Specifications

10
Schneider Electric
Review
(continued)
Protective earthing vs. grounding
(equipotential bonding)
Earth
The interconnection of the various exposed conductive parts of equipment by
earthing conductors (green/yellow or PE) represents a low-impedance path at low
frequencies.
The resulting equipotentiality avoids the presence of potentials that can be
dangerous to human beings (greater than 25 V AC or 50 V DC) between two
exposed conductive parts that can be touched simultaneously.
Earthing satisfies the requirements concerning the protection of persons.
This low-impedance path is connected to earth at a single point for each building
(e.g. by an earthing electrode, ring, grid, etc.) through which common-mode currents
flow.
The connection of the various exposed conductive parts by earthing conductors does
not however provide the high-frequency equipotentiality required for effective
immunity against interference because the impedance of the PE conductors is too
high (1 µH/m). High-frequency circuits therefore require special equipotential
bonding, referred to in Europe as grounding.
The various types of power system earthing arrangements (TT, TN, TI, etc.) affect
the security of persons but have little influence on the immunity of equipment to
interference.
Simultaneous access to two non-interconnected earths presents hazard and they
must therefore be interconnected.
Definition: An earthing network has the
following purposes:
b Divert equipment earth-fault and earth-
leakage currents to earth
b Divert common-mode currents of outside
cables (mainly power and
telecommunications cables) to earth
b Divert lightning currents to earth
b Meet requirements concerning the
protection of persons (25 V AC or 50 VDC).
E52276
U
Cabling Guidelines