I am dealing with helping our chemical plant facility technicians do the site ground grid resistance measurement for various grounding safety related matters. System grounding, Equipment grounding for ground fault return path, grounding for Lightning protection and grounding for static electricity mitigation. Some facilities have multiple buildings and I am not sure if all the building ground grids are tied to have a common ground grid.
From available drawings (these plants goes back to 1950s) and site visits, What I have observed is there are accessible ground wells and a ground wire is running around the perimeter of building/structure.
1) My first step in this process is establishing facility “Primary ground reference” and measure its resistance to ground using 3 point fall of potential method or Selective stake less method (If I choose not to isolate the ground well from ground grid my disconnecting the ground wire termination to the electrode in the ground well.).
2) Next I have to establish several “Secondary ground reference” points through out the plant where I am measuring resistance of equipment grounding conductor resistance to ground or process piping ground path integrity whose liquid flow can produce static electricity in a NEC 500 classified hazardous areas.
This “Secondary ground reference” points resistance I measure using a 2-point ground resistance test between “Primary ground reference” point and “Secondary ground reference” point and adding the result to Primary ground reference value measured in 1) above.
WITH THIS MUCH INTRODUCTION ABOUT WHAT I AM UP TO NOW THE QUESTIONS.
Q1) If I the site does no have visible ground well nor the drawing showing the grounding how can I determine or locate a “Primary ground reference” point. Is it correct to use Incoming Utility Power service substation transformer neutral grounding can be used as “Primary ground reference”?
Q2) To establish ground resistance value of this service entrance transformer neutral ground point I need to measure ground resistance using either the 3-point fall of potential after isolating the ground rod from service transformer neutral OR using selective stake less method.
Q3) When I have multiple power distribution transformers in the facility each transformer secondary neutral point is grounded to ground grid can be established as “Secondary ground reference” points for those areas of the plant right?
Thanks for your help.
Thank you for your questions regarding the grounding systems of a chemical plant, it is our pleasure to help.
We really get nervous when answering these types of complex questions regarding chemical plants and grounding, as there are really a great number of factors to be concerned about. While it sounds like you have a fair understanding of what needs to happen, we would just caution you that the subject of grounding is dealing with the safety of the facility. Improper analysis can result in not only equipment failures, but in catastrophic failures that could lead to massive fires and loss of life. Please consider bringing in an experienced consultant when you conduct your testing. The Engineers here at E&S Grounding Solutions have traveled around the world and are happy to help you with your project, but if not us, please get someone that has an expertise in this field.
Now to your questions.
- When you cannot find the primary grounding electrode, or any grounding electrode, we use the ground bar located inside the main electrical panel. This means that you will need to be able to open the panel while it is hot and connect a test lead to one end. Make sure you use proper test lead wires and a true direct-current (DC) meter. You will also need to account for test lead resistance. You will also need to design and propose a proper main grounding electrode(s) for the facility. They should be aiming for 5-ohms or less.
- It is generally not possible to measure the resistance-to-ground of an electrode at the transformers themselves. Especially if you can’t find the electrodes to begin with. The resistance-to-ground test (3-point or stake less) require that the electrode be either completely isolated or have one and one connection only. Usually you conduct a ground resistance measurement on the associated electrode and then a separate 2-point resistance measurement from the electrode to the XO of the transformer. This is how you confirm the fault path of a transformer. See the following link: http://www.esgroundingsolutions.com/about-electrical-grounding/how-to-do-a-3-point-method-fall-of-potential-and-induced-frequency-ground-resistance-test.php
- You are basically correct. Although some transformers can be different, generally speaking every time power is passed through a transformer a new “first service disconnect” is generated. However, the grounding systems of these sites must still be bonded together, generally below grade.
Don’t forget that it is one thing to measure an electrode, it is another thing to know what that electrode should be. In other words, just because you measure an earthing pit at 20-ohms, you won’t know if that is good or bad unless you have a computer model that agrees with you. If the computer says that the electrode should measure 5-ohms, and you get 20-ohms, it is likely that corrosion has taken out most of the electrode.
Some other issues you will want to ensure are taking place at the chemical factory include:
- How is the lighting protection system bonded back to the main grounding system?
- How are gas pipes bonded to the main grounding system?
- Have the water pipes been bonded?
- How are static control grounds bonded and what programs are in place for measuring added static control resistances?
- What types of equipotential bonding systems are in place and how are they bonded?
- Do the substations and power rooms have equipotential bonding systems for human safety?
- Is the main electrode system the concrete building slabs and do they have vapor barriers?
- Have fences, gates, and doors been properly bonded near DC rectifiers?
- Has the grounding systems for the computer server rooms been properly isolated and bonded back to the ground system?
This is just a small list of course. It would be impossible to know everything that is needed without analyzing the site.
Again, we highly recommend and encourage you to get a grounding specialist to consult with you during the testing phase. Please feel free to contact us at 310-318-7151 California time, and he will be happy to discuss your project with you, free of charge.
The Engineering Team at E&S Grounding Solutions
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