Environmental effects on the performance of electrical grounding ...

2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013

Environmental Effects on the Performance of Electrical Grounding Systems Siow Chun Lim#1, Lee Weng Choun#2 Chandima Gomes*3, Mohd Zainal Abidin Ab Kadir#4 #

Centre for Electromagnetic and Lightning Protection Research (CELP), Electrical & Electronic Engineering Dept, Universiti Putra Malaysia, Malaysia 1

[email protected] [email protected] 4 [email protected]

3

Abstract- Effects of surrounding environment of grounding electrode locations on the performance of overall grounding system has been studied. Copper rods were buried at 25 different locations with various environmental settings and their respective variations in grounding resistance were monitored for up to 4 months. It was found that there is a clear dependence of grounding resistance on the condition of surrounding environment, in addition to the average soil resistivity. Installation of grounding electrodes near running waterways, huge trees as well as slopes should be avoided wherever possible as the ground resistance of electrodes in such environments may fluctuate significantly with time.

soil, marshy land and sites prone to erosion may exist [1, 3]. Apart from these cases, it can in some other cases be inevitable to install copper rods near trees and soil near to either running or trapped water mass.

I. INTRODUCTION

As per the equations given in various standards, grounding resistance depended mainly on soil resistivity and dimensions of the grounding system [4, 5]. As aforementioned, presence of trees and proximity of grounding system to watery areas may or may not affect the grounding system performance which in this case is deep driven copper rod. A portion of the discussion was already available in [6]. In [6], the following results which were measured for a period of 3 weeks were discussed: 1. percentage difference of measured and calculated resistances for single rod 2. percentage difference of average resistivity of the site and specific resistivity of the exact location of the electrode

The effectiveness of a grounding system under transient conditions can accurately be evaluated only by measuring the in situ grounding impedance. However, out of convenience grounding resistance is usually used to characterized the effectiveness of grounding systems with fairly acceptable accuracy.

Grounding is a vital part for the protection of electrical systems whether as simple as a residential unit or as complicated as a building complex with numerous sensitive equipments. Grounding system deemed fit for lightning protection can be considered to be suitable for power grounding, signal grounding and also electrostatic discharge as the main principle behind it for all cases are the same which is to disperse charges as efficient as possible thus contributing to equipotentialisation [1]. However, integrated grounding systems should ensure equipotential bonding of all live and neutral parts under transient conditions via properly coordinated system of surge protective devices [2]. It is still a common practice to employ grounding via deep driven grounding rods in many countries, especially for housing units. Nonetheless, for structures with extensive electrical networks such as towers and high rise buildings other practices such as Ufer grounding and using the metal framework of the building itself or specially made reinforced concrete chunks are preferred [3]. However the focus of this paper is on issues which may be bugging the method of grounding via deep driven rods.

It was concluded in [6] that soil resistivity at any specific location is affected by the environmental factors considered in the study which are proximity of soil to water masses, built up area and vegetation. The research paper [6] also questioned the applicability of the average soil resistivity of the site in the calculations. In some cases the average resistivity and specific resistivity of the exact grounding location may vary by significant amount. In such cases, standards do not provide guidance to determine which soil resistivity is to be used in the calculations. In some cases, it has been found that neither of the soil resistivity values could provide estimations that are in good agreement with the measured electrode resistance. These observations expose the danger of using the formulae given in Standards in estimating the ground resistance of electrode

The issue of concern is the placement of grounding rods in various environments. The ideal condition for driving grounding rods which are typically copper rods are the areas with thick penetrable soil layers. However such convenience may not always be present. Extreme conditions such as rocky

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systems in the design stage. Grossly over-estimated or under estimated design may cause unforeseen financial losses. However as the results presented in [6] are based on a very short-term measurements, one may raise the question on the validity of the outcome.

Site-4 (Rod D4): furthest from road and vegetation Site-4 (Rod D5): near to solar panels Site-5 (Rod E1): near to a tarred road Site-5 (Rod E2): nearest to a tarred road and a road signage Site-5 (Rod E3): in the middle of the site Site-5 (Rod E4): near a giant tree Site-5 (Rod E5): nearest to a giant tree

Therefore in current study, a detailed investigation on the effect of the aforementioned environmental factors on grounding resistance will be elaborated. The measurements were done for up to 4 months and possible explanation as to why the behavior of grounding resistance of copper rod installed at specific sites is as such will be discussed.

Grounding resistance of each rod was measured on a weekly basis for up to 4 months using a digital grounding resistance meter KYORITSU MODEL 4105A. The meter functions base on fall of potential method but unlike Megger 3-pole device, the inter-probe separation has to be equal. In other words, if the potential probe is x m away from the measured rod, the current probe must be x m away from the potential probe and both probes must be in a straight line to maximize accuracy of measurement. In this study, 7 m separation was taken for all measurements.

