Thank you for your question regarding earth pit loads, it is our pleasure to help.
One of the more common aspects of electrical grounding that people tend to overlook is in regards to electrical loading. That is, how much electrical energy (in amps) can a given grounding electrode handle before the thermal load causes it to burn open like a fuse?
The answer isn’t as simple as saying, that X number of earth pits are required for Y level of energy. This is because soil conditions impact how the electrical energy will propagate across the earth, and how much the grounding system can handle. Highly resistive soils will conduct electrical energy far differently than conductive soils, and not in the way you may think. Grounding electrodes found in conductive soils are far more susceptible to thermal overload conditions than those found in resistive soils. This often means that even if you have a grounding electrode with a very low resistance-to-ground you may still need additional grounding just so your system can handle the fault currents without overloading.
All conductors have a maximum rated current load given a number of factors, usually temperature ratings. For example, a 4/0 AWG 75 C copper conductor is rated at 230 amps. But if you dig a little deeper there are actually a number of other specification associated with a 4/0 conductor. Turns out, you must also know the frequency, X/R factor, and duration (time in cycles) of the electrical fault. For example, a 4/0 AWG copper conductor handling a 60Hz, 60 cycle (one second) fault with an X/R factor of 20, is rated to handle a 30 kA fault. However, that same 4/0 AWG copper conductor dealing with a 6-cycle (0.1 second) fault with an X/R factor of 0 is rated for as much as a 99 kA fault!
We mention this, because a copper coated steel ground rod will also have a maximum current handling rating. Unfortunately, as there is no requirement for the manufacturers of ground rods to list ampacity ratings, and as such there are no electrode ampacity charts available. But if there were, they would be for a theoretical zero-resistance soil, which just isn’t the case.
Here is what you have to do in order to calculate the load on a given electrode:
- Measure the soil resistivity at your facility and develop a good soil model. Here is a link with some information: http://www.esgroundingsolutions.com/about-electrical-grounding/what-is-soil-resistivity-testing.php
- Calculate the fault current that will enter your grounding electrode, including the amperage, frequency, clearing time, and X/R ratio
- Calculate the electrical characteristics (impedance factors) of the grounding electrode. You will need the following factors:
- Relative resistivity
- Relative permeability
- Internal and external radius of the electrode
- Length and installation depths
- Working potential in volts
- Load inductance in henries
- Load capacitance
- One you have a soil model, the fault current characteristics, and the impedance factors of the grounding electrode, you can then calculate the Ground Potential Rise (GPR) and the Voltage Drop (Ground Potential Difference or GPD) of the electrode,
- Using the above factors you can then calculate the Leakage Current per segment length of your electrode which is important as you must subtract the current that enters the earth along the length of the electrode.
- You can now finally figure out the factor that you really want which is the maximum Longitudinal Current Flow in the electrode.
We recommend as a guideline that a standard 5/8-inch (8 or 10-ft) copper coated steel ground rod have a maximum of 10,000 Amps of Longitudinal Current Flow (ampacity) across its length in one second. Keep in mind, this is just a guideline and many factors could impact what the actual requirement for this is. You may wish to use a safety factor and cut this number in half, just to be safe.
Now, if your grounding system is comprised of a series of copper ground rings with copper coated steel ground rods, you are in for some serious math. We recommend using engineering software such as the CDEGS program from Safe Engineering Services. http://www.sestech.com/
So to answer your question, the number of earth pits required to properly handle the fault current form (ground) a 10 Kw office building all depends upon the characteristics of the fault, the soil model of the earth at the site, and the physics of the grounding electrode system.
We hope this answers your questions. If you would like to speak to someone about your project, please do not hesitate to call us at 310-318-7151 California time, and someone will be glad to speak with you, free of charge.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions