Ground Resistance 3

[Mbrooke]

Why does section 7.13 state that the max ground resistance in ohms must be no more than 5 ohms for in an industrial facility and and no more than 1 ohm in a generating station or transmission station?

What is meant by this and how is it measured? What is is the goal? Why is it so important?

As I understand it the ground has little to nothing to do with electrical safety in an industrial facility since bonding jumpers are opening OCPDs and not the earth soil (terra firma). Second, in utility substations bonding (brining everything near the same potential) is what protects people, not soil resistivity.


I'm really confused as to why earth resistivity is such a big deal or why its even brought up at all.

 

[FreddyNurk]

Step and touch potential?

EDMS Australia

 

[7anoter4]

I think you are talking about ANSI/NETA ATS-2009 STANDARD FOR ACCEPTANCE TESTING SPECIFICATIONS for Electrical Power Equipment and Systems
Here are indications of how to investigate the system grounding
7.13 Grounding Systems (continued)
3.2 Test Values – Electrical
1. Compare bolted connection resistance values to values of similar connections. Investigate
values which deviate from those of similar bolted connections by more than 50 percent of the
lowest value.
2. The resistance between the main grounding electrode and ground shall be no greater than five
ohms for large commercial or industrial systems and 1.0 ohm or less for generating or transmission station grounds unless otherwise specified by the owner. (Reference ANSI/IEEE Standard 142)
However, IEEE 142/2007 does not state this as a requirement - it is not mandatory- but only a finding See IEEE 142/2007 4.1.3 Recommended acceptable values.
I think for low voltage NEC requirements are sufficient, but for high voltage IEEE 80/2013 has to be follow.
In U.K. BS7430/2011- Annex A Guidance on typical HV/LV interfaces A.2. Legislation- states that up to 2002 1 ohm was limit but now the current advice of the Health and Safety Executive is that touch voltages should not exceed curve the recommendations of BS EN 50522, National Annex NA2

 

[Mbrooke]

@FreddyNurk: But doesn't bonding control step and touch potential, not soil itself?

@7anoter4: Thanks. Does IEEE 80/2013 make any mandatory requirements?

 

[cuky2000]

Historical Background: _{Before the original IEEE-80, the resistance of the ground grid was the primary concern of the design engineer, and still many utilities use that as a go/no-go design test parameter to meet with the intention to assure the system was effectively grounded. The resistance values on the IEEE Std It was based on typical resistance values for transmission and distribution substations throughout the United States in the 1950s. Years ago, the telephone system was based on hardwired with concern for dangerous transfer potentials to remote location or interference and noise generation.}

NOTE: _{Low or high resistances alone is necessarily an indication of a safe or unsafe design. The main safety criteria for acceptable touch and step potentials.}.

RECOMMENDED RESISTANCE RANGE:
_{a)IEEE Std 80 & IEEE Std 142….……...... <1 Ohm for large HV substation & <5 Ohms for distribution substation.
b) IEEE Std 142 clause 4.1.2 …….......... <1 Ohm large Indust.,HV sub & gen. & <5 Ohms for Indust. plant, & comm.
Go/No-Go Test Typ. US Utilities)…......... <1 Ohm (ConEd NY requires 0.5 Ohms)
c) Telecommunication facilities………......…<5 Ohms
d) Lightning protection…………………............<1 Ohm
e) Ground Rod per NEC 250- 84. ………........<25 Ohms And NESC (IEEE Std C2 096)
f) RR Pole grounding…………………….............<25 Ohms(f/stray current Corrosion Ctl)}

 

[7anoter4]

BS 7430/2011 has an explanation why 1 ohm limit was possible in the past:
The Electricity Supply Regulations (replaced by The Electricity Safety, Quality and Continuity Regulations in 2002) required that where in a substation the HV equipment earth and the LV neutral earth were common, that the resistance to earth has to not exceed 1 Ω. For most substations this value was provided by the un-insulated protective sheaths of the older types of cables in use and was normally sufficient to lower the impedance of these cable sheaths to ensure sufficiently low earth potential rise (EPR) for general combination of HV and LV earth systems even with very high earth-fault current. However this simple requirement is no longer adequate.

 

[basilasq]

For larger generating stations and transmission substations, step and touch voltages are dependent on the Ground Potential Rise (GPR) of the site which is directly proportional to the ground fault current and Ground resistance (IEEE 80 pg 5). Also check Section 4.2 of IEEE 80.

Basically the earth forms a poor conductor of electricity with a high ground resistance. Resistance of a conductor is dependent on its resistivity and thats why accurate measurement of ground resistivity is very important. Since a high resistance results in a large potential drop, a large fault current can result in a high Ground Potential Difference (GPD) between a person's feet resting on the ground (step potential) or between a hand (touching a grounded object) and feet (touch potential). Thats why a ground grid is necessary to equalize the GPD and thus reduce hazardous step and touch potentials.

Even though some codes specify a certain ground resistance requirement, its not always possible to meet a 1 ohm ground resistance (rocky/sandy terrain). And sometimes a station with even a 0.1 ohm ground resistance can result in hazardous touch voltages. Therefore the important thing is an accurate study and design of a grounding system to ensure personnel safety.

Most common method of measuring ground resistance of a large site is a Fall of Potential test. Details can be found in IEEE 80 and 81.