Megger Ground Testers

Megger Ground Testers

Ground Testing Instruments offer a comprehensive solution to ground rod resistance and soil resistivity (4 point) requirements. It is the verification that the electrical system ground is connected to the earth ground and meets safety requirements as defined by the National Electrical Code, (NEC), OSHA, NFPA (National Fire Protection Association), the IEEE and other safety and standards organizations.
 
The National Electrical Code has established minimum safety values. The National Electrical Code, Section 250-56 states that the “system impedance to ground (ground rod) is less than 25 ohms and that facilities with sensitive electrical equipment should be 5.0 ohms or less.”
 
Here is a list of several Ground Testing Applications

• Soil Resistivity- 4 Point Tester
• Ground Resistance Rods- 3 Point Tester
• Low Ground Resistance Grids or Mats or Plates- 3 or 4 Point Tester
• Ground Resistance for Multiple Grounds without disconnecting system- Clamp-On Tester
• Step and Touch Potential- 4 Point Tester
• Continuity on Bonding or Grounding -2 or 4 Point Tester
• Earth Coupling- 3 Point Tester

A grounding system is designed to provide safety for a variety of conditions that may exist

• Lightening strikes to protect personnel or system components
• Electrical faults
• Static Electricity Problems
• Voltage Regulations for Safety     

A good grounding system will improve the reliability of equipment and reduce the likelihood of damage due to lightning or fault currents. The primary reason that grounding is tested is the fact that resistance to an earth ground rod or system may vary based on changes in climate and temperature and over time. Such changes can be considerable. Over time, corrosion will degrade ground rods and their connections. It is reasonable to consider an annual maintenance program that checks grounds and bonding.

Features to consider when evaluating your selection for Ground Testers

• Types of Ground Testing to be Evaluated
• Number of Poles (points) for tests to be completed
• Display Required
• Graphics Required
• Memory Required and Capacity
• Enclosure IP Ratings
• Wiring Spools and Additional Rods Needed
• Wiring  Capable of Being Disconnected for Portable Clamp-On
• Rechargeable Battery or Field Battery Supply
• Test Current
• Safety Rating
• Resolution

Components of Earth Ground Electrode
 

1. Resistance of the electrode itself and connections to it, including the clamp and wire connection
2. Contact resistance between the electrode and the soil adjacent to it
3. Resistance of the surrounding earth

What affects the ground resistance measured?

• The length and depth of the electrode
• The diameter of the electrode
• The number of electrodes
• The total inherent system design

Ground testing systems to be tested also include Multiple Electrodes, Grounding Grids, Grounding Mats and Grounding Plates.

Traditional types of methods for ground testing include

• (a) Three Point Fall of Potential for Ground Rods and Grids
• (b) Four Point Soil Resistivity Testing
• (c) Touch Potential for Electrical Safety (high ground fault possibility)
• (d) Two-Point Tests (continuity tests and bonds)
• (e) Clamp-On

(a) Three-Point Fall of Potential

Three Point measurements are used when electrodes or grids can be easily disconnected. Three-point measurement is used to measure resistance to ground of ground rods and grids. The potential difference between rods X and P is measured by a voltmeter, and the current flow between rods X and C is measured by an ammeter. Using Ohm’s Law, R = E/I, we may obtain the ground electrode resistance R. It is not necessary to carry out all the measurements when using a ground tester. The ground tester will measure directly by generating its own current and displaying the resistance of the ground electrode under test (Z).

Earth Coupling. In order to measure Earth coupling, a 3 point system is required. It is primarily used to estimate the influence of two earth resistance systems that are not connected to each other.
 
(b) Four-Point Soil Resistivity (Wenner Method)

The primary purpose of this test is to locate the areas of lowest soil resistivity which is essential for achieving an economical grounding installation. For most applications, the most accurate method is the 4-Point method. The 4-Point method requires the insertion of four equally spaced electrodes in a test area. A known current from a constant current source is passed between the outermost electrodes and a voltage is then measured across the two innermost electrodes. Most ground resistance instruments will also read the resistance of the rod in test directly in ohms. Soil Resistivity is necessary when determining the design and location of the grounding system especially for new installations to meet acceptable ground resistance requirements.
 
Soil resistivity is a determining factor when measuring the resistance of a grounding site. This determines the electrode resistance and the depth required for acceptable resistance. The resistivity of the soil varies widely throughout the world and changes seasonally. Soil resistivity is determined largely by its content of electrolytes, consisting of moisture, minerals and dissolved salts. A dry soil has high resistivity if it contains no soluble salts.
  (c) Touch Potential (Three Pole) for Electrical Safety (high ground fault possibility)

Touch Potential is the term given to the potential difference a person might experience across their body if they were, for example, standing on the ground outside the perimeter fence of a substation and touching the fence at the time a fault to earth occurred. These tests are recommended when the ground cannot be disconnected, where ground faults are highly likely to occur, or when the “footprint” of grounded equipment (the outline of the part of equipment in contact with earth) is comparable to the size of the ground to be tested. Touch potential is defined as the voltage between an energized object and the feet of a person that may be in contact with that object. In cases close to power distribution sites and fences, the touch potential can be extremely dangerous and hazardous. A four pole tester is used for “touch potential” measurements.
 
Step potential is the potential difference a person would experience between their feet if they walked over the ground in which a fault current existed. A four pole tester is also used for this test.

(d) Two-Point and Four Point Tests (continuity tests and bonds)

Even though Micro-Ohmmeters with Kelvin probes are suggested to be used for primary accurate bond testing resistance values, a simpler and less accurate,  quick indication of resistance value may be performed utilizing a 2 pole (2 lead) method measuring DC resistance with less test current.
 
A 2 Point Test may also be used to measure the continuity resistance of any wired system. It also measure the integrity of all single or multiple ground system connections.

(e) Portable Clamp-On Method for Ground Testing

Clamp-on ground resistance testing offers the ability to measure the ground resistance without disconnecting the ground rods or the wiring of the system. This type of measurement also offers the advantage of including the bonding to ground and the overall grounding connection resistances. One of the most important considerations for the portable clamp-on system would be a series-parallel resistance path down-stream from the measurement point that is lower in resistance than the point being measured. Please note figures below for typical Clamp-on placement for measurement.
 
Applications for Clamp-on Ground Resistance Testers

• Measure electrical ground rod and grid resistance
• Use on multi-grounded systems without disconnecting the ground rod under test
• Measure resistance and continuity of grounding loops around pads and buildings
• Measure leakage current flowing to ground or circulating in ground systems
• Conduct field surveys
• Use on cell towers and telecommunication sites
• Use on pools, spas and other consumer installations

 

Ground Testing Resources

This documet from Megger is an excellent reference for basic Ground Resistance theory. Click the cover to download the PDF