CHAPTER
1
INTRODUCTION
Project background
Power
system protection is the most important requirement in the industrial or
domestic electrical to prevent equipment from damage cause by leakage current.
The ELCB is an important equipment to install at each of house, hospital,
factory, or every place that need the power supply. [1]
This
project is to develop an automatic tripping mechanism for the three phase
supply system. The project output resets automatically after a brief
interruption in the event temporary fault while it remains in tripped condition
in case of permanent fault. The electrical substation which supply the power to
the consumers, have failures due to some faults which can be temporary or
permanent. These faults lead to substantial damage to the power system
equipment. [1]
In
Malaysia it is common, The faults might be LG (Line to Ground), LL (Line to
Line), 3L (Three lines) in the supply systems and these faults in three phase
supply system can affect the power system. To overcome this problem a system is
built, which can sense these faults and automatically disconnects the supply to
avoid large scale damage to the control gears in the grid sub-stations. [2]
This
system is built using three single phase transformers which are wired in star
input and star output, and three transformers are connected in delta
connections, having input 220 volt and output at 12 volt. This concept low
voltage testing of fault conditions is followed as it is not advisable to
create on mains line. 555 timers are used for handling short duration and long
duration fault conditions. A set of switches are used to create the Line to Line
(LL), Line to Ground (LG) and Line to Line to line (3L) fault in low voltage
side, for activating the tripping mechanism. Short duration fault returns the
supply to the load immediately called as temporary trip while long duration
shall result in permanent trip. [2]
Earth
Leakage Circuit Breaker (ELCB) operates by measuring the current balance
between two conductors using a differential current transformer. The device
will open its contacts when it detects any difference in current between the
line conductor and the neutral conductor. The supply and return currents must
sum to zero, otherwise there is a leakage of current to somewhere else (to
earth/ground, or to another circuit, etc.). ELCB is designed to prevent
electrocution by detecting the leakage current, which can be far smaller
(typically 5-30 mill amperes) than the currents needed to operate conventional
circuit breakers or fuses (several amperes). RCD (Residential Current Device)
are intended to operate within 25-40 milliseconds, before electric shock can
drive the heart into ventricular fibrillation, the most common cause of death
through electric shock. [2]
In
the United States, the National Electrical Code requires GFCI (Ground Fault
circuit Interrupter) devices intended to protect people to interrupt the
circuit if the leakage current exceeds a range of 4-6 mA of current (the trip
setting is typically 5 mA) within 25 milliseconds. ELCB devices which protect
equipment (not people) are allowed to trip as high as 30 mA of current. In
Europe, the commonly used RCD have trip currents of 10-300 mA. Residual current
detection is complementary to over-current detection. Residual current
detection cannot provide protection for overload or short-circuit currents. [2]
ELCB
with trip currents as high as 500 mA are sometimes deployed in environments
(such as computing centers) where a lower threshold would carry unacceptable
risk of accidental trips. These high-current ELCB serve more as an additional
fire-safety protection than as an effective protection against the risks of
electrical shocks. For many years, the voltage operated ELCB and the
differential current operated ELCB were both referred to as ELCB because it was
a simpler name to remember. However, the use of a common name for two different
devices gave rise to considerable confusion in the electrical industry. If the
wrong type was used on an installation, the level of protection given could be
substantially less than that intended.[2]
Earth Leakage
Circuit Breaker (ELCB)
An Earth Leakage
Circuit Breaker (ELCB) is a device used to directly detect currents leaking to
earth from an installation and cut the power. It was mainly used in TT earthing
systems. In a TT earthing system, the protective earth connection of the
consumer is provided by a local connection to earth, independent of any earth
connection at the generator. The big advantage of the TT earthing system is the
fact that it is clear of high and low frequency noises that come through the
neutral wire from various electrical equipment connected to it. This is why TT
has always been preferable for special applications like telecommunication
sites that benefit from the interference-free earthing. Also, TT does not have
the risk of a broken neutral. In locations where power is distributed overhead
and TT is used, installation earth conductors are not at risk should any
overhead distribution conductor be fractured by, say, a fallen tree or branch.[3]
TT Network
In pre-RCD era, the TT
earthing system was unattractive for general use because of its worse
capability of accepting high currents in case of a live-to-PE short circuit (in
comparison with TN systems). But as residual current devices mitigate this
disadvantage, the TT earthing system becomes attractive for premises where all
AC power circuits are RCD-protected. Nowadays, ELCB have been mostly replaced
by residual-current devices (RCD). The RCD is the current operand ELCB type.
