Measuring low ground resistance is the key to a correct grounding system

Lightning protection is a key aspect of organizations operating sensitive electrical equipment, especially in the broadcasting industry. Related to the first line of defense against lightning and voltage surges is the grounding system. Unless designed and installed correctly, any surge protection will not work.
One of our TV transmitter sites is located on the top of a 900-foot-high mountain and is known for experiencing lightning surges. I was recently assigned to manage all of our transmitter sites; therefore, the problem was passed on to me.
A lightning strike in 2015 caused a power outage, and the generator did not stop running for two consecutive days. Upon inspection, I found that the utility transformer fuse had blown. I also noticed that the newly installed automatic transfer switch (ATS) LCD display is blank. The security camera is damaged, and the video program from the microwave link is blank.
To make matters worse, when the utility power was restored, the ATS exploded. In order for us to re-air, I was forced to switch ATS manually. The estimated loss is more than $5,000.
Mysteriously, the LEA three-phase 480V surge protector shows no signs of working at all. This has aroused my interest because it should protect all devices in the site from such incidents. Thankfully, the transmitter is good.
There is no documentation for the installation of the grounding system, so I cannot understand the system or the grounding rod. As can be seen from Figure 1, the soil on site is very thin, and the rest of the ground below is made of Novaculite rock, like a silica-based insulator. In this terrain, the usual ground rods will not work, I need to determine whether they have installed a chemical ground rod and whether it is still within its useful life.
There are a lot of resources about ground resistance measurement on the Internet. To make these measurements, I chose the Fluke 1625 ground resistance meter, as shown in Figure 2. It is a multifunctional device that can use only the ground rod or connect the ground rod to the system for grounding measurement. In addition to this, there are application notes, which people can easily follow to get accurate results. This is an expensive meter, so we rented one to do the job.
Broadcast engineers are accustomed to measuring the resistance of resistors, and only once, we will get the actual value. The ground resistance is different. What we are looking for is the resistance that the surrounding ground will provide when the surge current passes.
I used the method of “potential drop” when measuring resistance, the theory of which is explained in Figure 1 and Figure 2. 3 to 5.
In Figure 3, there is a ground rod E of a given depth and a pile C with a certain distance from the ground rod E. The voltage source VS is connected between the two, which will generate a current E between the pile C and the ground rod. Using a voltmeter, we can measure the voltage VM between the two. The closer we are to E, the lower the voltage VM becomes. VM is zero at ground rod E. On the other hand, when we measure the voltage close to pile C, VM becomes high. At equity C, VM is equal to the voltage source VS. Following Ohm’s law, we can use the voltage VM and the current C caused by VS to obtain the ground resistance of the surrounding dirt.
Assuming that for the sake of discussion, the distance between ground rod E and pile C is 100 feet, and the voltage is measured every 10 feet from ground rod E to pile C. If you plot the results, the resistance curve should look like Figure 4.
The flattest part is the value of the ground resistance, which is the degree of influence of the ground rod. Beyond that is part of the vast earth, and surge currents will no longer penetrate. Considering that the impedance is getting higher and higher at this time, this is understandable.
If the ground rod is 8 feet long, the distance of pile C is usually set to 100 feet, and the flat part of the curve is about 62 feet. More technical details cannot be covered here, but they can be found in the same application note from Fluke Corp.
The setup using Fluke 1625 is shown in Figure 5. The 1625 grounding resistance meter has its own voltage generator, which can read the resistance value directly from the meter; there is no need to calculate the ohm value.
Reading is the easy part, and the difficult part is driving the voltage stakes. In order to obtain an accurate reading, the ground rod is disconnected from the grounding system. For safety reasons, we make sure that there is no possibility of lightning or malfunction at the time of completion, because the entire system is floating on the ground during the measurement process.
Figure 6: Lyncole System XIT ground rod. The disconnected wire shown is not the main connector of the field grounding system. Mainly connected underground.
Looking around, I found the ground rod (Figure 6), which is indeed a chemical ground rod produced by Lyncole Systems. The ground rod consists of an 8-inch diameter, 10-foot hole filled with a special clay mixture called Lynconite. In the middle of this hole is a hollow copper tube of the same length with a diameter of 2 inches. The hybrid Lynconite provides very low resistance for the ground rod. Someone told me that in the process of installing this rod, explosives were used to make holes.
