Specifications

12
Schneider Electric
Review
(continued)
Grounding
The interconnection of all metal parts (building structures, pipes, cableways,
equipment, equipment enclosures, etc.) represents a low-impedance path for high
frequencies.
The resulting equipotentiality of this network of conductive parts provides effective
immunity to interference by reducing all the high-frequency voltages that could
otherwise exist between communicating equipment. This is true over a wide
frequency band.
Grounding satisfies the requirements concerning operation.
Metal parts must be interconnected (bonded) by screws, nuts and bolts or short, wide
metal braids (25 mm
2
cross-section and length less than 30 cm) or an equivalent
metal part.
The impedance of PE protective conductors is too high to serve the purpose of
grounding.
The grounding network must nevertheless be earthed.
Common-mode currents
As exposed conductive parts provide a reference potential for electronic equipment
and a return path for common mode currents, any current that penetrates a cable in
a piece of equipment insulated with respect to the exposed conductive parts exits via
the other cables.
When the equipotential bonding network is of poor quality, a cable carrying a
common mode current disturbs all the others. Effective equipotential bonding
reduces this phenomenon.
As opposed to earthing, which is a low-frequency requirement related to carrying
fault currents to earth for the safety of persons, grounding is designed to ensure
equipotentiality of equipment up to very high frequencies, in particular for the proper
operation of digital equipment.
Definition: An exposed conductive part is
an accessible metal part that is isolated
with respect to the live parts of the
equipment but which may accidentally
become live.
Exposed conductive parts ensure proper
operation of systems by providing immunity
to various types of disturbances.
For high-frequencies, the best way to
guarantee trouble-free operation is to
ensure equipotentiality between the
equipment. All equipment and electronic
systems must therefore be bonded together
(or grounded).
Cabling Guidelines