Equation 5.1
where: R = Ground resistance, in ohms ρ = Soil resistivity, in ohm-m
L = Length of the current path, in meters A = Area of current path, in square
meters R =ρ L
A
The easiest and best method to find the value of ground resistance is to measure it with a ground resistance tester. The reading is obtained directly in ohms. Soil resistivity is most accu- rately measured with a four-point earth resis- tance tester. Soil resistivity can vary widely over a small geographical area and is affected by the type of soil, moisture content of the soil, and soil ambient temperature. More information on field measurement of ground resistance, soil re- sistivity measurements, and the various elements that affect soil resistivity may be found in a later subsection,System Ground Resistance Measure- ment and Calculation.
PUBLIC SAFETY
A well-designed, -constructed, and -maintained grounding system is essential to the oper- ation of any electrical distribu- tion system to maintain all common points connected to it as close to ground potential
as practicable. Proper grounding of a four-wire, wye-connected, effectively grounded system provides the following functions:
• Limits voltage across line-to- ground insulation,
• Provides a path to shunt surge currents from the system,
• Allows ground faults to be isolated quickly,
• Reduces the shock hazard
to people by reducing touch voltages during faults on electrical equipment cases and frames to safe levels, and
• Improves the likelihood that ground faults will be isolated quickly.
Unlike an overhead system in which equip- ment is physically raised above public areas, most UD systems have equipment enclosures mounted on the ground within easy public ac- cess. If a phase conductor contacts an enclosure, no dangerous voltages should exist because the enclosure could be touched by a member of the general public or the cooperative’s crews. To decrease the chances of a shock, ensure that the enclosure is connected to the lowest possible ground resistance. Another way to reduce touch voltage on pad-mounted equipment is to install a buried counterpoise system around the system.
One way someone could accidentally come into contact with an energized conductor is by digging into a cable. All RUS-accepted UD pri- mary cable is manufactured with concentric neu- tral wires that provide some electrical protection for someone digging into it. The theory is that the metal digging tool would first contact the grounded neutral wires and then the conductor, thereby creating a low-impedance path between the conductor and the concentric neutral wires.
This low-impedance path shunts most of the fault cur- rent through the grounded system neutral.
Over the years that UD sys- tems have been in place, they have established an excellent safety record. One reason is that a good grounding system exists, resulting, in part, from the use of bare concentric neutral cable that provides a large neutral surface in di-
rect contact with the soil. However, because of corro- sion, changes in the water table, changes in facilities, and the increasing use of JCN cable, more careful attention should be paid to the installa- tion of the grounding system.
RETURN CURRENT PATH
The typical underground distribution system is a three-phase, four-wire wye with multigrounded neutral, which satisfies the definition of an effec- tively grounded system. The neutral circuit must be a continuous metallic path along the route of the primary feeder and must extend to every consumer’s location. For this requirement to be met, the concentric neutral of jacketed cable must be grounded at each distribution trans- former, at frequent intervals (specified below) where no transformers are located, and at driven ground rods at each user’s service entrance. Be- cause the concentric neutral is multigrounded, it is connected in parallel with the earth, which forms a relatively low resistance path to the flow of current. Under normal operating conditions, residual current caused by unbalanced phase-to- neutral loads on primary circuits returns to the neutral of the substation transformer along this parallel path. In no instance, even under emer- gency conditions, should the earth ever be used as the only path for the return of normal load current on a distribution system.
For typical overhead rural distribution lines, it has often been assumed that 40 percent of the return current is carried by the neutral with 60 percent returning through the earth. However, the current division will vary depending on earth
Proper grounding
increases
personal safety.
Pay attention to
how JCN installations
are grounded.
resistivity and the size of the neutral, especially in the case of JCN underground systems where the neutral is grounded only by ground rods or by counterpoise wires. If the neutral is the same size as the phase conductor, which is usually the case for single-phase underground circuits, the current in it will be almost as large as the phase current. As the size of the concentric neutral is reduced, the greater the current flow in the earth. However, this change in current distribu- tion does not have a linear relationship to the ratio change in the neutral size. On single-phase primary circuits, RUS specifies that the concen- tric neutral and phase conductor must have the same conductivity.
In a perfectly balanced three-phase system, no neutral or ground currents flow. However, as stated previously, unequal phase-to-neutral loads will cause an unbalanced current to flow in the return path. Normal practice is to try to keep loads balanced for the system to operate efficiently. For this reason, the concentric neutral size in a three-phase circuit can be much smaller than the phase conductor. Cooperatives may op- erate three-phase systems with three cables specified at 1/3 neutral each, or 100 percent of the conductivity of a single-phase conductor. Most engineers recognize that a 1/6 neutral, with a combined three-phase
conductivity of 50 percent of the conductivity of one phase conductor, is enough for most operating systems. Reducing the size of the neutral has the additional benefit of reducing the circulating currents in- duced in the concentric neu- trals when they are grounded
and connected to each other, which increases cable ampacity and reduces losses.
The grounding and neutral circuits also pro- vide a way to ground the neutral of both three- phase and single-phase pad-mounted distribu- tion transformers. The transformer neutral is connected to the cable concentric neutral and both are tied to at least one ground rod. The tank should be grounded at two points by sepa- rate connections to ensure that it cannot become ungrounded through accident or corrosion.
For secondary single-phase, three-wire, 120/240-volt systems, the two energized con- ductors plus the grounded neutral from the transformer are run to the user’s service en- trance where the neutral is again connected to a driven ground rod. The user’s ground circuit is directly connected to the grounded neutral of the transformer to ensure that no potential dif- ferences can exist between the two systems. Ef- fective grounding is especially important to protect 120-volt equipment connected across two halves of the 240-volt transformer sec- ondary. The solid neutral connection holds the neutral at a point halfway between the 240-volt conductors. If the user’s neutral becomes iso- lated from the transformer neutral point, unbal- anced voltages across the equipment will result. The voltages across the two 120-volt legs will split in proportion to the impedance of the load on each side of the circuit, possibly causing burned-out light bulbs or damaged appliances.
NEUTRAL CIRCUIT FUNCTION UNDER FAULT CONDITIONS
On distribution circuits, the principal means of fault protection are the overcurrent relay and fuse. For these types of devices to sense a short- circuit condition and act quickly to interrupt the
fault, the fault current magni- tude must be considerably higher than the maximum load current. The most probable type of fault on an under- ground circuit is the single- line-to-ground (SLG) fault. Simply stated, the amount of fault current depends on the following:
• The impedance of the source,
• The voltage at the source,
• The line impedance from the source to the point of fault,
• The impedance to ground at the point of fault, and
• The impedance of the fault.
Fortunately, in UD systems, unlike overhead, the cable concentric neutral is usually involved