Unit 5
Contents
Electrical Installations
Switch Fuse Unit (SFU), MCB, ELCB, MCCB.
The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear. The term ‘switchgear’ is a generic term encompassing a wide range of products like circuit breakers, switches, switch fuse units, off- load isolators, HRC fuses, contactors, earth leakage circuit breakers (ELCBs), etc...
A switchgear essentially consists of switching and protecting devices such as switches, fuses, isolators, circuit breakers, relays, control panels, lightning arrestors, current transformers, potential transformers, and various associated equipment.
SWITCH FUSE UNIT (SFU)
It is Switched Fuse Unit. It has one switch unit and one fuse unit.
When we operate the breaker, the contacts will get close through switch and then the supply will passes through the fuse unit to the output.
Whereas in Fuse Switch Unit there is no separate switch and fuse unit. There is only fuse unit which act itself as a switch.
When we operate the fuse unit will close the input and output of the breaker.
MCB
- A miniature circuit breaker automatically switches off electrical circuit during abnormal condition of the network means in over- load condition as well as faulty condition.
- Nowadays we use an MCB in low voltage electrical network instead of fuse. The fuse may not sense it but the miniature circuit breaker does it in a more reliable way. MCB is much more sensitive to over current than fuse.
- Handling a MCB is electrically safer than a fuse. Quick restoration of supply is possible in case of fuse as because fuses must be re- wireable or replaced for restoring the supply. Restoration is easily possible by just switching it ON. Let’s look at the working of the miniature circuit breaker.
ELCB
- Early earth leakage circuit breakers are voltage detecting devices, which are now switched by current sensing devices (RCD/RCCB).
- Generally, the current sensing devices termed as RCCB and voltage detecting devices named as Earth Leakage Circuit Breaker (ELCB).
- An ECLB is one kind of safety device used for installing an electrical device with high earth impedance to avoid shock. These devices identify small stray voltages of the electrical device on the metal enclosures and intrude the circuit if a dangerous voltage is identified.
- The main purpose of Earth leakage circuit breaker (ECLB) is to stop damage to humans & animals due to electric shock.
- An ELCB is a specific type of latching relay that has a structure’s incoming mains power associated through its switching contacts so that the circuit breaker detaches the power in an unsafe condition.
- The ELCB notices fault currents of human or animal to the earth wire in the connection it guards. If ample voltage seems across the ELCB’s sense coil, it will turn off the power, and remain off until manually rearrange. A voltage sensing ELCB doesn’t detect fault currents from human or animal to the earth.
MCCB
- Molded case circuit breakers are a type of electrical protection device that is commonly used when load currents exceed the capabilities of miniature circuit breakers. They are also used in applications of any current rating that require adjustable trip settings, which are not available in plug-in circuit breakers and MCBs. A molded case circuit breaker, abbreviated MCCB, is a type of electrical protection device that can be used for a wide range of voltages, and frequencies of both 50 Hz and 60 Hz. The main distinctions between molded-case and miniature circuit breaker are that the MCCB can have current ratings of up to 2,500 amperes, and its trip settings are normally adjustable. An additional difference is that MCCBs tend to be much larger than MCBs.
- As with most types of circuit breakers, an MCCB has three main functions:
- Protection against overload – currents above the rated value that last longer than what is normal for the application.
- Protection against electrical faults – During a fault such as a short circuit or line fault, there are extremely high currents that must be interrupted immediately.
- Switching a circuit on and off – This is a less common function of circuit breakers, but they can be used for that purpose if there isn’t an adequate manual switch.
- The wide range of current ratings available from molded-case circuit breakers allows them to be used in a wide variety of applications.
MCCBs are available with current ratings that range from low values such as 15 amperes, to industrial ratings such as 2,500 amperes. This allows them to be used in both low-power and high-power applications.
Wire is a single electrical conductor, whereas a cable is a group of wires swathed in sheathing.
Triplex Wires :
Main Feeder Wires :
Panel Feed Wires :
Non-Metallic Sheathed Wires :
Single Strand Wires :
Non-Metallic Sheathed Cable
Underground Feeder Cable :
Metallic Sheathed Cable :
Multi-Conductor Cable :
Coaxial Cable :
Unshielded Twisted Pair Cable
Ribbon Cable
Direct-Buried Cable
Twin-Lead Cable
Twin axial Cable
Paired Cable
Twisted Pair
Earthing is one of the main factor in electrical systems to protect the humans from geting electric shock. Earthing is used in alomst every equipment.
Earthing is system in which the part of the equipment is connected to the earth with the help of the wires or cables. Earthing is also known as the grounding system.
Earthing is defined as the disharge of electric current in the earth with the help of the wires or cables having low resistance. Mostly Galvanised iron (G.I) strips are used for the earthing. Earthing protects the humans from getting electric shock from the leakage current and when a live wire or cable comes in the contact of the body of the equipment or from the short circuit current.
