Nace CP 1, Course Manual

Page: 380 / 535

RP0177-2000
2 NACE International
Ground: An electrical connection to earth.
Ground Current: Current flowing to or from earth in a
grounding circuit.
Grounded: Connected to earth or to some extensive
conducting body that serves instead of the earth, whether
the connection is intentional or accidental.
Ground Electrode Resistance: The ohmic resistance
between a grounding electrode and remote earth.
Ground Mat (Gradient Control Mat): A system of bare
conductors on or below the surface of the earth, so
arranged and interconnected as to provide an area of
equal potential within the range of step distances.
(Metallic plates and grating of suitable area are common
forms of ground mats.)
Grounding Grid: A system of grounding electrodes
consisting of interconnected bare conductors buried in the
earth to provide a common electrical ground.
Inductive Coupling: The association of two or more
circuits with one another by means of the mutual
inductance of the circuits.
Lightning: An electric discharge that occurs in the
atmosphere between clouds or between clouds and the
earth.
Load Current: The current in an AC power system under
normal operating conditions.
Magnetic Field: One of the elementary energy fields in
nature. It occurs in the vicinity of a magnetic body or
current-carrying medium.
Polarization Cell: A DC decoupling device consisting of
two or more pairs of inert metallic plates immersed in an
aqueous electrolyte. The electrical characteristics of the
polarization cell are high resistance to DC potentials and
low impedance of AC.
Potential: See Electric Potential.
Potential Gradient: Change in the potential with respect
to distance.
Reclosing Procedure: A procedure which normally
takes place automatically, whereby the circuit breaker
system protecting a transmission line, generator, etc.,
recloses one or more times after it has tripped because of
abnormal conditions such as surges, faults, lightning
strikes, etc.
Reference Electrode: An electrode whose open-circuit
potential is constant under similar conditions of
measurement, which is used for measuring the relative
potentials of other electrodes.
Remote Earth: A location on the earth far enough from
the affected structure that the soil potential gradients
associated with currents entering the earth from the
affected structure are insignificant.
Resistive Coupling: The association of two or more
circuits with one another by means of resistance (metallic
or electrolytic) between the circuits.
Shock Hazard: A condition considered to exist at an
accessible part in a circuit between the part and ground
or other accessible part if the open-circuit AC potential is
more than 15 V (root mean square [rms]) and capable of
delivering 5 mA or more.
Step Potential: The voltage difference between two
points on the earth’s surface separated by a distance of
one pace, which is assumed to be one meter, calculated
in the direction of maximum potential gradient.
Surface Potential Gradient: The slope of a potential
profile, the path of which intersects equipotential lines at
right angles.
Switching Surge: The transient wave of potential and
current in an electric system that results from the sudden
change of current flow caused by a switching operation
such as the opening or closing of a circuit breaker.
Touch Potential: The potential difference between a
metallic structure and a point on the earth’s surface
separated by a distance equal to the normal maximum
horizontal reach of a human (approximately 1.0 m [3.3
ft]).