This article was posted 07/26/2007 and is most likely outdated.

Grounding in the Performance of Surge Protection Devices
 

 

Topic - Grounding and Bonding
Subject - Grounding in the Performance of Surge Protection Devices

July 26, 2007
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Grounding in the Performance of Surge Protection Devices

 

 

Mike,

A common misconception I once had was that a TVSS requires a good ground to operate. Since you pointed out several years ago this is incorrect, I have found a lot of this misconception. Click here to view a video from Eaton Cutler Hammer on a TVSS diverting a surge to ground. What are your thoughts?

 

Tom Baker

Code Moderator for www.MikeHolt.com

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Response by Mike Holt

 

Tom, a Surge Protection Device (SPD) at the service is designed to shunt induced lightning current to the earth as well as induced secondary current to the return neutral point of the secondary. How much lightning current does the resistance of the grounding electrode system (GES) to the earth at a premises impact the performance of a SPD is questionable in my mind. So this video clip is probably okay.

 

If they had indicated that the SPD strip in the house (point of use SPD) was trying to divert the lightning current to the earth, then I might have a more difficult time with the video, because the strip would be diverting lightning current to the equipment grounding conductor (EGC), that then leads it to the service neutral and ultimately to earth. Again how much does the GES resistance of the earth impact this? I don't know.

 

Induced lightning current is trying to get to the earth as well as the neutral point of the transformer and it has two paths:

1. A low resistive path to a high resistive earth connection and neutral point, such as a short grounding electrode conductor to the GES at the premises which has a contact resistance of 25 ohms.

 

2. A low resistive path to a low resistive earth connection and neutral point, such as a neutral conductor to the secondary utility transformer. Because the secondary neutral at the utility transformer is bonded to the primary neutral and the primary neutral is grounded to the earth at thousands if not millions of locations, there is practically no contact resistance at all.

 

SPDs are typically designed to shunt overvoltage from the ungrounded conductor to the neutral conductor, but some SPDs have ungrounded and neutral –to-earth connections as well. My discussion with SPD engineers is that this is done to give the impression that this SPD is better than the competitors.

 

I'm sure that if there was a study on the impact of the contact resistance of the earth of the GES on the performance of SPDs on a typically installation of a home, manufacturers of grounding fittings and devices would surely let us know. For now, they just make general claims that the low resistive grounding is important, practically for everything.

 

Grounding

Now don’t misunderstand me, grounding is important to for reducing overvoltage of electrical wiring and metal parts of electrical system [250.4(A)(1) and (2)]. What I don’t know is how to calculate the needed ground resistance for a grounding electrode system. What bothers me about grounding to reduce overvoltage from lightning is that lightning is a high-frequency event and I’ve never seen this taking into consideration when ground resistance is discussed.

 

Which works best for a 25 kA – 50 kA lightning event operating at a frequency of 5-10 kHz?

  1. Ten feet of 6 AWG to an eight-foot ground rod having a contact resistance of 25 ohms.
  2. Twenty to fifty feet of 3/0 AWG to a counterpoise consisting of three ten-foot ground rods have a combined contact resistance to the earth of 5 ohms.
  3. Fifty feet of 250,000 kcmil copper to the utility primary grounding system which has practically zero ohms (because of the thousands/millions of connections of the primary neutral connection to the earth).

 

I don’t have the knowledge to answer the above questions…

 

Mike Holt

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Comment by Dereck Campbell:

 

Mike, I will restrict my comments only as it pertains to residential applications with either single-phase or poly-phase because Ground Electrode System (GES) impedances do play a role in some industrial applications like a communications facility where cable plants and radio coaxes enter the facilities from multiple entry points that facilitate various arrestor devices.

 

After reviewing the Cutler Hammer video, I only have one comment about the statement, “Diverting the Charge to Ground”, which in my opinion is a half truth, misses the point, and miss-leading. The statement is OK for the layman public at large, but not for electrical professionals. I have said this many times so please excuse me if it is redundant.

 

According to IEEE 95% or more of lightning TVSS events occur ahead of the service utility transformer on the primary side in the Common Mode. Therefore the event appears on the secondary of the transformer in the Normal Mode (Differential) as that is what transformers are designed to do. Normal Mode means the effect is between the windings of the transformer on the secondary or put another way between L-L, and L-N.

 

At a residence we know only L1, L2, and the grounded circuit (Neutral) conductors are carried from the transformer to the service disconnecting means. At the service disconnect we are required by the NEC to bond the Neutral and Ground Electrode Conductor (GEC) together solidly via the Main Bonding Jumper (MBJ) to reference the system to earth. It is at this very point where we can clamp (limit) the potential differences to acceptable limits by placing an IEEE Class C (Service Entrance)  3-Mode TVSS device on a typical single-phase service (5-Mode for poly-phase). The three modes are L1-L2, L1-N, and L2-N. Note there are no modes connected to ground because there is no need for any since the N & G are bonded solidly together at the service disconnect device.

