Furse Total Solution to Earthing & Lightning Protection

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Earthing standards
TSC-0912 - 09.10.12
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
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 7430:2011 Code of practice for protective
earthing of electrical installations
BS 7354:1990 Code of practice for design of high
voltage open terminal stations
IEEE Std 80:2000 IEEE Guide for safety in AC
substation grounding
ENA TS 41-24 Guidelines for the design,
installation, testing & 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 7430:2011 - 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, under both normal and
fault conditions.
BS 7430 therefore defines selection parameters for the
earthing 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 to earth, 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 & 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, with additional 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.
Furse power earthing solutions have been specified
for many installations worldwide. For more
information, or to discuss a particular requirement,
please contact us.