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?
- Ten feet of 6 AWG to an eight-foot ground rod having a contact resistance of 25 ohms.
- 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.
- 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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