Monday, 21 October 2013
00:09
No comments
Nonlinear Load
This type of load is very much in demand all over the
world, regardless whether they
are industrial, commercial, or domestic users.
Computers, air conditioners, and televisions are examples of this type of load
and it is understandably impossible to eliminate them away, even their impact
to power system is already well documented. Nonlinear load, is define as,
“ A class of load that, when subjected to sinusoidal
voltage source, would draw
non-sinusoidal current
into the system.”
TYPE OF USER
|
TYPE OF LOADS
|
ELECTRICAL
CHARACTERISTICS
|
|
DOMESTIC
COMMERCIAL
INDUSTRIAL
|
Incandecent Lamps,
Water heaters
Computers,
Flourocent Lamps,
Water heaters.
Motors, ASDs Furnances, Pumps
|
Resistive
Resistive
Resistive
Resistive
Nonlinear
Inductive
|
Sunday, 20 October 2013
23:20
No comments
TN system neutral-connected
exposed conductive part
- Letter T depicts that
The neutral is
directly earthed.
- Letter N depicts that
The exposed
conductive parts of the loads are connected to the neutral conductor.
There are two types of systems, possibly depending on whether
the neutral
conductor and protective conductor (PE) are combined or not:
(Reference: IEC 60364-5, section 546-2)
Case 2: In this
case the neutral conductor and protective conductor are different. These type
of systems are recognized by a third letter S and is said as TNS.
Characteristics of different
earthing systems:
IT system:
Specific
characteristics:
- Switching upon occurrence of a double fault is usually generated by phase-to phase fault protective devices (circuit-breakers, fuses, etc.).
- If the short-circuit current is not sufficient to activate protection against phase-2-phase faults, In case if the loads are far away, protection should be done by residual current devices (RCD).
TT system:
Specific characteristics:
- The installation of RCDs is compulsory.
- All exposed conductive parts protected by the same protective device should be connected to the same earth.
- The neutral earth and the exposed conductive part earth may or may not be Interconnected or combined.
- The neutral may or may not be distributed.
Specific characteristics of TNS and TNC systems:
- Fault switching is obtained by devices providing protection against phase2 phase faults (circuit-breakers, fuses, etc.).
- both TNS and TNC systems can be used in the same installation. The TNC system (4 wires), however, must never be downstream of the TNS system (5wires).
Saturday, 19 October 2013
01:10
No comments
Because of fault there is a unbalance condition in the three phase stator currents and because of these unbalance currents dual frequency currents starts flowing in rotor core. these dual frequency currents heats up rotor and harms rotor. due to same unbalance currents damage of stator also takes place.
Negative arrangement channel furnished with the over current relay is utilized for the insurance against unbalance loading. From the hypothesis of the symmetrical components, we realize that unbalanced three phase currents contains the negative sequence part. This negative sequence current reasons warming of the stator.The negative warming accompanies the resistance law so it is corresponding to the square of the current. The warming time consistent normally rely on the cooling framework utilized and is equivalent to I²t=k where I is the negative succession current and t is the time of flow in seconds and k is the constant generally lies between 3 and 20. Its general practice to utilize negative current transfers which matches with the above warming aspects of the generator. In this sort of assurance three Cts are joined with three stages and the yield from the secondaries of the Cts is sustained to the curl of over current transfer through negative succession channel. Negative succession circuit comprises of the resistors and capacitors and these are joined in such way that negative succession currents moves through the transfer loop. The hand-off might be set to work at any specific quality of the unbalance currents or the negative succession part current.
00:29
No comments
Earthing systems are governed by standard IEC 60364-3. There are
three types of systems: IT, TT and TN.
The first letter defines the neutral point in relation to
earth:
- T stands for directly earthed neutral.
- I stand for unearthed or high impedance-earthed neutral (e.g. 2200 Ω).
The second letter defines the exposed conductive parts of
the electrical Installation in relation
to earth:
- T stands for directly earthed exposed conductive parts.
- N stands for exposed conductive parts directly connected to the neutral conductor.
IT system unearthed or
impedance-earthed neutral (see Figure-1)
Letter I depicts that
The neutral is unearthed or connected to earth by a high
impedance (an Impedance of 1600 Ω is often
used).
Letter T depicts that
The exposed conductive parts of the loads are interconnected and
earthed. A group of loads can be individually earthed if it is situated
far away from the Other loads.
TT
system directly earthed neutral
First letter T depicts that
The neutral is directly earthed.
Second letter T depicts that
The exposed conductive parts of the loads are interconnected
either altogether or by a group of loads. Each interconnected group is earthed. One
exposed conductive part can be individually earthed if it is far away from the
others.
Figure 2-3: directly
earthed neutral (TT system) in low voltage
Friday, 18 October 2013
23:48
No comments
In low voltage three-phase
system there are three single Phases measured between each
phase and a common
point called the “Neutral point”. In actual
fact, the neutral is the common point of three Star-connected windings
where
all three phases meet.
Meaning of Unearthed and Earthed System:
When there is an open circuit between the neutral point and
earth, we say that theNeutral is Unearthed.
On the other hand if there is a connection between
earth And neutral in this case system is called
Earthed.
A directly earthed neutral strongly limits over voltages but it
causes very high fault currents, whereas an
unearthed neutral limits fault
currents to very low values But encourages the occurrence of high over
voltages.
In any installation, service continuity in the event of an
insulation fault is also Directly related to the earthing system. An unearthed neutral
permits service Continuity during an insulation fault. Contrary to this, a
directly earthed neutral, or Low impedance-earthed neutral, causes tripping as soon as
the first insulation fault
Occurs.
However, in an unearthed or high impedance-earthed network, the
damage is Reduced, but the equipment
must have an insulation level
compatible with the level of over voltages able to cope in this type of network. The
choice of earthing system in both low voltage and medium voltage networks depends on the
type of
installation as well as the type of network. It is also influenced
by the type of loads and service continuity
required.
From above discussion it is clear that we have to choose a
compromise between The two, supply continuity and insulation protection.
Tuesday, 8 October 2013
08:02
No comments
There are some sentences which are very common in use with common mistakes.
RIGHT WRONG
1.) READ OUT READ
2.) SPEAK OUT SPEAK
3.) SHUT THE DOOR CLOSE DOOR
4.) BOLT THE DOOR LOCK THE DOOR
5.) MY NAME IS MIKE MYSELF MIKE
6.) PLEASE SHIFT A BIT PLEASE LET ME SIT
7.) SORRY WHAT
8.) I AM PURSUING MBA I AM DOING MBA
Thursday, 3 October 2013
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