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Total Solution to Earthing & Lightning Protection |
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Technical reference
Earthing standards
Installation of a well designed earthing system is a
fundamental requirement for all structures and electrical
systems (at all voltages).
Effective earthing safeguards people from risk of electric
shock, in that
‘hazardous-live-parts shall not be accessible
and accessible conductive parts shall not be hazardous live’
,
and ensures a low impedance route to the general mass of
earth for currents in the electrical system, under both normal
and fault conditions.
A number of national and international standards have been
published which define earthing system design parameters for
structures, electrical equipment and systems, including:
–
–
BS EN 50522:
Earthing of power installations
exceeding 1kVac
–
–
BS 7430:
Code of practice for protective earthing of
electrical installations
–
–
BS 7354:
Code of practice for design of high voltage open
terminal stations
–
–
IEEE Std 80:
IEEE Guide for safety in AC
substation grounding
–
–
ENA TS 41-24
Guidelines for the design, installation, testing
and maintenance of main earthing systems in substations
The design, specification, inspection and periodic testing
of earthing systems should follow the guidance and
recommendations provided by these standards.
BS 7430: Protective earthing of electrical installations
British Standard BS 7430 provides guidance on earthing of
general land-based electrical installations in and around
buildings in the UK, including:
–– Low voltage installation earthing and equipotential bonding
for general, industrial and commercial buildings, locations
with increased risk, rail systems etc
–– The interface between low voltage and high voltage substations
–– Earthing of generators and Uninterruptible Power Supplies
(UPSs) supplying low voltage installations
BS 7430 defines the elements for creating an appropriate
earthing arrangement for a low voltage installation, including a
main earthing terminal, protective conductors, earthing
conductors and circuit protective conductors, and the use of
earth electrodes to dissipate currents to the general mass
of earth. Extending the earthing arrangement through the use
of equipotential bonding measures to cover exposed and
conductive metal parts is further recommended to protect
against step and touch voltages, and to remove risk of
dangerous sparking. Five classes of low voltage electrical
installation are defined within the standard - TN-S, TN-C,
TN-C-S, TT and IT.
Performance requirements for earthing these low voltage
installations are defined in the IET Wiring Regulations,
BS 7671:2008(+A1:2011).
The earthing arrangement should be sufficiently robust to
ensure it lasts the lifetime of the installation, and be protected
from mechanical damage and corrosion so that it remains
capable of carrying the maximum expected current, it is
specified for under both normal and fault conditions.
BS 7430 therefore defines selection parameters for the earth-
ing arrangement, e.g. the size and material for conductors,
earth electrodes etc, and makes clear the need for careful
consideration of site conditions (soil composition and resistivity).
Taking actual measurements at the site is important to gauge
the expected effectiveness of the earthing arrangement,
and guidance is provided for measuring resistance
calculations for earth plates, earth rods, ring conductor and
foundation earth electrodes.
Where necessary in high resistivity areas or on rocky ground,
treatment of the soil through use of an earth electrode
backfill is recommended to improve earth contact resistance.
Substation earthing
BS 7354, IEEE std. 80 and ENA TS 41-24 reference the
requirements for earthing of substations.
The design and specification of an appropriate earthing
arrangement for substations is essential to provide a low
impedance path for earth fault, and lightning currents, and to
protect personnel on site from potentially fatal step and
touch voltages. These standards provide guidance on (but
not limited to):
–– Maximum permitted step and touch voltages
–– Methods for calculating earthing system design
–– High voltage earth electrode selection, including type,
material and size
–– Switching and busbar arrangement
–– Equipotential bonding
–– Insulation co-ordination
Primary to these standards is limiting earth potential rise (EPR)
under earth fault conditions so that step and touch potential
limits are not exceeded, and earth resistance remains as
low as possible. Essentially, use of an earthing grid consisting
of horizontal cross-bonded earthing conductors is
recommended, augmented by earth rods where the site
includes low resistivity layers beneath the surface. These earth
rods mitigate seasonal variations in earth grid resistance at
the grid’s burial depth.