Few years ago, there was voltage operand ELCB, but its utilization was
currently had been abolished because it was less effective. So, the voltage
operand ELCB was replaced with the current operand ELCB. The RCD is an
electrical wiring device that disconnects a circuit whenever it detects that
the electric current is not balanced between the phase conductor and the
neutral conductor. Such an imbalance is sometimes caused by current leakage
through the body of a person who is grounded and accidentally touching the
energized part of the circuit.
ELCB has become one of
the home safety systems in our life today. ELCB has reset button which is to
reclosed circuit breaker when the tripping occur. Today, many of people busy
with work and usually not at home. The problem are during the over current,
short circuit or current leakage at live conductor, it can trip the circuit
breaker “OFF” and cut off the whole house power supply. This situation can make
certain important component or equipment cannot be operated. Most household
ELCB need to be reclosed manually during tripping, hence is a troublesome thing
for user who is not at home and may be would take long time to reset on back
the button at circuit breaker. The main mechanism in operation is tripping coil
which is it can operation either in live or off condition. This ELCB will
operate when current is exceeding the rating of the current ELCB. This high
current not flows into equipment after ELCB tripped. It will flow directly into
ground by using ground rod. This ground rod must has the lower resistance it
because easy to flow high current. There are two types of ELCB:-
i. Voltage Earth Leakage Circuit Breaker (vELCB)
ii. Current Earth Leakage Circuit Breaker (iELCB)
Voltage Earth Leakage Circuit
Breaker (vELCB)
vELCB is a voltage
operated circuit breaker, the device will function when the current passes
through the ELCB. vELCB contains relay loop which it being connected to the
metallic load body at one end and it is connected to ground wire at the other
end. If the voltage of the load body is rise which could cause the difference
between earth and load body voltage, the danger of electric shock will occur.
This voltage difference will produce an electric current from the load metallic
body passes the relay loop and to earth. When voltage on the load metallic body
raised to the danger level which exceed to 50Volt, the flowing current through
relay loop could move the relay contact by disconnecting the supply current to
avoid from any danger electric shock.
Current Earth Leakage Circuit
Breaker (iELCB)
iELCB is current
operated circuit breaker. Current-operated ELCBs are generally known today as
RCD (residual current device). These also protect against earth leakage,
through the details and method of operation are different. The device will
function with when the Current passes through ELCB. This current admitted to
current transform device and on the load. Current from the load also admitted
again to transform device. In normal state, total current applied to load is
equal with total current out of the load. Because of the balance of in and out
of current, it does not affect the current transform device. If there is any
earth current leakage caused by earth damage, then the in and out current will
no longer in balance. This unbalance current phenomenon will generate the
current and if the current exceeded the prescribed rate, the ELCB will jerked
and cut off the supply. The device also being called RCD, Residual Current
Device in IEC or RCCB, Residual Current Circuit Breaker.
Operation of ELCB Trip Situation
There are two types of
fault normally detected by ELCB, which are permanent fault and temporary fault:
Permanent Failure or Permanent
Damage
It usually trip when
have any leakage current in circuit to earth or ground. For permanent failure,
the damaged must to repair first or remove the damage from current before
automatically trigger back ELCB. If the damage not to repair or remove the
damage from circuit, it will trip again when the ELCB become automatically
trigger. If this happen many times, it will damage the ELCB. For example is
electrical, electronic device or short circuit.
Temporary Failure or Temporary
Damage
It can automatically
trigger ELCB without to repairs first or remove the damage from supply circuit.
If usually lightning and over loading occurs in resident or industrial, it can
give more problems to user to automatically trigger by itself. For example is
lightning.
