Soil resistivity directly imapacts the resistance or performance of an electrical grounding
system.
It is also the starting point of any electrical grounding design.
A well designed ground system ensures that protective devices will
operate correctly during a fault event and reduce any excessive voltages developed during a
fault,
which could damage equipment and endanger the safety of anyone near the fault.
Prior to installing a ground system the resistivity of the surrounding soil should be measured. Inaccurate resistivity tests can lead to unnecessary costs in additional system design and construction costs. After installation it is vital to check that the electrical grounding system meets the design criteria and should be measured periodically to ensure corrosion or changes in the soil's resistivity do not have an adverse effect. Ground networks may not appear faulty until a fault occurs and a dangerous situation arises.
Grounding of electrical equipment serves several purposes. The first is to protect personnel and equipment from overvoltages, faults and lightning. Grounding also ensures stability of system voltages by providing a solid reference to earth, and establishes a reference to control electrical “noise” that might interfere with the proper operation of electronic equipment. Acurate measure of a facility's ground resistance is essential in preventing costly downtime due to service interruptions caused by poor grounds.
| Terminology | |
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| Grounding: |
Connected (connecting) to ground or to a conductive body that extends the ground connection. An example of a conductive body that extends the ground connection is the first 5 feet of a water pipe entering a building. It is permitted to connect at this point even though it is not a grounding electrode, as the first 5 feet of the pipe is not physically in contact with the earth.
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A system or circuit conductor that is intentionally grounded. These are often regarded as “neutral” conductors in the trade. Section 200.6 requires these conductors to be identified as continuous white, continuous gray, or having three continuous white or gray stripes along the conductor’s entire length on other than green insulation. Be mindful that grounded conductors can also be phase conductors in certain applications. Connected (connecting) to ground or to a conductive body that extends the ground connection. An example of a conductive body that extends the ground connection is the first 5 feet of a water pipe entering a building. It is permitted to connect at this point even though it is not a grounding electrode, as the first 5 feet of the pipe is not physically in contact with the earth. The conductive path(s) that provides a ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. These are often called “ground” conductors in the trade. Section 250.119 requires these conductors to be bare, covered, or insulated. Covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes. |
| Grounding Electrode: |
A conducting object through which a direct connection to the earth is established. Section 250.52(A)(1–8) lists the permissible grounding electrode options. While all the available grounding electrodes are required to be bonded per 250.50, it’s fair to say the most common are typically rod and pipe electrodes (i.e., ground rods), which are listed in 250.52(A)(5), and concrete-encased electrodes, which are listed in 250.52(A)(3). Often, concrete-encased electrodes consist of rebar that is embedded in the concrete footings of a building.
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| Ground-Fault Current Path |
An electrically conductive path from the point of a ground fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source.
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| Bonded (Bonding) — |
Connected to establish electrical continuity and conductivity.
