This Work Aid is designed to help the Participants in performing Exercise 3. Exercise 3 requires the Participants to design a substation/plant ground grid for a hypothetical installation. The Participant must complete the following steps that are covered in the designated Work Aid to design a substation/plant ground grid.
_ Determine need for Ground Grid Protection - Work Aid 3A _ Determine Step Potential - Work Aid 3B
_ Determine Touch Potential - Work Aid 3B
_ Determine Required Grid Spacing - Work Aid 3B _ Determine Number of Grounding Rods - Work Aid 3C _ Determine Ground Wire Sizes - Work Aid 3D
WORK AID 3 (Cont'd)
Work Aid 3A: Checklist for Evaluating the Need for Ground Potential Rise (GPR) Protection
This Work Aid is designed to help the Participant in performing Exercise 3A.
To evaluate the need for ground potential rise protection, perform the following steps:
_ Calculate Rg; the impedance to remote earth of the grounding electrode.
where: _ = Earth resistivity of substation in ohms-m.
A = Area occupied by ground grid in m2. h = Depth of the ground grid in m.
L = Buried length of conductors in m not including ground rods.
_ Calculate Ig; fault current flowing to the grounding electrode.
Ig = Ground fault current current division factor
_ Calculate GPR; Ground potential rise
GPR = Rg Ig
_ If the GPRis less than 300V, the area is classified as a low risk site. No protection is required for communication equipment.
_ If the GPR is between 300V to 1500V, the area is classified as a moderate hazard site. Protection must be applied to all communication equipment circuits.
_ If the GPR is above 1500V, the area is classified as severe hazard site. Protection must be applied to all communication equipment circuits.
WORK AID 3 (Cont'd)
Work Aid 3B: Formulas and Procedures for Determining Step and Touch Potentials and Grid Spacing
This Work Aid is designed to help the Participant in performing Exercise 3B.
_ To calculate the step voltage limit for a man weighing 50 kg, use the formula:
where: Cs (hsiK) = factor for surface soil
_s = resistivity of surface soil material
ts = duration of shock current in
seconds
_ To calculate the touch voltage limit for a man weighing 50 kg use the formula:
_ Compare the resultant ESTEP 50 limit and the ETOUCH 50 limit to the calculated GPR. If both the ESTEP 50 limit and the ETOUCH 50 limit are above the GPR, no further actions are required, the ground grid design is complete. If either (or both) of the ESTEP 50 limit or the ETOUCH 50 limits are below the GPR, further design improvements of the ground grid are required.
_ If the ground grid design uses the normal soil as the surface soil, consider addition of a layer of crushed rock to the surface, which will increase the resistivity of the surface.
_ If crushed rock is added recalculate ESTEP 50 and ETOUCH 50.
_ A further reduction in ESTEP 50 and ETOUCH 50 can be accomplished through an increase in the length of the ground grid conductor and through an increase in the number of ground rods. (Note: An increase in the length of the ground grid conductor and the number of ground grids can only be accomplished through change to the grid spacing because the overall dimensions of the grid have already been established). The following equation will give an estimation of the required length of the ground grid conductor to obtain the maximum voltage below the ESTEP 50 and ETOUCH 50 limits:
WORK AID 3 (Cont'd)
here: L = Total length of grounding grid including length of
grid conductor and ground rods in m.
Ki = 0.656 + 0.172n.
n = Number of parallel conductors in one direction; also equals for equally spaced rectangular grids where
NA is the number of conductors running in one
direction, and NB is the number of conductors
running in the other direction.
_ = Soil resistivity in ohm-m.
IG = Maximum grid current that flows between the ground
grid and the surrounding earth.
ts = Duration of shock current in seconds. Cs(hs1K) = Factor for surface soil.
_s = Resistivity of the surface soil.
Km = Mesh factor determined by formula below:
where: D = spacing between parallel conductors in m.
h = depth of ground grid in m.
d = diameter of ground grid conductor.
Kh =
WORK AID 3 (Cont'd)
Kii = 1 if ground grid has rods along perimeter. or
_ When the required length of ground grid (L) is known, determine how much of L will be Lc (total grid conductor length) and how much will be LR (total ground rod length). L can be expressed by the equation:
L = Lc - LR
_ For the new value of LC and LR, determine the maximum mesh voltage. The maximum mesh voltage should be less than (or equal to) the ETOUCH 50 limit. The maximum mesh voltage (Em) is calculated through use of the following equation:
_ Use the new value of L to calculate the maximum step voltage. The maximum step voltage should be less than (or equal to) the ESTEP 50 limit. The maximum step voltage (Es) is calculated through use of the following equation:
_ If Em and Es are equal to or less than the ETOUCH 50 and ESTEP 50 limits, the length of LC and LR are acceptable.
