Terms that are often used loosely. The distinctions below are the ones that
change what you are required to do.
Earth (ground)
The conductive mass of the earth, whose electric potential at any point is
conventionally taken as zero.
Earth electrode
A conductive part in direct contact with the soil, providing an electrical
connection to it. A rod, a plate, a buried tape, or the reinforcement of a
foundation.
Protective conductor (PE)
A conductor provided for safety, connecting exposed conductive parts to the
main earthing terminal. It carries current only during a fault.
Combined conductor (PEN)
A single conductor performing both the neutral and the protective function.
It carries load current continuously, which is what makes its integrity
critical.
Exposed conductive part
A conductive part of equipment that can be touched and is not normally live,
but may become live under fault conditions — a metal enclosure, for
instance.
Extraneous conductive part
A conductive part not forming part of the electrical installation, but
liable to introduce a potential — typically a metal water or gas pipe, or
structural steel.
Equipotential bonding
Electrical connection maintaining various exposed and extraneous conductive
parts at substantially the same potential. Its purpose is to remove the voltage
difference, not to carry fault current to the source.
Earth fault loop impedance
The impedance of the path taken by fault current from the point of fault
back to the source. It determines the magnitude of the fault current and
therefore whether, and how quickly, a protective device operates.
Residual current device (RCD)
A device that disconnects when the vector sum of the currents in the live
conductors departs from zero by more than a set value, indicating that current
is returning by some other path.
Functional earthing
Earthing for reasons other than safety — reference potential for
signals, screening, or interference control. It does not substitute for
protective earthing, and confusing the two is a recurring source of trouble in
mixed installations.