1. It provides a means of dissipating electric
current into the earth without exceeding the operating limits of the equipment.
2. It provides a safe environment to protect
personnel in the vicinity of grounded facilities from the dangers of electric
shock under fault conditions.
The grounding system includes all of the
interconnected grounding facilities in the substation area, including the
ground grid, overhead ground wires, neutral conductors, underground cables,
foundations, deep well, etc. The ground grid consists of horizontal
interconnected bare conductors (mat) and ground rods. The design of the ground
grid to control voltage levels to safe values should consider the total
grounding system to provide a safe system at an economical cost.
The following information is mainly concerned
with personnel safety. The information regarding the grounding system
resistance, grid current, and ground potential rise can also be used to
determine if the operating limits of the equipment will be exceeded.
Safe grounding requires the interaction of two
grounding systems:
1. Intentional ground, consisting of grounding
systems buried at some depth below the earth’s
surface
2. Accidental ground, temporarily established by
a person exposed to a potential gradient in the vicinity of a grounded facility
It is often assumed that any grounded object can
be safely touched. A low substation ground resistance is not, in itself, a
guarantee of safety. There is no simple relation between the resistance of the
grounding system as a whole and the maximum shock current to which a person
might be exposed. A substation with relatively low ground resistance might be
dangerous, while another substation with very high ground resistance might be
safe or could be made safe by careful design.
There are many parameters that have an effect on
the voltages in and around the substation area. Since voltages are
site-dependent, it is impossible to design one grounding system that is
acceptable for all locations. The grid current, fault duration, soil
resistivity, surface material, and the size and shape of the grid all have a
substantial effect on the voltages in and around the substation area. If the
geometry, location of ground electrodes, local soil characteristics, and other
factors contribute to an excessive potential gradient at the earth surface, the
grounding system may be inadequate from a safety aspect despite its capacity to
carry the fault current in magnitudes and durations permitted by protective relays.
During typical ground fault conditions, unless proper precautions are taken in
design, the maximum potential gradients along the earth surface may be of
sufficient magnitude to endanger a person in the area. Moreover, hazardous
voltages may develop between grounded structures or equipment frames and the
nearby earth.