WORK AID 3 (Cont'd)
Work Aid 3C: Formulas, Procedures, and References for Determining Number of Ground Rods
This Work Aid is designed to help the Participant perform Exercise 3C.
_ To calculate the total resistance for a group of ground rods, the resistance of one rod must be calculated first.
_ Through use of SADP-P-111, the resistance of one rod can be determined from Figure 13.
_ Ratio the value taken off of Figure 13 to the actual soil resistivity through use of the following equation:
_ Find the group ratio for the spacing of the ground rods for Figure 14. _ To find the resistance for the group of ground rods, take the value
calculated as the actual value for a single rod, and the group ratio; use the following equation to calculate:
WORK AID 3 (Cont'd)
Resistance of a Single Rod
WORK AID 3 (Cont'd)
Ground Ratio
WORK AID 3 (Cont'd)
Work Aid 3D: Table of Wire Sizes and Ampacity
Use Work Aid 3D to complete Exercise 3D.
The size of ground grid conductors is determined by the magnitude of the fault current and the time of flow, based on the maximum allowable temperature rise.
Figures 15 and 16 show wire sizes vs. short-time ampacity for Saudi Aramco Systems.
WORK AID 3 (Cont'd)
Impedance Grounded Systems over 600V
WORK AID 3 (Cont'd)
Work Aid 3E: Procedures and References for Adjusting Ground Wire Sizes for Fault Times
Use Work Aid 3E to complete Exercise 3E.
_ After the size of the ground wire has been determined, the ability of the ground wire to handle the expected short-time current without exceeding the temperature limit must be verified. The temperature limit of 450oC is for use when the ground grid has brazed connections. The 250oC limit is for use when the connections are bolted.
_ Use Figure 17 to find the minimum allowed circular mils for the expected fault current, given the time duration.
WORK AID 3 (Cont'd)
Nomogram for Conductor Sizing
GLOSSARY
circuit breaker A device that is designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.
electric potential The potential difference between the point and some equipotential surface, usually the surface of the earth, which is arbitrarily chosen as having zero potential (remote earth).
electrical noise The disturbance in an electrical system that interferes with the normal transmission of signals carrying information.
equipment ground A ground connection to non-current carrying metal parts of a wiring installation or of electric equipment, or both.
fault time The duration for which a fault current flows prior to being interrupted.
ground A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body, of relatively large extent and that serves in place of the earth.
ground bus A bus to which the grounds from individual pieces of equipment are connected and that, in turn, is connected to ground at one or more points.
ground circuit A circuit in which one conductor or point (usually the neutral conductor or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through a grounding device.
ground conductor A conductor or system that is intentionally grounded, either solidly or through a current-limiting device.
ground grid A system of grounding electrodes consisting of interconnected bare cables that are buried in the earth to provide a common ground for electric devices and metallic structures.
ground potential An AC potential difference between remote earth and local rise
(GPR) ground.
grounded Connected to earth or to some extended conducting body that serves in place of the earth, whether the connection is intentional or accidental.
grounding conductor The conductor that is used to establish a ground and that
(ground conductor) connects an equipment, device, wiring
system, or another conductor (usually the neutral conductor) with the grounding electrode or electrodes.
grounding electrode A conductor that is used to establish a ground (for instance,
(ground electrode) ground grids, ground rods, or groundwells). grounding transformer A transformer that is primarily intended to provide a neutral
point for grounding purposes.
impedance grounded Grounded through impedance.
neutral ground An intentional ground applied to the neutral conductor or neutral point of a circuit, transformer, machine, apparatus, or system.
reactance grounded Grounded through impedance, the principle element of which is reactance.
resistance grounded Grounded through impedance, the principle element of which is resistance.
resistivity (material) A factor such that the conduction-current density is equal to the electric field in the material divided by the resistivity.
service ground A ground connection to a service equipment or a service conductor or both.
solidly grounded Grounded through an adequate grounded connection in which
(directly grounded) no impedance has been inserted
intentionally.
static electricity The accumulation of electrostatic charges on the surfaces of conducting and non-conducting bodies that are insulated from their surroundings.
step potential The potential difference between two points on the earth's surface, separated by a distance of one pace, that will be assumed to be one meter, in the direction of maximum potential gradient.
surface soil The resistance of the upper layer of soil in a ground-grid area.
resistivity
transfer potential The relocation of a hazardous potential from a ground-grid area to outside points.
touch potential The potential difference between a grounded metallic structure and a point on the earth's surface separated by a distance equal to the normal maximum horizontal reach (approximately one meter).
ungrounded A system, circuit, or apparatus without an intentional connection to ground except through potential indicating or measuring devices or other very high impedance devices.
voltage to ground The voltage between any live conductor of a circuit and the earth.
zig zag transformers A special grounding transformer that has two-phase windings on each core leg.