Chapter 1 -Introduction-

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]