Substation Earthing 2

[bluenose72]

Any advice where I can source (BS's/Standards etc) the earth value of a sub-station. My thoughts are that it should be less than 1 ohm, but getting conflicting reports that it can be up to 5 ohms???
Also being advised that a common earth can be used, being linked between sub-stations. Surely if this is the case and a cable is isolated/disconnected no protection is then provided.

 

[cuky2000]

As a reference, in the ANSI/IEEE marketplaces, the IEEE Std 80 provide a general guidelines for overall ground (earth) grid resistance of 1 Ohm for large substations and 5 ohms for small distribution substations.

Many utilities companies adopt similar criteria than the IEEE Std 80 for a go/no-go test after installation is completed.

It is good idea to interconnect neighboring substations to maintain a uniform potential in the areas. Many design call for insulated cable if is routed in area with low resistivity such as native soil or concrete slabs where there is a good chance to exceed the allowable step and touch potentials

The use of insulated interconnecting conductors may not be necessary if insulating surfacing materials such as crushed rock or asphalt is provided between the interconnected substations.

 

[Deansharafi]

I confirm the above posting. For large substations the good earthing resistance value is below 0.5 ohm. I have carried out dozens of tests in transmission substations in Australia, the average value is 0.25 Ohms.

 

[jghrist]

The substation ground grid resistance is mostly dependent on the soil resistivity [ρ] and the grid area A. As an approximation, the minimum value of grid resistance R_g in ohms is:

R_g = ([ρ]/4)·sqrt([π]/A)

where [ρ] is in ohm-m and
A is in m²

(IEEE std 80-2000 eq. 50)

Using this, a 100' x 100' grid in 100 ohm-m soil would have a resistance of around 1.5 ohms.

 

[goodchoi]

I think IEEE Std-80 isn't discussing about the ground resistances (1 or 3 ohms) as a guideline (or a limit value) like a safety limit of mesh(step) voltage.

 

[cuky2000]

Goodchoi,

Here is what IEEE-Std 80 say about resistance.

14. Evaluation of ground resistance

14.1 Usual requirements

A good grounding system provides a low resistance to remote earth in order to minimize the GPR. For most transmission and other large substations, the ground resistance is usually about [red]1 ? or less[/red].
In smaller distribution substations, the usually acceptable range is from [red]1 ? to 5 ?[/red], depending on the local conditions.

 

[ccamts]

I am a small fish but just a casual suggestion the whole story of earthing depends upon ground fault current which is then related to GPR and then touch and step potential. SO
we start it from back we get GPR and step and touch potential and see the value of resistance and if we realize that this value is not good. we need to reduce it. Saying that for distribution is < 5 ohms, it works but again we have to make sure the potentials are with in limits.
cheers !

 

[cuky2000]

Hello CCAMTS,

A good ground design should satisfy the entire safety requirements. This include not only the GPR (resistance)but also the prospective step and touch potentials must be less than the allowable design values.

Although there is a way to perform actual field tests of step and touch potential, only few utilities perform this test routinely opting to measure the ground resistance unless there are large discrepancies with the calculated values.

Here is additional information in this subject.

http://standards.ieee.org/reading/ieee/interp/80.html

 

[cflatters]

BS 7430 says that 1 ohm is the maximum earth electrode resistance for LV & HV earths to be interconnected, otherwise they should be isolated. ENA TS 41-24 and
ENA S34 also provide useful assistance for UK applications

IEC 61936 provides guidance for HV substation applications

IEEE80 Also refers