Earthing, Grounding and Lightening
Protection
Grounding or Earthing and Lightening
Protection are part of basic structural
components of an electrical installation.
An appropriate earthing system provides
stability in normal conditions and supports
activation of protective relaying during
fault conditions. Similarly, lightening
protection shields the installation from
strikes of lightening that involve extremely
high energies and considerable damages.
It is obvious that these, once installed
with adequate design and due care, are
expected to work efficiently for years
and require little attention except for
periodical testing and health check. However,
these are also the ones that are often
" taken for granted " and overlooked
till some serious trouble surfaces demanding
more attention and review.
Grounding or earthing serve
various basic purposes and may be classified
as
Neutral earthing :
Star point of Wye connected transformers
and alternator windings are earthed primarily
to provide voltage stability to system
on account of imbalance in loading resulting
in neutral current flow. Neutral point
allows vector sum of three phase currents
displaced at 120 degrees nullifying it
to zero. Secondary purpose served is detection
of earth faults or earth leakages and
allow the protective gear to operate in
desired time.
In the view of high harmonic contents
found in modern facilities, the zero sequence
or triplen harmonic currents overload
the neutrals if not adequately sized.
This is because these are in phase and
hence do not cancel out at neutral point.
So neutrals are required to be sized equal
to or sometimes double the size of line
conductors to carry "algebraic sum"
of triplen components in the lines.
Neutral earthing for applicable to low
voltage distribution systems upto 1000
V ac comprise of solid earthing with no
intentional resistance connected in the
path.
Neutral earthing for higher voltages
employ resistance or reactance earthing
to limit ground fault currents and allow
operation of protective relaying at appropriate
settings.
Safety earthing : As
the term suggests, purpose of safety earthing
is to minimize hazards arising out of
earth leakages that lead to shock hazard
and electrical fires on account of insulation
failure.
Safety earthing is the most simple and
easily practiced form of earthing and
its absence is realized immediately. A
number of applications that are met through
safety earthing are metallic enclosures
& chasis of switchgear, transformers,
motors, alternators, UPS, etc. Moreover,
secondaries of CTs & PTs are grounded
to eliminate possibilities of over voltages.
Poor grounding of PTs of outdoor switchyards
is a common cause of inadvertent failures,
particularly during rains when lightening
is a possible hazard.
Dedicated earthing :
Also known as isolated earthing is an
extension of Safety earthing. Whereas
Safety earthing utilizes a grid or mesh
of bare flat conductors earthed at a number
of locations, dedicated earthing utilizes
isolated or insulated conductors in parallel
loops connected to a number of flat bars
which in turn are connected to earth stations.
This is to eliminate possibility of noise
and other abnormalities being transferred
to sensitive equipments like servers and
workstations.
Lightening protection :
Lightening protection arrangement follows
the most simple and rugged design which
comprises of a number of earth spikes
situated at strategic locations on the
taller structures of a facility and are
connected solidly to a separate grid of
earth stations dedicated for lightening
protection away from other earth stations
of the facility. The earth spikes offer
reliable protection from lightening by
diversion of lightening stike from lower
structures and discharging to earth.
Lightening Arrestors in sub-stations,
continuous earth conductors on overhead
lines are also part of lightening protection
scheme.
Energy Integra offers complete solutions
that include
