I am checking the resistance to ground for equipment/instrumentation installed in a processing plant. I seem to remember that an acceptable resistance value is less than five ohms but am unable to find any reference to what this value should be.
It all depends upon the specific equipment, and you need to check with the manufacturer for that equipment. As far as the electrical code goes (for grounding an electrical service) - If a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less, a supplemental electrode shall not be required. But this still may not be good enough for certain installations - a communication tower, for example. It all depends upon the specific equipment.
We are talking about a variety of equipment including:
instruments, level, temperature, pressure, flow; transmitters, switches, etc.
valves, solenoids, pneumatically actuated modulating valves
There is no magic answer to your question, that can be found in a book. It all depends... Maybe you need an electrical engineer to evaluate your various equipment and come up with a good recommendation. Communication towers and such usually go with 5 ohms or less, but you will not find that value stated in the electrical code.
Resistance of a ground conductor is a function of it's circular mill area, length, and material composition. In most instances, any "increases" in resistance will occur at the connection points, not the conductor itself.
The maximum resistance of a ground electrode system, beyond the reference to NFPA 70 above, is determined by the AHJ.
OP uses the term "ground conductor" in his title. But he is actually asking about the resistance to earth, not the resistance of a conductor.
While an AHJ could certainly dictate what they will accept for resistance to earth, most of them don't (I'm going to estimate over 90% or more). That leaves us with the code. If certain electrodes are used, then a supplemental electrode must be provided. But if 25 ohms or less, then no supplemental electrode is required. But one better be able to prove 25 ohms or less.
And NEC only requires one supplemental electrode and then the contractor has complied with code, no matter what the actual reading may be.
Actually I am referring to the ground conductor to each device in the system. I am checking with a meter and a wire tied back to the MCC ground bus that feeds all of the equipment. The intent is to determine if the ground is in fact intact and effective.
Sorry about that, I misunderstood. I did not mean to put words in your mouth or anything. I don't have any suggestions for you on this issue.
It sounds to me that you are looking for the earth fault loop impedance. I am an American electrician but have many British friends that are electricians and in the UK this is a standard practice. I don't know if the NEC has requirements but what I can tell you from my understanding is what you are looking for is to see if and how fast your breaker will trip in a fault type condition.
So according to Schneider for example the Zs = Uo/ Ia where Zs= the earth fault loop impedance Uo is the voltage to ground and Ia is the amperage it takes to trip the breaker under a certain time taken from the Trip time curve from the manufacturer. In the UK they use 5 seconds for fixed equipment and .4 seconds for appliances.
Having said that what you would do is say you have a 20 amp breaker feeding your circuit you'd look up the amperage it would take that breaker to trip in .4 seconds. Then take your voltage (120) divided by the amperage you just looked up. This would give you the maximum impedance value of your earth fault loop path. Now you are probably using just a standard ohm meter which would only give you a resistance reading but... it would be very close I believe.
As I said i've never had to do any of these tests in the U.S. so i'm guessing in part. But in the UK it is really standard and they have to test every installation for earth fault loop impedance. I personally think it would be a good thing for us to adopt as standard practice.
Not sure this helped at all and sorry it was long winded. I'd like to hear what you finally come up with on this.
I think you are asking for point to point. NETA MTS requires investigation of anything greater than 0.5 ohms. We typically use an earth resistance tester, like a DET2, not a multimeter.
Do you know how much current is passed through the circuit under test by your earth resistance tester? I am looking for a test instrument that will verify the adequacy of the equipment grounding conductor path.
The Resistance to Ground value in the New Mexico Electrical Industry has been 5 Ohms or Less for at least the last
40+ years. The Question is one of a Standard that specifically states a value. I have specified the 5 Ohm value
without the Knowledge of the source; However, in research, I believe the 5 Ohm value came from Goverment
requirements and was established as a Norm for the NM Electrical Industry. Note: NM has the Los Alamos, Sandia/
Kirtland infrastructure, White Sands Facility and many more such facilities which required a low resistance to ground
due the nature of there testing equipment.
I also note that I have provided Engineering Studies for Aviation Instrument Repair Facilities that included separate grounding of the instrument testing bench with the instrument itself and a separate ground for the Technician. I also
provided a grounding source of multiple Wells as the "Earth Ground". Why?, to achieve the lowest "Resistance to Ground" path. This grounding method has become a desire for "Critical" facilities as above along with other facilities
that deal with similar low voltage equipment [Data, etc]. So, the response to your inquiry is that I would not specify
any value higher than the 5 Ohms, especially in the current Computer based world we live in.
I hope this gives you some background/parameters to make a decision on your part.
Have A Great Day
Harvey Peel, PE
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