Electrical Faults
A fault is any abnormal
situation in an electrical system in which the electrical current may or may
not flow through the intended parts. Equipment failure also attributable to
some defect in the circuit, example is loose connection, insulation failure or
short circuit etc. The type of faults in a distribution network that is
detected by an ELCB is:
i. Over-Current Fault
ii. Short-Circuit Fault
iii. Lightning Fault
Over-current Fault
The National Electrical
Code defines over current as any current in excess of the rated current of
equipment or the amp city of a conductor. It may result from overload, short
circuit, or ground fault. Current flow in a conductor always generates heat.
The greater the current flow, the hotter the conductor. Excess heat is damaging
to electrical components. For that reason, conductors have a rated continuous
current carrying capacity or amp city. Over current protection devices are used
to protect conductors from excessive current flow. These protective devices are
designed to keep the flow of current in a circuit at a safe level to prevent
the circuit conductors from overheating. In term of over-current fault when a
current greater than that which a circuit or a fuse is designed to carry, the
fuse or wire may melt or damage the other elements of the circuit
.
Short-Circuit Fault
A short circuit in an
electrical circuit is one that allows a current to travel a long a different
path from the one originally intended. The electrical opposite of a short
circuit is an “open circuit”, which is an infinite resistance between two
nodes. It is an abnormal low-resistance connection between two nodes of an
electrical circuit that are meant to be at different voltages. This result in
an excessive electric current (over-current) and potentially causes circuit
damage, overheating, fire or explosion. Although usually the result of a fault,
there are cases where short circuits are caused intentionally, for example, for
the purpose of voltage-sensing crowbar circuit protectors. In circuit analysis,
the term short circuit is used by analogy to design at a zero-impedance
connection between two nodes. This forces the two nodes to be at the same
voltage. In an ideal short circuit, this means there is no resistance and no
voltage drop a cross the short, in simple circuit analysis, wires are
considered to be shorts. In real circuits, the result is a connection of nearly
zero impedance, and almost no resistance
.
Lightning Fault
Lightning is the
visible discharge of static electricity within a cloud, between clouds, or
between the earth and a cloud. Scientists still do not fully understand what
causes lightning, but most experts believe that different kinds of ice interact
in a cloud. Updraft in the clouds separate charges, so that positive charges
flow towards the top of the cloud and the negative charges flow to the bottom
of the cloud. When the negative charges moves downwards, a “stepped leader” is
created. The leader rushes toward the earth in 150-foot discrete steps,
producing an ionized path in air. The major part of the lightning discharges current
is carried in the return stroke, which flows along the ionized path. One of the
temporary faults is cause by direst lightning phenomena. Where example of
permanent fault is fault on electrical equipment.
Scope of Project
An Earth Leakage Circuit
Breaker (ELCB) is a safety device used in electrical installations with
high earth impedance to prevent shock. It detects small stray voltages
on the metal enclosures of electrical equipment, and interrupts the circuit if
a dangerous voltage is detected. Once widely used, more recent installations
instead use residual current circuit breakers which instead detect
leakage current directly.
An ELCB is a specialized type of latching
relay that has a building's incoming mains
power connected through its switching contacts so that the ELCB disconnects the power in an
earth leakage (unsafe) condition.
The ELCB detects fault currents from live to the earth (ground) wire within the
installation it protects. If sufficient voltage appears across the ELCB's sense coil, it will
switch off the power, and remain off until manually reset. A
voltage-sensing ELCB does not sense fault currents from live to any other
earthed body.
ELCBs have one
advantage, they are less sensitive to fault conditions, and therefore have
fewer nuisance trips. (This does not mean they always do, as practical
performance depends on installation details and the discrimination enhancing
filtering in the ELCB.) Therefore by electrically separating cable armor from
the cable circuit protective conductor, an ELCB can be arranged to protect
against cable damage only, and not trip on faults in down line installations.
ELCBs have some limitations such as they do not detect
faults that don't pass current through
the CPC to the earth rod. They do
not allow a single building system
to be easily split into multiple sections with independent fault protection,
because earthing systems are
usually bonded to pipework. They may
be tripped by external voltages from something connected to the earthing system
such as metal pipes, a TN-S earth
or a TN-C-S combined neutral and earth. As with RCDs, electrically leaky
appliances such as some water heaters, washing
machines and cookers may cause the ELCB to trip. ELCBs introduce
additional resistance and an
additional point of failure into
the earthing system.[1]