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A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected. Connected (connecting) to ground or to a conductive body that extends the ground connection. An example of a conductive body that extends the ground connection is the first 5 feet of a water pipe entering a building. It is permitted to connect at this point even though it is not a grounding electrode, as the first 5 feet of the pipe is not physically in contact with the earth. The conductive path(s) that provides a ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. These are often called “ground” conductors in the trade. Section 250.119 requires these conductors to be bare, covered, or insulated. Covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes. |
| NEC 240(A) | |
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Grounding Systems are connected to Earth, to limit voltage spikes from lightning, line surges, umintentiaonal contact to with higher voltage lines, stabilize normal voltages Normally non-current carrying conductive materials, shall be connected to earth to limit voltage spikes. Normally non-current carrying conductive materials, enclosing conductors of materials enclosing conductors or equipment, or forming part of equipment shall be connected togetherand to supply to form ground fault path. |
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| Grounding Test Methods | |
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The fall-of-potential method is the fundamental method for measuring the ground impedance of large grounding systems (see 8.2.1.5 of IEEE Std 81-1983 [2]), It is extremely reliable, highly accurate and can be used to test any size ground system. Additionally, the operator has complete control of the test set-up and can check or proof his/her results by testing at different probe spacings. Magnetic-trip-only breakers have no thermal element. Such breakers are principally only used for isolating the circuit and short-circuit protection. Molded-case breakers with magnetic only trips find their application in motor circuit protection. MCP's can be found inside Motor Control Center (MCC). They are typically placed inside a cubical or enclosure, along with motor control elements and a motor overcurrent device; commonly knows as a heater. In addition, earth resistivity may be used to indicate the degree of corrosion to be expected in underground pipelines for water, oil, gas, gasoline, etc. In general, spots where the resistivity values are low tend to increase corrosion. This same kind of information is a good guide for installing cathodic protection. TThe clamp-on ground testing method, although it does not conform to IEEE 81, does provide the operator with the ability to make effective measurements under the right conditions. The clamp-on methodology is based on Ohm’s Law (R=V/I). A known voltage is applied to a complete circuit and the resulting current flow is measured. The resistance of the circuit can then be calculated. The clamp-on ground tester applies the signal and measures the current without a direct electrical connection. The clamp includes a transmit coil that applies the voltage and a receive coil that measures the current. |
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| Low Voltage Circuit Breaker Testing | |
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Lorem ipsum dolor sit amet consectetur adipisicing elit. Fugit sit nihil voluptatibus illo vitae cumque! Quia ipsum soluta consequatur corrupti aperiam voluptas possimus, voluptate ex ullam porro neque dicta laudantium. Perform a mechanical check on the trip device to assure a successful tripping operating just before the armature reaches its fully closed air gap position. (1) Metal Underground Water Pipe. A metal undergroundwater pipe in direct contact with the earth for 3.0 m (10 ft) or more (including any metal well casing bonded to the pipe) and electrically continuous (or made electrically con tinuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductor(s) or jumper(s), if installed.
(2) Metal Frame of the Building or Structure.
(3) Concrete-Encased Electrode.
(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 2 AWG.
(5) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 2.44 m (8 ft) i n length and shall consist of the following materials.
The purpose of this test is to determine that the trip device will open the circuit breaker to which it is applied. This test can usually be performed by injecting 150%–300% current of the coil rating into the trip coil. The test equipment used should be able to produce the required current and be reasonably sinusoidal. The magnetic (INST) trip should be checked by selecting suitable current to ensure that the breaker magnetic feature is working. The diffi culty in conducting this test is the availability of obtaining the required high value of test current.
Ground Fault Protection Tests
A ground fault is an inadvertent contact between an energized conductor and ground or equipment frame. During a fault condition, current will return through the grounding system. Ground faults are frequently the result of insulation breakdown. It’s important to note that damp, wet, and dusty environments require extra diligence in design and maintenance. Since water is conductive it exposes degradation of insulation and increases the potential for hazards to develop. Ground fault sensing and relaying equipment utilizes ground return or vectorial summation sensing methods Ground fault protection can be checked in the field by passing a measured test current directly through the sensing transformer or test windings. To confirm the proper functioning of the equipment while it is installed in the switchboard or panelboard. The following tests can be performed.
Zero SequenceCircuit feeders are fed through a current transformer. A zero sequence relay or circuit breaker trip unit ensures that the circuit current feeding the load will return to the source returns on those same conductors . If the current is returning to the source through a different path (usually ground), the ground-fault relay will detect this difference. If the diffrence in current exceeds a pre-determined amount for a pre-determined amount of time, the ground-fault relay will operate.
\[I_{a}+I_{b}+I_{c}-I_{n}=0\]
Summation MethodDirect measurement of all phase and neutral current through a protection device's current transformers is evaluated. All phase currents feeding a load should return to the source. A ground fault some where downstream will create a difference in the returning current.
\[I_{a}+I_{b}+I_{c}=I_{n}\]
Megger The secondary current injection test of solid-state units can be performed by a specially designed power supply unit. It should be noted that the secondary current injection method only tests the solid-state trip unit logic and components, and does not test the current sensors, wiring, or the breaker’s current carrying components as is done during primary current injection method |
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