There are two types of earthing in electrical system:-
Pipe earthing
Plate earthing
Chemical earthing
Pipe earthing
A galvanized steel and a perforated pipe of approved length and diameter is placed vertically in a wet soil in this kind of system of earthing. It is the most common system of earthing.
Plate earthing
In plate earthing system, a plate made up of either copper with dimensions 60cm x 60cm x 3.18mm (i.e. 2ft x 2ft x 1/8 in) or galvanized iron (GI) of dimensions 60cm x 60cm x 6.35 mm (2ft x 2ft x ¼ in) is buried vertical in the earth (earth pit) which should not be less than 3m (10ft) from the ground level.
Chemical earthing
Chemical earthing are recently introduced. In this type of earthing arrangements are same like pipe earthing but the charcoal and salt replaced by the chemical. In this type of earthing there is less chance of loosing resistivity and also demands less maintenance as compared to other type of earthing.
Electrical safety checks and tests
Regular informal 'before use' visual checks and more formal visual inspections improve safety. Remember to include cables and transformers in any checks.
Take into consideration the following about your equipment
- Where it's used
- How often it's used
- The type of equipment
- If it's portable or transportable
- If it's used in a harsh environment.
Preventing electrical risks
To remove some electrical risks by using tools powered by air, hand or hydraulics. However, be aware that these tools could introduce other hazards for the user.
Lower voltages can reduce or remove the risks of shocks and burns. Battery powered tools are safest. Use lower voltage portable tools at 110 volts. Temporary lighting can also run at lower voltages.
You should use a residual current device or lower voltage tools in harsh environments.
Basic safety precautions
There are simple ways to reduce risks.
- Check all equipment is in good working order.
- If you find or suspect a fault, stop using the equipment, disconnect from the electrical supply and label 'do not use'.
You should also
- Avoid overloading sockets by providing enough socket-outlets
- Where possible switch off all appliances at the mains at the end of the working day
- Switch off and unplug equipment before you clean it or make adjustments
- Provide an accessible and clearly identified switch near fixed machinery to cut off power in an emergency.
Types of batteries
Batteries are the most common power source for basic handheld devices to large scale industrial applications. A battery can be defined as; it is a combination of one or more electrochemical cells that are capable of converting stored chemical energy into electrical energy.
There are 2 Types of Batteries
1) Primary Batteries: As the name indicates these batteries are meant for single usage. Once these batteries are used they cannot be recharged as the devices are not easily reversible and active materials may not return to their original forms. Battery manufacturers recommend against recharge of primary cells.
Some of the examples for the disposable batteries are the normal AA, AAA batteries which we use in wall clocks, television remote etc. Other name for these batteries is disposable batteries.
2) Secondary Batteries: Secondary batteries are also called as rechargeable batteries. These batteries can be used and recharges simultaneously.
They are usually assembled with active materials with active in the discharged state. Rechargeable batteries are recharged by applying electric current, which reverses the chemical reactions that occur during discharge. Chargers are devices which supply the required current.
Some examples for these rechargeable batteries are the batteries used in mobile phones, MP3 players etc. Devices such as hearing aids and wristwatches use miniature cells and in places such as telephone exchanges or computer data centre’s, larger batteries are used.
Types of Secondary (rechargeable) Batteries:
Sealed maintenance free (SMF), Lead Acid battery, Lithium battery, Nickel cadmium (Ni-cd) , Lithium ion battery
To calculate the consumption of an electrical appliance in kWh, you have to take into account three factors:
- The capacity of your electrical appliance, expressed in watt.
- The number of hours that the appliance is in use in one day.
- The number of days per year when the appliance is in use.
The calculation is as follows:
[number of hours’ use] x [number of days’ use] x ([capacity of appliance expressed in watt] / 1,000) = number of kWh
The capacity should be divided by 1,000 to convert the number of watts into the number of kilowatts. This finally gives us the number of kWh (kilowatt hours).
The term power factor comes into the picture in AC circuits only. Mathematically it is the cosine of the phase difference between the source voltage and current. It refers to the fraction of total power (apparent power) which is utilized to do the useful work called active power.
Cosɸ = Active power/ Apparent power
- Real power is given by P = VIcosφ. The electrical current is inversely proportional to cosφ for transferring a given amount of power at a certain voltage. Hence higher the pf lower will be the current flowing. A small current flow requires a less cross-sectional area of conductors, and thus it saves conductors and money.
- From the above relation, we see having poor power factor increases the current flowing in a conductor and thus copper loss increases. A large voltage drop occurs in the alternator , electrical transformer and transmission and distribution lines – which gives very poor voltage regulation.
- The KVA rating of machines is also reduced by having higher power factor, as per the formula:
KVA = KW/ cos ɸ
Hence, the size and cost of the machine is also reduced.
References:
Getting Started in Electronics Book by Forrest Mims
Practical Electronics for Inventors, Fourth Edition Book by Paul Scherz and Simon Monk
Electronics for dummies Book by Cathleen Shamieh and Gordon McComb
