use the TNCS system then drive a rod in and combined them. ?

 

[telectrix]

alternative..... rip out old bathroom ffxtures. install a large jacuzzi bath, forget hot tub.

benefits.....
1, cheaper to run when heating with gas.
2. no dangers electrical wise.
3. no pigeon shyte on your bald patch.

 

[pc1966]

Going TT with a double pole RCD or RCBO is the simplest solution.

Make sure the TT rod is close to the tub, then what matters for voltage difference (tub CPC to nearby soil) is going to be fine irrespective of other buried conductors. As you are looking at a 30mA RCD the rod only needs to meet 200 ohm or less so should be easy enough.

Just check for buried services one way or another!

 

[stevethesparks]

Agree with PC1966, DP RCD/rcbo should be the default.

 

[Buzz1980]

Thanks for all replies much appreciated.

have spoke with the hot tub supplier and they have stated that they are plug and play. They either put a 13amp plug top on it at one setting or 16amp plug top on it and change the setting. Said that they don’t TT the system just plug straight in, as it was used for hiring. Is this enough info to go by as looking up threads some say plug and play still have to be TTd ?

 

[marcuswareham]

Thanks for all replies much appreciated.

have spoke with the hot tub supplier and they have stated that they are plug and play. They either put a 13amp plug top on it at one setting or 16amp plug top on it and change the setting. Said that they don’t TT the system just plug straight in, as it was used for hiring. Is this enough info to go by as looking up threads some say plug and play still have to be TTd ?

Alot of people would say a plug and play would not be TTd
and it would be unrealistic for a hot tube hire company to require a TT supply for it,


If it were my hot tub (and i got it second hand so no warranty to void) I would have a little look inside and see how the heating element is assembled and what construction the pumps are. I would image most use plastic pumps and i have seen some tear down videos online of the lazy spa hot tubs it seems some models have class 1 heating elements and other models have an insulating jacket between the water and the element. If all of the exsposed parts are double insulated and the water is insulated from the CPC then there is no risk from PEN faults (class 2)

This might not be possible if it is not your hot tub, I was just saying that what I would do if was mine.

Best bet is to contact the hot tub manufacturers they will hopfully be able to tell you