Acceptable value for earth resistan

Acceptable value for earth resistance

When measuring the resistance of the large earth electrode (earth mat) what is the acceptable values depending on soil resistivity.I also want to know the relevant SABS code that can be kept as reference for future earthing practise in my company.Please coment on the best methods I can use to measure earth resistance on substations.

Having performed this very function/testing and consulting in the past I would suggest that you approach the problem by looking at the potential rise of the ground mat under various fault conditions and not the resistivity of the earth/soil you are dealing with.
Use software (ETAP) or others to do this calculation.
Further suggest that you look in the Brown and Green books of IEEE. These may also help.

For safety purposes, potential gradients (step and touch potentials) are more important than ground electrode resistance.

See IEEE Std 80, "IEEE Guide for Safety in AC Substations Grounding."

For measuring resistance, see IEEE Std 81, "IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System (Part 1)" and IEEE Std 81.2, "IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems (Part 2)."

Suggestion: Contact SABS for the document. There are some available on web, e.g.
http://www.cbi.co.za/papers/26/CBI4.pdf
http://www.promech.co.za/architect/archive.htm
(check with SA architects)
http://www.asosh.org/Standards/standards.htm
(check for standards)

10 ohms should suffice on a dry warm summers day when the water table is low

The lower the ground resistance, the safer for people and better protection for the equipment is but possible more expensive.

The response to your question appears to have a wide range of variation depending of the accepted engineering practice, application, compliance with code and standard. The follow are some of the acceptable resistance value that I remember:

IEEE Std 80 & IEEE Std 142….……  <1 Ohm _{for large HV substation}
            `                                      <5 Ohms _{for distribution substation.}
                                      
IEEE Std 142 clause 4.1.2   …….  <1 Ohm _{for large Indust., HV sub & gen. st.}  
                                               <5 Ohms _{for Indust. plant, & comm. facilities}

Go Not Go Test (Typ. US Utilities)…  <1 Ohm

Telecommunication facilities…………<5 Ohms

Lightning protection…………………....<1 Ohm

Ground Rod per NEC 250- 84. ……….<25 Ohms
And NESC (IEEE Std C2 096)

RR Pole grounding……………………...<25 Ohms
(f/stray current Corrosion Ctl)

As per our standard each Earth pit resistance should not be more than 5 ohm & Earth grid Earth resistance should not exceed 1 ohm

The earth grid has two major functions:

1. Complete circuit for earth fault current return
2. Safety

Of which 2. is the most important. Providing ohm values as guideline values for earth grid safety ignores basic safety issues and is irresponsible.

Providing safe ground resistivity values ignores the fact that most safety criteria equations result in lower limits for lower ground resistivity.

Touch and step voltages depend on fault current magnitude, a fact that ohm guidelines ignore. In addition, safety criteria usually depend on fault duration.

To provide a safe earthing design you must use a standard that addresses touch and step potentials. Satisfy yourself that a standard has a sound theoretical basis before using it.

Need some advice on PSM and TSM setting for 2 nos. 10 MVA, 22kV incoming which is stepped down to 6.6kV with 14 outgoing feeders of 2MVA each which are further stepped down to 400 Volts. I need to know the fundamental principle in selecting the PSM and TSM values for the Overcurrent and Earth fault relays for discrimination.

Suggestion: Please, notice that the earth resistances may be specified more stringent than indicated by the industry standards such as those posted above. This is often mentioned in the Client specifications or Requests for Proposal.