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Grounding & Testing A Communications Perspective Ed Rousselot National Telecom Sales Engineer 2

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Agenda What is a Ground and why is it important to have a good one Soil resistivity and soil resistance Considerations with installing a Ground Electrode System (for our purposes, rods ) Measure the effectiveness of the ground electrode system by means of ground testing 4

What is a Ground? A ground is a conducting connection by which an electrical circuit or equipment is connected to the earth or some conducting body. Source: IEEE Standard 81 Low impedance conductor used to provide a safe path for the dissipation of: - fault currents - lightning strikes - static charges - EMF/RFI signals Simply put. 5

Simply Put The characteristic of a grounded system is that there is a steady flow of current that is going to the path of least resistance. We do not want that path of least resistance to be through someone or some delicate equipment. We want it to be through a ground electrode system. This current can be measured using an ammeter that measures milliamps. 6

Brief Look at Benefits of Proper Grounding Safety Us and our Equipment Noise on our Circuits 7

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Ground Resistance vs. Soil Resistivity Ground Resistance: The resistance (opposition to current flow) of an installed electrode system Measured in Ohms Measured using three or four-point stake testers or a clamp-on tester Soil Resistivity: The electrical properties of the soil for conducting current Measured in Ohm-cm (Ohm centimeters) Measured using a four-point stake tester 14

Resistivity Soil Resistivity: The electrical properties of the earth for conducting current 15

Why Test Resistivity? Tells you how good (conductive) your soil is Good indication on whether or not a generic ground specification design will work Helps reduce surprises at the end of the installation An indication of the degree of corrosion to be expected on components of the ground system 16

Resistivity of Different Soils 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 Granites, Basalts, etc. 100,000 Evershed & Vignoles Bulletin 245 17

Soil Resistivity Ranges 100-15,000 Ohm cm Standard Design OK 15,000-25,000 Ohm cm - Standard Design Maybe 25,000-50,000 Ohm cm - Special Circumstances 50,000 + Ohm cm Perhaps not practical 18

Measuring Soil Resistivity Use a 4-terminal earth resistivity tester Space the rods an equal distance apart a in next slide Insert the rods a distance of 1/20 th into the ground a Measures the average soil resistivity to a depth equal to the rod separation 19

Measuring Earth Resistivity a a a C1 P1 P2 C2 a a/20 20

Actual Site Testing Procedures Test at multiple locations across the site 21

Four-Point Resistivity Tester Also Tests Resistance 22

Why Measure Soil Resistivity Periodically? To check that climatic conditions have not changed the ground such that it no longer meets the requirements To check for seasonal variations To check that changes to buildings, the lot, the streets, etc. have not affected resistivity 23

Ground Resistance Ground Resistance: The resistance (opposition to current flow) of an installed electrode system 24

Typical Permissible Ground Resistance Values Telecom - varies depending on the standard used: - Electronic equip mfg. vary from fraction of an ohm upward -At premise must equalize with power ground which is 25Ω Typical values from an insurance company: - Industrial plant: 5 - Chemical plant: 3 - Computer system: 3 Typical values for a power company: - Generating station: 1 maximum - Large sub-station: 1 maximum - Small sub-station: 5 maximum Why 25 Ohms? 25

25 Ohm ground keeps US safe NEC Section 250 IEEE STD 81 26

Ground Electrode System Components Ground Electrodes Ground Conductors Ground Bars Bonding Connectors Mechanical Compression Welded 27

Ground Electrodes 28

Ground Electrodes Do Not Have To Be Rods Ground Electrode Types - Ground Plates Ground Rods: Copper Clad Steel Solid Copper Galvanized Stainless Steel Enhanced Ground Mesh 29

Ground Electrodes Types of Grounding Systems Multiple-Rod Electrode Single Rod Electrode Multiple Pole Grounds 30

Ground Electrodes Utility Pole Ground Wire 12 below ground Butt Plate? Ground Rod 31

Another type of butt ground 32

Ground Electrodes Considerations Soil Resistivity - Some soils, (such as sandy soils), have such high resistivity that conventional ground rods or ground electrode systems may be unable to attain the desired ground resistance. Enhanced ground electrodes or ground enhancement materials may be required to meet the grounding specification. Some of these are 33

Enhanced Ground Electrodes Ufer Grounds - Concrete encased electrode tying into tower footing or building-pad rebar 34

Enhanced Grounding Material Low Resistance Reduces seasonal variation Carbon-based Not permanent Washes away over time Only consider when deep or multiple rods are not practical 35

Enhanced Grounding Material Soil treating material placed in circular trench and covered with earth Appx 1 ft. Ground rod Appx 18 in. 36

Enhanced Grounding Material 37

Ground Electrodes Considerations Soil Ph - affects the rate of corrosion of metal ground components that are in contact with the soil cont. 38

Ground Electrodes Considerations Soil Characteristics - Some sites may have only a few inches of soil (or none) sitting on top of bedrock. Consider- Ground mesh & plates Horizontal rods (Don t drill into bedrock such as granite. Such rock does not make a good ground.) cont. 39

Ground Rods Considerations Ground Rod Diameter Doubling diameter of ground rod reduces resistance only 10%. Using larger diameter ground rods is mainly a strength issue. In rocky conditions, a larger diameter ground rod might be advantageous. Cont. 40

Ground Rods Considerations Ground Rod Length - Doubling length theoretically reduces resistance 40%. Actual reduction depends on soil resistivities of multi-layered soils. Cont. 41