II. METHODOLOGY The same experimental setup and sites as reported in [6] were used in this study as well. 25 copper rods were hammered for a depth of 1.58m into the ground at 25 different locations in 5 different sites. The inter-rod separations for each site can be obtained in [6]. The physical conditions of each site are retold here [6]:

The soil resistivity profile of the selected site has been measured by a 4-pole ground resistivity meter (MEGER DET5/4R). Measurements were repeated for better accuracy. Earth resistance measurements of the electrodes were taken by a digital earth resistance meter KYORITSU MODEL4105A, which works on fall of potential techniques. Each measurement was repeated in perpendicular directions and the average value has been taken for analysis.

Site-1: A flat lowland in close proximity with a lake and a small building. Site-2: A flat land near to a building complex and half of the site is surrounded by a river. Site-3: A highland with very thin layer of loamy soil with barely any vegetation. A quarter of the site is neighboring a slope of about 45° and 10m high. Site-4: A flat lowland with rocky soil and no vegetation except for a few medium-sized trees and there are a few isolated solar panels nearby. Site-5: A highland with quite heavy vegetation.

III. RESULTS AND DISCUSSIONS Fig.1 to fig. 5 illustrated the variation of grounding resistance for each copper rod at each site for 4 months. Before analyzing the results for each specific site, the average soil resistivity for each site was recalled from [6]:

As above mentioned, there are 5 rods driven into each site. The exact location of each rod will be described again :

TABLE I Average Soil Resistivity of Each Soil Site Average soil

Site-1 (Rod A1): nearest to a lake Site-1 (Rod A2): near to lake and a cement walkway Site-1 (Rod A3): nearest to a building Site-1 (Rod A4): in the middle of the site Site-1 (Rod A5): nearest to a cement walkway

1 2 3 4 5

Site-2 (Rod B1): nearest to a building complex Site-2 (Rod B2): nearest to a river Site-2 (Rod B3): in the middle of the site Site-2 (Rod B4): nearest to medium sized trees Site-2 (Rod B5): near to medium sized trees and river

resistivity (Ωm) 32 43 1685 184 442

As the dimension of grounding system is the same for all 25 settings, clearly the main governing factor of grounding resistance is the soil resistivity. This is reflected by the trends of grounding resistance in Fig. 1 to Fig.5 with Site 3 depicting the highest average grounding resistance of all 5 rods since it possessed the highest average soil resistivity.

Site-3 (Rod C1): nearest to a tarred road and a drain Site-3 (Rod C2): near to a hill Site-3 (Rod C3): near to medium sized trees Site-3 (Rod C4): nearest to slope Site-3 (Rod C5): in the middle of the site

In site 1, rod A1 which was nearest to trapped water mass consistently reflect the highest readings with the exception of A4’s spike in the 4th week of measurement. Although

Site-4 (Rod D1): near to medium sized trees and road Site-4 (Rod D2): near to medium sized trees Site-4 (Rod D3): in the middle of the site

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hammering in A1 required the least effort compared to other rods indicating existence of thick layer of loam, the result measured clearly contradicted one’s initial assumption that grounding rod driven in loamy soil is a good practice as it indicates absence of rocky layer. Possible reason to this result is osmotic gradient which in this case is the difference in concentration of water trapped in the soil with the water trapped in the lake. Since A1 is driven at the soil which formed the boundary between the lake and the soil mass, the osmotic gradient would be greatest causing water to seep to the soil mass further away from the lake. As grounding resistance depends on soil resistivity which in turn depends on moisture content of that particular soil, A1 should exhibit the highest grounding resistance since moisture has lower possibility to be retained in the soil which houses A1. Proximity of rod to building as demonstrated by A3 does not seem to have any detrimental effect on the grounding resistance. A4 exhibits significant fluctuation in grounding resistance variation possibly owing to the fact that it was driven in a soil with the highest specific soil resistivity measured at the soil mass near the rod. Rocky layer of soil was encountered after hammering the rod for more than 1 meter.

Figure 2: Site 2

Site 3 is the resemblance of an experimental site with extreme soil resistivity extending to the range of kΩm. Hammering copper rods into the site is the most difficult compared to other sites owing to the extremely thin layer of soft loamy soil. The most important finding derived from Fig. 3 is the rod (C4) which is closest to a slope exhibits the highest grounding resistance consistently. This could again be due to movement of moisture in the soil but this time caused by gravity pull rather than osmotic pressure. This gravity pull could possibly reduce the duration that moisture can stay within the soil mass surrounding and housing C4. C1 and C2 are furthest away from the slope and this explanation may be accurate in describing their constantly relatively low grounding resistance.

