Measuring Earth Resistance - What Affects Grounding Resistance?

By Chris Dodds on 23rd September, 2014

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Earthing & Lightning Protection - Measuring Earth Resistance

Originating from an online LinkedIn Discussion titled Measuring Earth Resistance and started by Edvard Csanyi, Founder and Electrical Engineer at EEP.

Soil composition, moisture content, and temperature all influence the soil resistivity, so it is recommended that the copper earthing ground rods be placed as deep as possible into the earth to be most effective.
 

What Affects Ground Resistance? 

The NEC code (987, 50-83-3) requires a minimum ground electrode length of 2.5m to be in contact with soil.

The following four variables affect the ground resistance of a ground system:

 

1. The Length/ Depth Of The Ground Electrode

Earth and Grounding Resistance - Grounding Electrodes

Placing the ground electrodes as deeply as possible is one effective way of lowering the ground resistance.

It is critical that the ground electrode is deeper than the frost line so that the resistance to ground will not be greatly influenced by the freezing of the surrounding soil. The ground resistance can reduce by an additional 40% by doubling the length of the ground electrode.

*Alternative methods, including grounding cement, can be used when it is physically impossible to place the ground rods deeper - occasions where the ground is composed of rock, granite, etc.

 

2. The Diameter Of The Ground Electrode

Diameter of the ground electrode

Image: showing a reduced diameter of an earthing electrode.

Making the diameter of the ground electrode bigger has very little effect on reducing the resistance. For example, the diameter of the electrode could be doubled and the resistance would decrease by 0%.

 

3. The Number Of Ground Electrodes

Using multiple ground electrodes can lower the ground resistance - more than one electrode is driven into the ground and connected in parallel to lower the resistance. The spacing of additional rods needs to be at least equal to the depth of the driven rod. Without the correct spacing the ground electrodes spheres of influence will intersect and the resistance will not be reduced.

 

4. The Ground System Design

Simple grounding systems consist of a single ground electrode driven into the ground. This is the most common form of grounding and can be found outside your home or place of business.

Complex grounding systems consist of multiple ground rods, connected mesh or grid networks, ground plates and ground loops. Complex networks dramatically increase the amount of contact with the surrounding earth and lowers ground resistance. These complex systems are usually installed at power generation substations, central offices and cell tower sites.

 

A Selection of Comments...

 

Timothy Shaw - Supervising Engineer at Electrical Reliability Service

"One other item.....Location, Location, Location....If you are in the desert, your resistance will be much higher, typically, then if you are located near a water table (such as the ocean). The other things that people don't take into account, is the location of underground piping which can greatly affect the readings that you get while performing a three point fall of potential."

 

Chris Dodds - LV, HV & Hazardous Area Electrical Group Manager

"Marconite and Bentonite conductive compounds replace sand and aggregate to provide suitable earthing resistance whatever the ground conditions."

 

Esmail Afshari - Consultant Private Company

"The best base for a grounding condition is a silicon base material or in another word gravel and sand mixed as deep as 8 inches (20 cm) where grounding is required."

 

Ian Griffiths - Principal

"In my experience, success has been more than a little 'mixed' to say the least when using Marconite and/or bentonite as a blanket one-size fits all solution. Altho extremely useful for a lot of situations, these compounds have their limitations."

 

Dave Wilson - Product Manager: James Durrans & Sons Limited

"To draw distinction between the two, our product Marconite®, originally developed by the Marconi company, is specifically manufactured for grounding. Whereas Bentonite is just a clay material that is dug out of the ground and happens to have limited moisture retaining properties.

With regards to its performance, many organisations look only to use ground enhancement materials whilst attempting to overcome problems only after work has been started. Marconite®'s performance is reliable, predictable and permanent but it is not a panacea. Ground enhancement is a proven technique, its just considered by many as an unnecessary on-cost for projects until its too late and they look for cure-all miracles."

 

Kieron King - Electrical/Electronic Manufacturing Professional

"Another solution would be to offer "ULTRAFILL". Ultrafill contains no Bentonite or Concrete. For more information see the attached link. Harger offers complete solutions for earthing and grounding systems."

 

Mark Johnson - European Marketing Manager at Megger

"Megger, the maker of earth resistance testers, offers The Megger guide to earth testing "Getting Down To Earth" which can answer these questions. To download it all you have to is register at http://www.megger.com/uk/login/Register.php and go to publications." Or Download Below.

