Earthing

[HappyHippyDad]

They have stolen the 'Edit button'... all my mistakes will be seen!! I loved the 'edit button'!!

I would have liked to change the last paragraph to :

"because both figures will not allow a large enough fault current to flow to operate the Supply fuse within the required disconnection times"

 

[GMES]

Do you want it changing steve??????????

 

[Leesparkykent]

They have stolen the 'Edit button'... all my mistakes will be seen!! I loved the 'edit button'!!

I would have liked to change the last paragraph to :

"because both figures will not allow a large enough fault current to flow to operate the Supply fuse within the required disconnection times"

I noticed that, Said something about only being able to edit a post up to the maximum of 60mins after it was posted.

 

[HappyHippyDad]

Do you want it changing steve??????????

I edit nearly all of mine all the time Glen! I reread through them and correct spelling mistakes, add bits, take out silly bits etc etc!! I shall now be seen as the fraud I am! :smile5:

 

[Richard Burns]

Earth rod stability and low Ra values are obviously a subject of much debate on here so I am only supplying my take on things.
Most of the intent of getting sub 20Ω or sub 1Ω values stems from earthing for higher power applications where these results would be standard.

Generally (but only generally) in the UK if you are putting a rod in a reasonable location (not rock or builders rubble) then you should be able to get less than 200Ω with a 4ft single rod.
However as conditions vary wildly even in a small area this cannot be taken as read.
So if you were to put a single rod in place and get an Ra of 300Ω then it is likely that there is a problem with the ground conditions (e.g in rock or penetrating fixed voids) and so your value will be unstable as there will not be a good contact of the rod with earth. This could get worse (hopefully it would improve in the long term), especially if you were installing it on a warm wet winters day, as the soil conditions change.
As said above if a 12' rod gave 300Ω then it is likely that the soil is high resistivity but you have a good ground contact so it would be stable.

Whether it would be possible to get above 1667Ω from a 200Ω start I do not know, I would doubt it in general if you have used a good rod to start with, however there is some consideration of best practice and ensuring the longevity of the system you are installing, also the trip time will improve (a bit) at higher currents and would be safer.
I found a 300mm "earth rod" that was providing an Ra of 500Ω and just pulled out as I was seeing if it was solidly in place, still compliant with 1667Ω, however I decided to cause the customer more cost! by installing a decent rod and ensuring that they were safe.
In general I am reiterating the posts above, it is better to aim for as low as practicable so that you have covered yourself in case the resistance increases.
A high resistance with a short rod is indicative (in the UK) of poor earth contact and instability.

 

[davesparks]

Thanks for the explanation Dave.

One last question on the matter.

I know you would not be happy with an Ra of 200Ω, but could you answer if you believe that a value of 200Ω (even if achieved through one 3/8" rod) could ever increase above 1667Ω? Have you ever known such a large change to occur? if you have would you say this is extremely unlikley or quite possible?


If such a large variance is pretty much impossible then would you say 200Ω will offer the same degree of security as >1Ω on a TT installation which is protected by an RCD, because both figures will not allow a large enough fault current to flow to operate the Supply fuse, and both figures will be sufficient to operate a 30mA RCD (limiting fault voltage to below 50V)?

You've missed my point a bit,

You can't set a blanket figure across the country for what is or is not acceptable. You have to consider the soil conditions of each installation.
I have installed a TT system at a house which ended up with a 16' rod at each corner of the house and still had an Ra of ~250ohms, I was happy that with four long rods it was going to be stable and so signed it off
Other installations I have installed 2x 8' rods and got ~20ohms on each rod which I have also been happy with.
The ~250ohm installation is going to be more stable than the ~20ohm installation, but would be 'non-compliant' according to those who have got this 200ohm nonsense stuck in their heads.

