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Low Voltage Plant
Protection and
Discrimination (ACB,
MCCB and BS88 Fuses)
The primary function of
the protection device
system is to protect the
Power Distribution
System which needed to
supply electrical power
with the required
voltage level and energy
demand. The power
distribution system and
its components can be
protected by various
types of Fuses, Overload
relays, Low voltage
circuit breakers (ACB,
MCCB, MCB) and trip
units, etc.
q Three basic
considerations during a
protection device system
design are:
q The system has
to be safe for the
personnel
q The system has
to be capable of fully
protecting electrical
power equipment such
power cables,
Transformers, Motors,
etc.
Explanation
q The system has
to be selective
(co-ordinated) short
circuit study should be
performed to provide
short circuit fault
levels on each bus in
the system
q The
interrupting ratings of
all of the breakers and
fuses should be greater
than the calculated
fault levels available
at the point of
application.
Study Objective
q The objective
of coordination study is
to coordinate system so
that the protective
device closest to the
fault on the
power-source side has
the first chance to
operate; but each
preceding protective
device upstream should
be capable, within its
designed settings of
current and time, to
effect the isolation if
the fault persists.
q The settings of
the protection devices
must be so selected to
first, satisfactorily
protect electrical
equipment from overloads
as rapidly as possible.
q If for example,
you have an new MCC
rated at a FLC of say
1250A and as we know the
MCC will only achieve
this when all the Plant
is fully commissioned,
to ensure the maximum
safety the main MCC
Incomer should be set
for a proportional load
as commissioning moves
on.
Data needed to compile a
coordination study, if
requested Electrowave
could generate the main
elements on your behalf;
q System single
line diagram
q Short circuit
fault levels
q The electrical
equipment ratings and
withstand damage
characteristic
protective devices
q Ratings,
settings and
time-current
characteristic.
When we are in
possession of this Data
we will;
q By changing the
ratings and adjusting
the settings of the
protective devices,
greater protection of
the distribution system
and coordination of the
protective devices can
be obtained
q To accomplish
this, the tripping
characteristics of the
over-current devices
should not overlap but
should maintain a
minimum time interval
between them for all
current values
q To assure
complete coordination,
the respective
time-current trip
characteristics should
be simultaneously
displayed on a log-log
curve to determine
adequate time-delay
separation between
operation of the various
devices
Data
q This display of
characteristics can also
include devices at a
different voltage level
(11kV) on the system,
provided proper
consideration of the
relative voltage levels
is made.
q In this manner,
those fuses on the
primary side of
Transformers and high
voltage breakers which
are controlled by relays
can be coordinated with
low-voltage breakers.
q If the ground
fault current protection
is incorporated in the
system, it should be
also included in the
time-current
Co-ordination plots.
q Once, the
Co-ordination of
protective devices is
performed, that is not
guarantee that the
system will remain
Co-ordinated forever
system changes and
additions plus power
source changes
frequently modify the
protection requirements.
q This leaves
some of the protective
devices unable to
properly interrupt the
available fault currents
consequently, periodic
study of
protective-device
settings is as important
in preventing power
outages as is periodic
maintenance of the
distribution system
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