 

Rakesh Kapila - Professor Electrical Engineering / Professional Engineer at GNIT

Accuracy of the Ground Resistance tests is some what better, if the ground carrying conductors are well insulated, hence avoiding they stray capacitive couplings with the local ground surfaces.The real challenge remains, how to measure the Ground Resistance under actual Electrical Fault Conditions. It should be both stable and must have sufficient Short Circuit Capacity and Voltage Recovery ( 2-cycles), which rarely exists in deep wells. They cause more low level fault conditions than an average electrical engineer can ever cope up with for any existing electrical distribution system. All measurements are done for the actual point of nearest coupling with the ground, and if you slightly withdraw and step back from the point of actual measurement, then you may land up measuring the perceived ground resistance of the electrical system behind you rather than the ground resistance of the grounding grid ahead it self. You can have lot of fun with these permutations and combinations at times and finally one has to go back and study and refresh the basic theory and the recommended techniques associated for such measurements! Go ahead and have lots of fun. Try to disconnect the cables before you start measuring GR.

 

Terry Mulligan - Consultant at TPP

You summarise well Stephen, these are exactly the common problems with testing protocol and the electrode design. However, Equally Rakesh is correct, the effectiveness of electrodes diminish with depth and the improvement in resistance falls away after around 25 feet, or 9/>10 meters or so. Limitations in available working footprint further encourages the wasteful placement of electrodes with overlapping zones of influence and consequential diminished effectiveness. A solution can be to install at an angle, though this is limited by ground type for well applications.

Rakesh, again you are correct Nd detailed. We I fact injection test our deep wells to prove the resistance, and this has demonstrated the inadequacy of the wenner tests and the meters. However, we also test with longer leads and probe separation and we find even with the proportionally correct voltage and current probe spacing the accuracy drifts out. Over 60m, the inaccuracy increases dramatically. We have our "home made" variable frequency high current injection test apparatus which provides excellent scientific accuracy for the deeper electrodes requested by Supply Authorities for their HV installations. Clearly they are not fully aware of the mattress effect of current dissipation and the diminishing benefit and effect of depth.

 

Stephen Matthews - Electrical Engineer, Excellen Consultancy

Rakesh, I think you are missing Terry's point. The point is the 3 electrode test is often carried out in the wrong manner with people either not understanding the test or taking short cuts. Often they fail to prove their test results are on the plateau or to calculate the accuracy of their results. Also no check has been made to ensure the potential 'shells' of each electrode do not overlap.It is not so much the type or depth of an electrode system, as why we should carry out the test in a prescribed manner.

 

Rakesh Kapila - Professor Electrical Engineering / Professional Engineer at GNIT

Terry: Nearly all electrical activity in the soil ends between 6 to 7 feet(93%) and nearly ends between 22 to 23 feet(99%) in common soil conditions. In higher resistivity soils and rocky conditions the depth for ending electrical activity is much lower. When you deal with deep wells like 110Meters you rarely measure the actual ground resistance. Most of the Ground Resistance Meters have technical limitations by the length of the conductor they are placed on to make these measurements. Ground resistance meters normally are not accurate for grounding conductors having high capacitance, as they actually use the principle a a reflection of a high frequency signal from the grounding points.

Deep wells are another Cup of Tea, as they have very poor or really no short circuit capacity, both in terms of the SC of the connecting conductors and the technical inability of the water/Metal interface to carry high SC under fault conditions. What you may have measured is the impedance of the connecting conductors to the deep well and not the actual ground resistance you were supposed to have measured. These variations in measured values are due to the varying reflection points for the signals generated by the actual transmitter of the Ground Resistance Meter and nothing to do with the actual Ground Resistance of the deep well on your site! I think it may all be due to high capacitance interference for these conductors going in the well. Unfortunately, the old Wenner Equations also fail to mathematically model the Ground Resistance for deep well accurately! You have always ongoing challenges with these structures all the time. Just try through a concentrated solution of Epson Salt in the well your measured results will change for few weeks at least.

 

Terry Mulligan - Consultant at TPP

Steve, I couldn't agree more. Most people using the system don't realize what they are doing and don't know how to recognize the 61.&% plateau or what it means when they have attained it.

 

Steve Hamilton, PE Systems Consultant

The three point method is VERY often performed wrong. Those with little understanding just record the value at 62% distance (and various other errors as well).

 

Terry Mulligan - Consultant at TPP

Rakesh, our engineers and technicians spend many hours using meters each week. Many of these tests are to prove test conducted by others are valid or invalid. We often find that tests, 3 point mainly, are not conducted correctly. People read instructions and don't understand them, leading to errors. E.g, in September we visited a site which allegedly had installed a deep well earth and still could not get the required low earth resistance. Two 110m wells produced 2 .3 ohms. When we conducted the tests, correctly, we proved 0.65 ohms. The previous tests, with calibrated meters had been conducted using the wrong protocol. Suggesting a meter is used, sadly is not enough.

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