Figure 1: Site 1

In site 2, rod closest to water source again exhibits the highest grounding resistance in a consistent manner with the exception of B4 at week 5 of measurement. This further validates the explanation to results obtained in site-1. Proximity of medium-sized tree seems to cause fluctuation of grounding resistance of B4. However, there is minimal fluctuation of grounding resistance of B5 which is located close to medium sized trees and at a distance further away from the river compared to B2. The grounding resistance of B3 which is at the middle of the site is the most stable. Hence, it is very evident that the presence of trees may significantly affect the fluctuation of the electrical resistance of grounding electrodes. However, we have not analyzed the botanical nature of the trees pertinent to this study (type of the tree, the dimensions of the branches, dimensions and nature of root span, distribution of moisture content in the root span etc.)

Figure 3: Site 3

In site 4, D2 was driven into a relatively rocky soil compared to other rods hence explaining its high grounding resistance. However the interesting point is D5 which was planted closed to built up structure namely solar panel. There seems to be an influence of proximity to built up structures be it building or solar panel. Both A3 and D5 demonstrated the lowest grounding resistance and B1 (second lowest) consistently in each respective site although the measured specific resistivity in all 3 cases is not the lowest relative to

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other rods installed in the respective sites [7]. These results seem to suggest that driving grounding rod close to building is an advisable practice. However further validation has to be done to confirm this suggestion.

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Vertically driven electrodes of grounding systems placed in the vicinity of vegetation (trees of medium to large scale) may be subjected to significant fluctuations in grounding resistance with time. Vertically driven electrodes of grounding systems placed near water masses (whether flowing or stagnating) may show unexpectedly higher ground resistance and also be subjected to significant fluctuations in grounding resistance with time. Estimation of grounding resistance of electrodes that placed in the vicinity of vegetation and water masses (calculated with formulae given in Standards), may significantly be different from actual values (measured). Behavior of grounding electrodes close to buildings is similar to that in open spaces.

Based on the above observations we may recommend that; 1. Placement of grounding rods near to big trees should be avoided as the root activity of trees may cause fluctuation of grounding resistance. 2. If it is inevitable, the rods installed near large trees are advisably be installed together with backfill material such as bentonite to minimize the fluctuation of earth resistance. Information on the application of bentonite as backfill material is available in [8]. Further research is required with regard to these conclusions for solid validation.

Figure 4: Site 4

Site 5 is another interesting site as the grounding resistance of the grounding rods installed here are on the average most fluctuative compared to other sites. This site is relatively heavily vegetated compared to other sites. Most outstanding observations from Fig. 5 are from E4 and E5 which fluctuates the most and the commonality is that both are driven close to a big tree. This suggested that the activity of the roots of a large tree greatly affected the flow of moisture as well as ions in the soil and this translated to more fluctuative grounding resistance. The other 3 rods are nowhere near to any trees and are just surrounded by bushes.

ACKNOWLEDGMENT This work was supported by Grant No: 05-01-11-1195RU/FRUGS. Facilities provided by the Department of Electrical and Electronics Engineering, Universiti Putra Malaysia are greatly acknowledged. REFERENCES [1]

S.C. Lim, C. Gomes, M.Z.A.A. Kadir., “Electrical earthing in troubled environment”, International Journal of Electrical Power & Energy Systems, Volume 47, May 2013, Pages 117-128

[2]

C. Gomes. “On the selection and installation of surge protection devices in a TT wiring system for equipment and human safety”, Safety Science, Vol. 49, 861–870, 2011

[3]

C. Gomes and A G Diego, “Lightning protection scenarios of communication tower sites; human hazards and equipment damage”, Safety Science, Vol. 49, 1355–1364, 2011

[4]

IEEE SDT-142 (Green Book), IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, 2007

[5]

IEEE Guide for safety in AC substation grounding, IEEE Std.80-2000

[6]

W.C. Lee, C. Gomes, M.Z.A.A. Kadir, W.F.W Ahmad, "Analysis of earth resistance of electrodes and soil resistivity at different environments," in Lightning Protection (ICLP), 2012 International Conference on , vol., no., pp.1-9, 2-7 Sept. 2012

[7]

W.C. Lee,” Variation of Earth Resistance Between Natural and BuiltUp Environment,”BSc. dissertation, Faculty of Engineering, University of Putra Malaysia, Serdang, Malaysia, 2012

[8]

S.C. Lim, C.Gomes, M.Z.A.A. Kadir, “Characterizing of Bentonite from Chemical, Physical and Electrical Perspectives for Improvement of Electrical Grounding Systems”, International Journal of Electrochemical Science, May 2013

Figure 5: Site 5

IV. CONCLUSION Based on the outcome, several interesting conclusions were drawn that could be useful to designers of electrical grounding systems:

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