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September 2019
Let's Dig In

Let's Dig In

05 September 2019

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Last week, I introduced you to team telecom. As promised, this week we are getting in to the details of good grounding and bonding in a telecommunications environment. If you've read the Secrets of the Soil Series, you might be thinking, haven't we already talked about ground testing enough? Nope, never. That was ground testing methods, this is ground testing for telecom - a whole different ball game. Let’s dig in. 

There are two things to consider before you get started: soil resistivity and ground resistance.

Aren’t resistivity and resistance just synonyms?

No, nice try. Soil resistivity, our first consideration, is how good the soil is at conducting electricity. You can also think of it as how well (or poorly) the soil resists the flow of electricity. That probably makes more sense, since resist is literally in the name.

You probably already know that some materials are better conductors of electricity than others. Or maybe you don’t, which is fine too, since I am going to tell you all about it. If you are holding a conductive material, it is going to conduct electricity much better when it is wet, than when it is dry. For example, loamy topsoil (which is a mixture of sand, silt, and clay) conducts electricity significantly better than dry sand, and it’s all due to the moisture content. By measuring the resistivity, we can differentiate the grounding potential of different materials, based on their ability to conduct (or resist electricity). Then, we can design a grade-A grounding system, since we know exactly what we are dealing with.

Keep in mind though, soil resistivity is like a fruitcake (or how I imagine a fruitcake to be, since I’ve never actually eaten one). When you cut into it, it’s a surprise; you are never certain what you are going to find. In one slice, there might be some unidentifiable dried fruit, whereas another piece may be home to a random assortment of roasted nuts. You never know, so you just hold your breath, cut into it, and find out for yourself. Soil resistivity is the same. Bet you weren’t expecting that analogy! You need to measure at several places and depths to get a full illustration of the true resistivity of the system you are working with.

Why do we measure the resistivity though?

Well, when you are setting up a grounding system, it’s not like throwing a javelin into a field with your eyes closed. You, unfortunately, cannot just put the ground rods wherever you want. By measuring the soil resistivity, you can find the best location and depth for your ground rods, so that you know exactly where to place them. Many companies already have standard or generic grounding plans in place, which is great. Way to be prepared.

However, what if you install the standard plan and it is not living up to your expectations? Ouch, you should have measured the resistivity first. Now, you don’t know whether to put in longer rods, more rods, or use another method all together. What a mess! Unless you are fueled by frustration, we recommend breaking the ice with your grounding system by measuring the soil resistivity first. If you thrive off of frustration too, we also recommend switching your fuel to maybe, I don’t know, coffee?

If you are still not convinced, at the very least, soil resistivity values can give you valuable insight into the equipment you should be buying for your system. For example, if you are looking at pockets of low resistivity values, throw some galvanized rods and hardware in your shopping cart because the potential for corrosion is looming.

So, we have a value of resistivity. Now what?

There is no golden rule of ground testing, but we can still give you some go-to guidelines. Let’s say you are trying to build a grounding system of 25 ohms (or less). Before doing anything, you measure the soil resistivity (units of ohm-cm), of course. If you get a value of less than 15,000 ohm-cm, you are good to go with the standard or generic design. It’s your lucky day! If your measurement is in the 15,000 to 25,000 ohm-cm range, you might be okay. But, if you are getting resistivity readings between 25,000 and 50,000 ohm-cm, you have some work to do to augment the ground. At the very worst, if you get a reading above 50,000 ohm-cm, a 25-ohm ground may not be in your foreseeable future. We hate to break the news to you, but the outlook is not good guys.

For your convenience, we also have a handy table for you to look at. I don’t know, but maybe, this will make things easier for you. The following table came straight from the prestigious Evershed & Vignoles Bulletin 245. Sounds fancy, right? Way back when, Evershed & Vignoles was one of the companies that eventually became Megger, by the way. 

Resistivity of Different Soils: Wet and Dry

Soil Type

Resistivity Range (ohm-cm)

Loam

100 – 5,000

Clay

200 – 10,000

Shales

500 – 10,000

Limestone

500 – 400,000

Surface Limestone

10,000 – 1,000,000

Slates

1,000 – 10,000

Sandstone

2,000 – 200,000

Sand & Gravel

5,000 – 100,000

Granite, Basalt, etc..

100,000

 

Hopefully, you are still reading this blog because the table alone does not tell the full story. If you just glance at the table and slam your laptop shut or throw your phone across the room, you’re going to be confused.

Let’s look at surface limestone, for example. Is that a big enough range for you? What you need to know is that the lower number (10,000) is when it is wet and the higher value (1,000,000) is for dry surface limestone. If you were to read between the lines (of the table), this is a good reminder to consider the range of dampness or wetness the site will experience when you are installing a grounding system. Also, some materials, like granite, will just never make a good ground. I am not sure why they even included it in this table, but like that failed relationship from freshman year of high school, it’s never going to work out. Sorry, we hate to be the bearer of bad news.

What about resistance? Did you forget about that little topic?

No, of course not. Let’s chat about resistance. Resistance is the opposition to current flow and it’s what you probably think of, if you think about ground testing. You probably don’t think about ground testing though, so that’s likely a reach. In the telecommunications industry, there are a lot of standards or “goals” for ground resistance, depending on the equipment manufacturer you are working with. If ground testing falls under your job title, then you’ve likely heard the recommendation for a 25-ohm ground.  

Why do we need a 25-ohm ground, though?

You are so inquisitive! Well, a 25-ohm ground is enough to protect us, humans. I think we can all agree that no one wants to get struck by lightning. Feel free to send a thank you note or a dozen roses to your nearby grounding system for the constant protection. I am, personally, very grateful for the comfort blanket that ground rods provide, and I am sure the feeling is mutual. The National Electric Code and IEEE also specify that grounds must be 25 ohms or less, so it’s supported by the industry’s biggest (and smartest) players, as well. This standard has been around for a long time too. Today, we are usually looking for a resistance of less than 25 ohms because we want to protect both ourselves, as well as the expensive, delicate, and sensitive equipment involved.

Now, you’ve considered everything there is to consider. You are basically a ground rod placement expert. Printing your certificate of excellence now.

What about measuring these things? Seems like a lot of work…

There were only two things, come on. For soil resistivity, there is no getting around it, you must have a 4-terminal ground tester. Sorry, 3-terminals, you’re going to have to sit the bench on this one. If you have do have a 3-terminal tester though, all hope is not lost. You can still measure the ground resistance and bonding, you’ll just be on your own for resistivity. On the other hand, a 4-terminal instrument can do it all – ground resistance, bonding, AND soil resistivity. Of course, the choice is always yours. But, if the previous 1000 words didn’t sell you on measuring the soil resistivity though, I don’t know what will.

 

  • Meredith Kenton, Digital Marketing Assistant, Megger Valley Forge