Ground Rods Considerations Multiple Ground Rods Two well-spaced rods driven into the ground provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly. (The resultant resistance is not one-half of one of the rod s resistance.) The reduction for equal resistance rods is about: 40 percent for 2 rods 60 percent for 3 rods 66 percent for 4 rods Spaced apart greater than their length 42

What Else Can YOU Control? Torque Bullet Bonds 43

Which kind does your locate contractor use? 44

Causes of Ground System Deterioration Weather influences exert mechanical strain on ground rods Cable locates Corrosion over time Catastrophic events like lightning strikes or large fault currents can cause instant degradation that may not be visible Soil resistivity can change over time due to environmental conditions Facility expansion can create different needs in the ground system 45

Risks from Ground System Deterioration Potentially deadly electrical shock situations Plant-wide equipment damage Disruption in the performance of sensitive equipment with tight voltage parameters Heat build-up on a single piece of electrical equipment and, eventually, fire 46

Ground Electrode System Testing Ok, the system is designed and installed. Let s Test! 47

Choose the Proper Instruments Use a dedicated ground tester - designed to measure grounds Don t use a generalized ohmmeter or multimeter Don t use an insulation resistance tester Don t use the ground test of a telecom multifunction test set 48

Safety First! This is not a safety course 49

Three-Terminal Ground-Resistance Testing 50

Review Ohm s Law Resistance = Voltage Current Ohms = Volts Amps If we know Voltage and Current, we can calculate Resistance So, If we have 50 Volts and 2 Amps, we have 25 Ohms (25 = 50 2) 51

Three-Terminal Ground Resistance Tester Current Supply Ammeter (I) Ground Rod Under Test (Isolated) Voltmeter (E) Potential Probe P Current Probe C X Earth Earth 52

Fall of Potential Ground Rod Under Test (X) (Isolated) Ground Rod Position 53

Resistance in Ohms Resistance Curve True Resistance X Ground Rod Position Distance of Potential Probe from X C Current Probe Position 54

Rules of Thumb on C Probe Spacing From Ground Rod X Single ground rod - 50 feet Small grid of 2 ground rods - 100-125 feet Large system (several rods or plates in parallel) >200 feet Complex systems (large number of rods or other electrodes and other metallic structures bonded together) - far greater distances are required 55

Resistance in Ohms Insufficient Probe Spacing Ground Rod Under Test (X) (Isolated) Potential Probe (P) Current Probe (C) No Flattening No True Resistance Distance of Potential Probe from X 56

Advantage: Advantages/Disadvantages of Fall of Potential Testing Conforms to IEEE 81 - only approved method. Disadvantage: Time consuming 57

61.8% Rule Ground Rod Under Test (X) (Isolated) 61.8% Ground Rod Position 58

61.8% Rule/Method Based on the full Fall of Potential method Take measurement at only one point Advantage Quick and easy Disadvantage Assumes that conditions are perfect with respect to: Adequate spacing of C and P probes Resistivity of the soil being the same 59

Ground Testing on Asphalt Lazy Spike Does not have to be wet (Isolated) MEGGER DET5/4R 3 POLE 4 POLE MEASURE 60

Ground Testing Methods Fall of Potential Method* 61.8% Rule Method* Simplified Fall of Potential Four Potential Method Intersecting Curves Method Slope Method Dead Earth Method Star-Delta Method *Covered here 61

Stake-less or Clamp Ground Testing 62

Review Ohm s Law Resistance = Voltage Current Ohms = Volts Amps If we know Voltage and Current, we can calculate Resistance So, If we have 50 Volts and 2 Amps, we have 25 Ohms (25 = 50 2) 63

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CAUTION Ammeter does not measure the value of the ground 65

Considerations When Using Stake-less/Clamp-On Method Effective only in situations with multiple grounds in parallel such as pole grounds 66

Multiple Grounds in Parallel 67

Considerations When Using Stake-less/Clamp-On Method Effective only in situations with multiple grounds in parallel such as pole grounds Requires a good return path so it cannot be used on isolated grounds 68

Clamp Won t Work No Return Path CATIV n600v Megger 69

Considerations When Using Stake-less/Clamp-On Method Effective only in situations with multiple grounds in parallel such as pole grounds Requires a good return path so it cannot be used on isolated grounds Cannot be used if an alternate, lower resistance, return path exists not involving the soil 70

Must Measure at the Correct Part of the Loop Connecting Wire Lightning Rods CATIV n600v Megger 71

Power Company Feed NID What is being measured? Clamp-on set must be BELOW this point. To power company ground (Has many parallel connections to power company feed.) 6 Rods are 6 to 8 apart Multiple ground rods bonded underground 72

Reality 73

What is Being Measured? Grounding Conductor Bonded to MGN Utility Pole Butt Plate Ground Rod 74

What is Being Measured? Only Connectivity Not Ground Resistance Wire Connecting Each Leg to Rod Ground Rods At Each Leg CATIV n600v Megger Buried Wire Connecting the Rods 75

Pedestal Ground Bus SHIELD CONNECTION Ground Bar 76

CATIV n600v Megger Ground bar Measuring continuity 77

This is what the ground wires look like that we have to clamp around to test. Right? 78

Sometimes It s Not Easy 79

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What Now? Could be worse 81

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Quick Review 83

Addressing Ground System Problems Use longer ground rods Use multiple ground rods Chemically treat the soil Place the system in lower resistivity soil if possible 84

So, we tested our ground system and we passed! Do we ever need to test it again? 85

Why Measure Ground Resistance Periodically? (You have seen this before but ) To determine the effectiveness of ground rods and connections Seasonal changes Water table changes Changes in the site and/or building To check that standards set by codes are still being met To check that specific design parameters have been met To check that the ground rods and bonds are still present 86

Questions 87