Once the voltage and current piles are implanted in the ground, a wire is connected from each pile to the meter in turn, where the resistance value is read.
I got a ground resistance value of 7 ohms, which is a good value. The National Electrical Code requires the ground electrode to be 25 ohms or less. Due to the sensitive nature of the equipment, the telecommunications industry usually requires 5 ohms or less. Other large industrial plants require lower ground resistance.
As a practice, I always seek advice and insights from people who are more experienced in this type of work. I asked Fluke Technical Support about the discrepancies in some of the readings I got. They said that sometimes the stakes may not make good contact with the ground (perhaps because the rock is hard).
On the other hand, Lyncole Ground Systems, the manufacturer of ground rods, stated that most of the readings are very low. They expect higher readings. However, when I read articles about ground rods, this difference occurs. A study that took measurements every year for 10 years found that 13-40% of their readings were different from other readings. They also used the same ground rods we used. Therefore, it is important to complete multiple readings.
I asked another electrical contractor to install a stronger ground wire connection from the building to the ground rod to prevent copper theft in the future. They also performed another ground resistance measurement. However, it rained a few days before they took the reading and the value they got was even lower than 7 ohms (I took the reading when it was very dry). From these results, I believe that the ground rod is still in good condition.
Figure 7: Check the main connections of the grounding system. Even if the grounding system is connected to the ground rod, a clamp can be used to check the ground resistance.
I moved the 480V surge suppressor to a point in the line after the service entrance, next to the main disconnect switch. It used to be in a corner of the building. Whenever there is a lightning surge, this new location puts the surge suppressor in the first place. Second, the distance between it and the ground rod should be as short as possible. In the previous arrangement, ATS came in front of everything and always took the lead. The three-phase wires connected to the surge suppressor and its ground connection are made shorter to reduce impedance.
I went back again to investigate a strange question, why the surge suppressor did not work when the ATS exploded during the lightning surge. This time, I thoroughly checked all ground and neutral connections of all circuit breaker panels, backup generators, and transmitters.
I found that the ground connection of the main circuit breaker panel is missing! This is also where the surge suppressor and ATS are grounded (so this is also the reason why the surge suppressor does not work).
It was lost because the copper thief cut the connection to the panel sometime before the ATS was installed. The previous engineers repaired all the ground wires, but they were unable to restore the ground connection to the circuit breaker panel. The cut wire is not easy to see because it is on the back of the panel. I fixed this connection and made it more secure.
A new three-phase 480V ATS was installed, and three Nautel ferrite toroidal cores were used at the three-phase input of the ATS for added protection. I make sure that the surge suppressor counter also works so that we know when a surge event occurs.
When the storm season came, everything went well and the ATS was running well. However, the pole transformer fuse is still blowing, but this time the ATS and all other equipment in the building are no longer affected by the surge.
We ask the power company to check the blown fuse. I was told that the site is at the end of the three-phase transmission line service, so it is more prone to surge problems. They cleaned the poles and installed some new equipment on top of the pole transformers (I believe they are also some kind of surge suppressor), which really prevented the fuse from burning. I don’t know if they did other things on the transmission line, but no matter what they do, it works.
All of this happened in 2015, and since then, we have not encountered any problems related to voltage surges or thunderstorms.
Solving voltage surge problems is sometimes not easy. Care must be taken and thorough to ensure that all problems are taken into account in wiring and connection. The theory behind grounding systems and lightning surges is worth studying. It is necessary to fully understand the problems of single-point grounding, voltage gradients, and ground potential rises during faults in order to make the right decisions during the installation process.
John Marcon, CBTE CBRE, recently served as the Acting Chief Engineer at Victory Television Network (VTN) in Little Rock, Arkansas. He has 27 years of experience in radio and television broadcast transmitters and other equipment, and is also a former professional electronics teacher. He is an SBE-certified broadcast and television broadcast engineer with a bachelor’s degree in electronics and communications engineering.
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Post time: Jul-14-2021
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