 

In the event of a lightning strike on the utility distribution, extremely high potential differences between all three conductors and earth will be present as expected. With a three-mode device installed at the service entrance will limit potential differences between the downstream feeder cables to acceptable limits if the SPD’s (Surge Protection Device like MOV’s and Avalanche Diodes) are properly installed and sized accordingly. A minimum SPD of 100 KVA per mode in a typical 200-amp service is a good place to start. Each mode will limit voltages between its respected conductors.

 

So you might ask; where the discharge current is going? Two places: some sent back to the utility on all the service conductors and some to earth, all of it through the SPD’s in the TVSS. The SPD’s dissipate the energy as heat acting as a simple load device. This is the reason the SPD’s at the service entrance should be as large as practical. Adding modes to ground would only take up more material, space, and add to cost not affording any added benefit. Manufactures do supply TVSS units with the ground modes installed, but this is from demand of customers who have miss-conceptions about TVSS operation on a grounded service. NOTE: Ground modes are important in Point-of-Use devices or IEEE Class A, but are not within scope of the discussion of service entrance devices

 

So what does earth have to do with the process? Not a lot except provide a reference point, and a poor one at that. What is important is the N-G bond at the service entrance, and the two SPD connected from L1-N, and L2-N. It is this point in which the down stream conductors are fed from and referenced too, not earth during a lightning event. True during a lightning event the potential differences between earth and the N-G bond point will be extremely high into the 10’s of thousands of volts, but all the conductors rise and fall at the same time with respect to the N-G bond point, NOT EARTH. So all the downstream conductor potential differences are clamped to acceptable limits with respect to the service N-G bond point, even though it may float into the 10’s of thousand of volts with respect to earth. The potential between N-G should still be around 0 volts, and if the SPD’s installed between L-G are going their job should be clamped to a few hundred volts depending on their UL SVR rating of the SPD.

 

Does the GES impedance really matter? Not in my opinion for a residence because the GEC impedance will be significantly higher and in series with the GES. Here is a good example from IEEE Emerald Book Std 1100-1992, Table 4-1

 

Let’s take a look at two GEC’s impedances at 100 MHz (a good lightning frequency), each 10-foot in length. One is copper #4 AWG, the other 4/0 AWG and connect them in series to say a mythical 5-ohm GES.

 

The #4 AWG will exhibit 2.6 K-ohms, and the 4/0 will exhibit 2.3 K-ohms. Use either conductor you want in series with the 5-ohm GES. Doesn’t make a difference, the GEC is the road block, not the GES. It wouldn’t make one bit of difference if the GES is 5 or 100 ohms, the limiting factor is the free-air Impedance of the GEC. With that said GES impedances are determined by very low power frequencies of 200-Hz or less, which has no correlation to high frequencies found in components of lightning. So the power frequency impedance is irrelevant and HF and RF.

 

Question: Wouldn’t the GES impedance also increase as it relates to high frequency current of lightning?

 

Answer: You are absolutely correct about the GES impedance at high frequency. I don’t know how to calculate the effect of high-frequency current on the different types of GES, but would imagine 10, 100, or 1000 times higher depending on what frequency was used.

 

Dereck Campbell, Licensed Professional Engineer with a BS in electrical Engineering from Oklahoma State University. Started his career with an electric utility company in sub-station relay control, switched disciplines to RF and digital transmission engineering for 10 years, and for the last 10 years switched back to electrical engineering as a Power Protection Engineer for a large telephone company.

 
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Comments

  • Mike,

    The performance and behavior of the Transient Voltage Surge Suppressor (TVSS) and/or the Surge Protective Device (SPD) has truly been characterized, by many manufacturers, as "magic". Lightning itself is perceived as a form of "black magic" by many individuals who have witnessed its power and unpredictability. Many individuals who have used surge protection in their homes have noticed less failures of their electrical equipment from light bulbs to expensive electronics. All of this combined has allowed many entrepreneurs to "cash in" on these and other misconceptions.

    The problem is that lightning and man-made transients are just that, transients. Transient behavior can not be perceived in the same way we perceive steady state power; so grounding, circuit impedances, and other “normal" circuit parameters do not act the way we naturally expect them to behave. The transient is a relativistic, three-dimensional phenomenon that often appears not to adhere to Ohm's Law. But, with a little understanding we can predict how transients will affect our power systems and, yes, transients STRICTLY obey Ohm's Law.

