Substation : grounding resistance

[dalas]

Hi,

I've been trying to find any standard on the net (IEC, NEM, etc) about the maximum (or minimum) value of grounding resistance for a substation.
I appriciate any help.

Thank you.

 

[davidbeach]

What you want is IEEE-Standard 80, IEEE Guide for Safety in AC Substation Grounding.

 

[jghrist]

There is no ANSI or IEEE maximum grounding resistance for a substation. IEEE Std 80 requirements are based on keeping touch- and step-potentials within safe limits.

Some utilities impose their own limits on grounding resistance. If you have communication lines coming into the station, ground resistance may have to be limited to keep the ground potential rise (GPR) within the limits of the communication line interface.

 

[Zogzog]

<25 ohms for large industrial and <5 ohms for generation.

 

[cuky2000]

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 1 Ohm or less. In smaller distribution
substations, the usually acceptable range is from 1 Ohm to 5 Ohm, depending on the local conditions.

 

[lz5pl]

Standard values for Bulgaria:
Less than 0.5 Ohm for substations (transmission and distribution), less than 10 Ohm for individual groundings (like earthing rod for OHTL towers for example).

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[sslobodan]

Like they said there is no limited but recommended value. In general cases it depends from the area of application. The main point in grounding is to create path for an unwanted current to flow on it rather than it should flow trough you or gear. Of course due to an law U=Z*I you have many different aspects of grounding because although the current is searching the shortest way (grounding) to close circle when surfaces like metal housing of pump comes under unwanted voltage and thereby produces current trough you when you touch it, we sometimes want deliberately to set up a high ohm grounding resistance to save our equipment by limiting ground fault amperage to a reasonable level (100-400A) You can read more of that in a tread(s) here considering high resistance grounding (HRG) but such grounding is limited to an application of substations where there is minimum risk of people getting killed from high Amperage touch (remember it is not the voltage that kills you but the current trough you) because the path for closing circuit was better (less ohm) trough you than trough grounding system. I am certain that you are familiar with the formula for current flow trough two parallel circuits of ohm value R1 and R2 it can be applied in your calculations where you model your circuit as R1 and your body ohm value as R2
If you like to know a bit more you should try link

http://www.tlc-direct.co.uk/Book/3.4.2.htm

and of course legendary site (I suggest reading all posts) considering what you should not do when doing human ohm resistance test:

http://www.darwinawards.com/darwin/darwin1999-50.html