Earth impedance metering 2

[Weldinspect]

Before anything else, Happy New Year!

I am supervising an earth network installation and after the contractor didn´t reach the desired impedance of 5 Ohms stated by the designer he asked to meassure hsi network after connecting to an old one, is this allowed? I didn´t find it in my NEC and I guess it shall be stated in IEEE
Thanks in advance

 

[waross]

You should refer this back to the designer. He may accept it or he may want to re-evaluate his touch and step potentials.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

There is a general reference in the IEEE Std 80 suggesting that smal distribution substation should be around 5 Ohms and for large HV facility the ground resistance is suggested to be 1 Ohm or less.

Many utilities in the USA use a similar criteria and usually more stringent requirement. Indirectly the grid resistance will limit the maximum GPR allow in the station.

If the ground resistance value in the spec, it is suggested to measure other important safety parameters such as the step and touch potentials in selected critical areas.

 

[Weldinspect]

Thanks cuky I will have to study further IEEE80. Happy New Year!

 

[Weldinspect]

Dear friends, I found it in IEEE80, thank you

9.5 Design in difficult conditions

In areas where the soil resistivity is rather high or the substation space is at a premium, it may not be possi- ble to obtain a low impedance grounding system by spreading the grid electrodes over a large area, as is done in more favorable conditions. Such a situation is typical of many GIS installations and industrial substations, occupying only a fraction of the land area normally used for conventional equipment. This often makes the control of surface gradients difficult. Some of the solutions include



a) [highlight #FCE94F]Connection(s) of remote ground grid(s) and adjacent grounding facilities, a combined system utiliz- ing separate installations in buildings, underground vaults, etc. A predominant use of remote ground electrodes requires careful consideration of transferred potentials, surge arrester locations, and other critical points. A significant voltage drop may develop between the local and remote grounding facil- ities, especially for high-frequency surges (lightning).

 

[waross]

A predominant use of remote ground electrodes requires careful consideration of transferred potentials, surge arrester locations, and other critical points. A significant voltage drop may develop between the local and remote grounding facil- ities, especially for high-frequency surges (lightning).

This was my concern in my previous post.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Weldinspect]

Thanks waross, shamely there are no surge arresters designed and the 2 networks are close to each other.

 

[waross]

Transferred potentials during fault events are the issue. Surge arresters are not the only cause of transferred potentials.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Weldinspect]

I should study more this subject, thanks again waross