    First, there are several rules that we must accept if we are to successfully deal with transients. 1 - There is no such thing as a "singular ground". 2 - EVERY conductor has significant impedance regardless of its size. This includes the earth itself. 3 - Very small parametric values can be JUST AS IMPORTANT as very large parametric values. For example, we understand that kilovolts and kiloamps are large and potentially deadly. But in the transient world milli-henrys can be just as dangerous. 4- It is important to consider what initiated the transient, where the transient is coming from, and where the transient wants to go. Lightning transients must equalize the charge between cloud and earth, locally developed transients (for example a motor turning on and off) must dissipate the field energy within a local loop. 5 – Ground or conductor resistance is not enough; in the transient world reactance must be considered in addition to the DC, or low frequency, ground.

    For the purpose of brevity, I am going to limit my comment to the topic of Residential Grounding as it relates to surges.

    Let's start at the utility phase wire on top of the pole and for simplicities sake, let's imagine a single phase line that connects to a utility transformer and a utility high voltage surge arrester. A bolt of lightning strikes the phase wire, immediately the current divides in half. One half of the surge heads back to the utility, in search of an earth ground, and the other half proceeds to the transformer feeding the home, again looking for earth ground. The utility surge arrester reacts to the surge by diverting much of the surge current to earth. The surge arrester develops a discharge voltage based on the surge current (maybe 25,000 volts). At this point two things happen; first this "discharge voltage" is developed across the arrester is presented to the transformer to "step-down" to send to the home. Secondly, a very large surge current is sent down the utility ground wire causing a rise in the local voltage at the utility ground/neutral connection with respect to the home and the next pole span.

    Let's take a moment and consider how high (in volts) the neutral/ground connection has risen at the base of the utility transformer. If there is 10 ohm ground impedance at the utility transformer and there was a 1500 amp surge diverted by the arrester, then the neutral/ground voltage would be 15,000 volts. (E=IR) The numbers for ground impedance, surge current, arrester discharge voltage, etc are rather arbitrary and arguable.

    At the home then, the 120 volt line (170 volts peak) has risen to at least 400 volts (25,000 volts (discharge voltage) / 60 turns (transformer turns ratio)) and the ground/neutral (at the home) has also risen to a value between 0 and 15,000 volts based on the neutral impedance of the neutral wire and the ground impedance at the home. So at the home, the L1 & L2 to Neutral is 400+ volts, the ground to "far ground" is between 0 and 15,000 volts, the L1 to "far ground" 0 to 20,000 volts peak. It is important to note that the likely neutral/ground point, at the home, is going to be considerable less than 15,000 volts peak since grounding at the home and the impedances connecting the utility transformer to the home will probably split this voltage at in half.

    At this point a couple of observations and comments: 1 - Since the home "rides" on the ground/neutral plane, Mike is correct in saying that the value of the ground is not important for proper operation of the TVSS. It will clamp the 400 volts regardless of the ground condition. 2 - That being said, if an outbuilding, pool, or spa has its own ground (intentionally or otherwise), then there can be a significant transient voltage between Line and spa ground creating a hazard. This indicates the need for additional surge protection at the spa. 3 - The lower the ground resistance at the service entrance the lower the ground rise will be at the home. The TVSS (SPD) still does its job and protects line connected equipment even with poor grounds. 4 - UL SVR ratings are the maximum clamp values, for that product. Actual SVR values can be less. UL SVR test values are based on a 500 amp surge. 5 – The transient voltages seem, and in fact are, quite high. This is why one will notice small weld (or burn) marks around the home on electrical equipment enclosures in high lightning areas like Tampa, FL. A high transient causes a spark-over which results in power current being drawn until the circuit breaker trips or the arc clears itself. 6 – The Neutral to Ground bond at the service entrance is extremely important to provide a common reference point for the rest of the home. Without this bond, the difference in between the power line neutral and the Earth ground at the home could rise to thousands of volts. This would be very disruptive to every piece of line connected line connected equipment in the home.

    Other connections to the outside world also can cause problems in the home. Telephone and Cable TV connections are notorious for providing a different potential for “far ground” In these cases, there can be Line to Cable TV Ground arcs and/or large currents flowing between the Outlet (neutral or ground) to Cable TV Ground. This is a good reason to bond other services together at the service entrance.

    I think it should be said that the Cutler-Hammer (Eaton) video makes no mention of the magnitude of the ground impedance. Just that surges should be diverted as soon as possible before they have the chance to travel, unabated, through the house. This is a philosophy that is consistent with the IEEE C62 and UL1449.

    Thomas C Hartman TVSS Technology Manager Eaton Corporation

    Tom Hartman
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