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Ground (earth) Resistance in Low Voltage (<600 volts) System

Ground (earth) Resistance in Low Voltage (<600 volts) System

Ground (earth) Resistance in Low Voltage (<600 volts) System

Can any one tell me when we need to be concerned with the resistance to ground (earth) of a grounding electrode system in an industrial or commercial installation with system(s) operating less than 600 volts? It appears that the NEC is not concerned with it, however, it requires to connect the electrical system to ground with a specified grounding electrode conductor size as appropriate for the electrical service. The NEC goes on to say that you connect to earth in order to protect against lightning, line surges or accidental contact with the higher voltage as well as providing equal potential. With that said, the code is not concerned with the earth ground) resistance of the electrical system as it may be the industry as well as IEEE standards (80 and 81) practice for high voltage systems in substations. Any feed back on this topic is appreciated.

Thank You

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

I'm not quite sure I follow your question.  NEC Article 250.56 requires supplemental grounding electrodes if measured resistance to ground of a single electrode is greater than 25 ohms.  

So if you are under juridiction of NEC, you must always be concerned with resistance to ground of your grounding system.  

As a practical matter, as a responsible engineer, you should be concerned about earth resistance regardless of what is required by code.  In most cases, it should be possible to achieve a resistance of 5 ohms or less for any industrial or large commercial system without too much difficulty, unless soil conditions are poor.  

The NEC provides minimum requirements and attempts to cover all possible installations.  It really can't be used as a design guide.  Many times it is necessary to go beyond the NEC requirements in designing a reliable system.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

Well said, dpc.  Star for you.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

    While the NEC does require a second ground rod or plate electrode if the first one is greater than 25 ohms, it does not require any additional setps to be taken to reach the 25 ohms.  All the code requires is that you install a second grounding electrode.  In most cases there is already some other type of electrode such as a metal under ground water pipe or effectively grounded building steel and under the code you are not even required to test those electrodes.  
   As a practicle matter, what do we actually gain for the under 600 volt building electrical system when we require the installation of grounding electrode systems with 5 or even 1 ohm?

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

Yes, I agree that the NEC requirements for remedial action if the 25 ohms is not achieved leave a lot to be desired.  But you have to remember that this code applies to residential and light commercial installations, and historically that is what the NEC was written to address.   It could definitely be stronger in this area -- maybe someday...

When we write construction specs related to grounding, we specify a minimum grounding resistance that we feel is appropriate to the installation. This is typically five ohms for industrial facilities.

What you gain from a low resistance to ground is reduced voltage differential between earth and your ground system during faults as well as a lower impedance path to ground for surge events such as lightning.  It also helps ensure that a ground fault will draw enough current to be detected and isolated.   The IEEE Green Book on Grounding (IEEE-142) is a good reference.  

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

  Exactly how is the voltage difference between earth and the grounding  system reduced by having a better grounding electrode system?  The voltage difference is the voltage drop in the fault return path caused by the high current flow in this path.  I don't see how a better grounding electrode system could change the impedance of the fault return path.  If the grounding electrode system is playing ANY  real part in the clearing of electrical system ground faults, the system has not been installed per the requirements of the NEC.   Even at 5 ohms, the grounding electrode system won't cause a 480 volt 100 amp breaker to trip on a ground fault.  I do agree that you will get some additional protection under lightning or accidental contact with a primary voltage system by having a better grounding electrode system.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

As mentioned by dpc, the code does mention maximum of 25 ohms resistance to ground limitation for made electrode system. The code goes on to say that if that is the case, you need to supplement it with another electrode installed not less than 6 feet from the first one. Since the code does not require further testing, I have seen that many designers just specify installation of two electrodes installed within 10 feet from each other, thus meeting the requirement of the code without testing!

What concerns me is that the code is apparently not concerned with the magnitude of the shock potential due to lightning or a surge with outdoor installations as compared to indoor installations due to some outdated assumptions. For example, if an individual was within the vicinity of an outdoor equipment(outside of a building, hence outside of the Faraday cage) he is exposed to the lighting or surge potential that has been conducted by the electrical system bonding or grounding (green conductor or conduit) system.

In retrospect with the primary intend of NEC by requiring to connect an electrical system to earth to protect individuals (or perhaps the equipment) against "shock" potential from lightning, line surges or  accidental drop of a high voltage line on a low voltage line, it appears that earth resistance testing should have been the first required step by the NEC toward building a safer system, but magically forgotten.

Any comments on this subject is appreciated. Thank You.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

"600-volt" grounding, and particularly system grounding, have been hotly contested issues since before the first edition of the NEC.  I believe IAEI used to publish or sell a ~50-page paperback booklet on the history of the NEC.  Does anyone remember that publication?  

Probably the next hottest Code issue over the years was the passionate arguments for and against vinyl {thermoplastic} cable insulation versus {thermoset} rubber—id est, “T” versus “R” {and “X”} formulations.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

   The Soares book on grounding published by the IAEI has an appendix titlrd "The History and Mystery of Grounding".  

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

For resqcapt19

I think the concern is the possibility of a substantial voltage difference between "earth" and a grounded surface such as the outside of a conduit.  The lower the resistance of the grounding system to earth, the lower the voltage difference.  This helps protect the person leaning up against a metal enclosure during a fault.

Also, while I agree that the fault return path often does not include earth, it sometimes does.  Grounding conductors are often not properly bonded, steel conduit fittings corrode, and things are mis-wired.  Under these conditions, a lower resistance to earth is important in providing adequate short circuit current and to limit potential rise of "grounded" equipment.  I have experience first-hand a miswired lighting pole energized laying on the ground for testing with 277 volts accidentally applied to the steel pole itself.  The resistance to earth was too high to allow the breaker (30A) to trip, but it was possible to get a nasty shock when the pole was touched, even through my rubber-soled shoes.  If the pole had been properly mounted and bonded to earth, there's a good chance the breaker would have tripped.  Certainly, if the pole had been bonded to a ground rod at the pole, the shock hazard would have much less.


RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

   I don't agree.  Please explain to me how a lower resistance in the grounding electrode system can reduce this touch potential.  I don't think that there is any way that a better grounding electrode system can lower the touch potential between the metallic electrical equipment and the earth under fault conditions.  While the fault exists, all of the noncurrent carrying parts of the electoral system will be raised to a potential above earth equal to the voltage drop in the fault return path.  A better grounding electrode system cannot lower this voltage drop. If the drop is not changed, then the touch potential is not changed.  If the fault return path (code required equipment grounding conductor) has a low enough impedance, the fault will be quickly cleared and danger will go away when the OCPD opens the circuit. Having a better grounding electrode system does not change this voltage between the metallic parts and "earth".  The only way that this could happen is if the earth was a good enough conductor so that all of the "earth" in the are would be raised to the same potential as the electoral equipment.  The earth just isn't that good of a conductor.
   In the case of the light pole, if the ground rod had a low enough impedance to clear the fault, the hazard would be gone, but that is not very likely in most soils.  Even if the rod had a 10 ohm resistance, that would not clear a fault on a 30 amp 277 volt circuit.  If the fault is not cleared, there will be 277 volts between the "earth" and the pole unless the person is standing right on top of the ground rod.  According to the IEEE Green Book about 68% of the voltage is dropped in the first 1' around a ground rod and 75% in 2'.  A person touching a pole with a 277 volt ground fault and no EGC would receive a 188 volt shock if he was touching the "earth" only 1' away from the ground rod.  The only way to protect people is to provide a fault clearing equipment grounding (bonding) conductor back to the system main bonding jumper.  Grounding electrode systems do not protect people from the hazards of a ground fault.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

The low resistance of the grounding will help you if you using a ground for safety purposes. However if sensitive electronic is connected to the system or the EMI (Electro Magnetic Interference) shall be reduced one should worry about the ground impedance which includes the inductance of the grounding system. When the grounding has to ensure Electro Magnetic Comparability, it has to be sufficiently low impedance in the high frequency range as well.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

   Please explain exactly how a lower resistance on the grounding electrode system improves the safety ground.  Show me the numbers that prove this.  A lower resistance on the equipment grounding (bonding) conductor will improve the safety of the system, but I see no way that lowering the resistance of the grounding electrode system of a code compliant installation provides any increase in safety under system ground fault conditions.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System


The following excerpt is from the IEEE Green Book “IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems”

4.1.2 Recommended Acceptable Values

The most elaborate grounding system may not perform satisfactorily unless the connection of the system to earth is adequate for the particular installation. It follows, therefore, that the earth connection is one of the most important parts of the whole grounding system. It is also the most difficult part to design. The connection to earth or the electrode system, needs to have a sufficiently low resistance to help permit prompt operation of the circuit protective devices in the event of a ground fault, to provide the required safety from shock to personnel who may be in the vicinity of equipment frames, enclosures, conductors, or the electrodes themselves and to limit transient overvoltages.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

I believe we are digressing from the main issue, which is exposure of an individual against "shock" potential due to lightning, line surges or  accidental drops of a high voltage line on a low voltage line, if the person is inside or outside of a building. What resqcapt19 says is true for a fault condition. However, the question has to do with exposure to a shock potential that is generated due to  lightning, line surges or  accidental drops of a high voltage line on a low voltage line. What is everyone s’ opinion or suggestion? Do we need ground resistance testing in this case? Why does anyone think the code tries to avoid it?

Thanks for your input.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System


You said:

(much snipping)
...While the fault exists, all of the noncurrent carrying parts of the electoral system will be raised to a potential above earth equal to the voltage drop in the fault return path.  A better grounding electrode system cannot lower this voltage drop.

I don't agree with your last sentence.  You seem to be assuming there is never fault current flowing in the earth, which will not always be the case.  There will always be a certain voltage drop between the fault point and remote earth.  A lower ground resistance can shift some of this voltage drop to the earth, under some conditions.  It can also increase fault current which will increase the chances of clearing the fault.  If we had a perfect system with a low resistance grounding conductor from the fault to the transformer neutral, you're right, the connection to earth does not play a major role during ground faults.  But if we had a perfect system, we wouldn't have had the fault in the first place.  

But I don't think I'm going to change your mind on this so maybe we should just let it drop.  It doesn't sound as though my ground rod is hurting anything, even by your analysis, so I think I'll leave it in.  

As to why the NEC is silent on this particular issue, I have no idea.  I'm not even sure I agree that is silent.  But if the 12000 volt line falls on your 240V line, I don't think any grounding system is going to protect someone unlucky enough to be holding onto a ground wire at the pole.  This would also apply to lightning strikes.  Nothing in the NEC says that any particular value of earth resitivity will offer complete protection in every situation.

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

   It is interesting to note that the Green Book was last revised in 1991.  Up until the 1993 edition the NEC said almost the exact same thing in Article 250.  Starting with the 1993 NEC the words stating that the grounding electrode system facilitates the operation of the overcurrent protective device under ground fault conditions were removed from Article 250.  

RE: Ground (earth) Resistance in Low Voltage (<600 volts) System

  you said:
 "...A lower ground resistance can shift some of this voltage drop to the earth, under some conditions. ..."

How?  There is no connection in a code compliant system between the grounding electrode system and the equipment grounding conductors until you get to the main bonding jumper.  


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