 

[HappyHippyDad]

You've missed my point a bit,

You can't set a blanket figure across the country for what is or is not acceptable. You have to consider the soil conditions of each installation.
I have installed a TT system at a house which ended up with a 16' rod at each corner of the house and still had an Ra of ~250ohms, I was happy that with four long rods it was going to be stable and so signed it off
Other installations I have installed 2x 8' rods and got ~20ohms on each rod which I have also been happy with.
The ~250ohm installation is going to be more stable than the ~20ohm installation, but would be 'non-compliant' according to those who have got this 200ohm nonsense stuck in their heads.

Thanks for the explanation Dave.

 

[davesparks]

Thanks for the explanation Dave.

No worries, I hope what i've said makes sense. On bigger installations you would carry out soil resistivity testing before starting and assess the requirements mathematically, but on everyday jobs it is normal to install the first set of rods and see what you get then take it form there.
I normally start with 2x 8' rods spaced 10'/12' apart to begin with and take it from there.

On our firework storage sites we have to bond every metal container to earth electrodes for lightning protection and they have to have no more than 10ohms to earth.

Wheras when I am setting up generators for outdoor events/festivals etc I only ever use single 4' rods as I am not aiming for long term stability, just a low value, as the installation will only be in place for 4/5 days usually, but I do have to pull them out again!

 

[NDG Elecs]

I have had a couple of crap TTs in the last six months. On one of them I only had a 5x5ft concreted floor to use for the rods. No where else was available so had to grind out space for the pit and sink the rods. Using a heavy duty SDS Max with rod driver I only managed to get down approx six feet and a Ra of 50 ohms. The rods point blank refused to sink any lower. I decided to stop when the rods were going in at a rate of 1mm per minute. Gets a bit tedious after 30 mins!! This install has a 100mA time delay upfront due to switch fuse and a standard dual RCD at the board, so I would hope these suffice.

The other crap one I had was I managed to sink three rods coupled together (12ft) into sand. This took less than twenty mins to sink them but the Ra was again around 50 ohms.

Best I have had was 13 ohms off one 4ft rod which dropped down to around 3 ohms with the second rod. I wanted to go further on this one but the client didn't want to pay. And I am a business not a charity.

Always enquire about the availability of PME. It takes a quick phonecall and is much the better solution. Certainly cuts down on blisters!

 

[mechelec]

Just to throw a small spanner in the works of those trying to achieve sub 1 ohm Ra readings, the DNO sub station LV Earth has to be below 20 Ohms. That could be 20 ohms or less so even if you are close to the sub station you will not get any lower !
I have always worked on the theory that you should achieve the lowest reading that is reasonable practicable.

 

[davesparks]

Just to throw a small spanner in the works of those trying to achieve sub 1 ohm Ra readings, the DNO sub station LV Earth has to be below 20 Ohms. That could be 20 ohms or less so even if you are close to the sub station you will not get any lower !
I have always worked on the theory that you should achieve the lowest reading that is reasonable practicable.

Where do you get that information from? The various documents from DNOs I have got set the limit quite a lot lower than 20 ohms. And irrespective of that, how on earth is the substation earth going to affect your Ra?
Ra is the resistance to earth of an electrode or network of electrodes, it does not have anything to do with the DNOs earth connection. The only time the substation earthing would come in to the equation is if the bodge-it method of testing with a loop tester is used rather than a proper electrode tester.

 

[mechelec]

Where do you get that information from? Please feel free to read the document I have linked to below 9.6.1. UKPN are not the only DNO to use this figure. The various documents from DNOs I have got set the limit quite a lot lower than 20 ohms. And irrespective of that, how on earth is the substation earth going to affect your Ra? It won't but it will prevent the resistance of the earth path back to the neutral point of the supplying transformer dropping below that figure. https://library.ukpowernetworks.co....0014+Secondary+Substation+Earthing+Design.pdf
Ra is the resistance to earth of an electrode or network of electrodes, it does not have anything to do with the DNOs earth connection. The only time the substation earthing would come in to the equation is if the bodge-it method of testing with a loop tester is used rather than a proper electrode tester.

by bodge it method I assume you are referring to the alternative method referred to in the OSG ? Usually used on subsequent tests.

https://library.ukpowernetworks.co....0014+Secondary+Substation+Earthing+Design.pdf