Question Bank

Module 4

Electrical Installations

1. What are the components of LT Switchgear?

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 equipments.

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.

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.

2. What is contractor?

Electrical contractor is a business- person or firm that performs specialized electrical construction work. This work is related to the design, installation, and maintenance of the electrical system. They are responsible for making sure that those systems work effectively, safely, and most of the time in an environmentally friendly way. 

3. What is wire ? What are the types of wires?

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 :

4. What are the types of cables?

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

5. What is earthing?

Earthing is one of the main factor in electrical systems to protect the humans from getting electric shock. Earthing is used in almost 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 discharge 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.

6. What are the types of earthing?

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.

7. What are the 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

8. What are the important characteristic of battery?

The characteristics of batteries are:

Voltage:

The theoretical standard cell voltage is determined by

Eo (cathodic) – Eo (anodic) = Eo (cell)

This is the standard theoretical voltage. The theoretical cell voltage is modified by the Nernst equation, which takes into account the non-standard state of the reacting component.

The actual voltage produce will always be lower than the theoretical voltage due to polarisation and the resistance losses (IR drop) of the battery and is dependent upon the load current and the internal impedance of the cell.

These factors are dependent upon electrode kinetics and thus vary with temperature, state of charge, and with the age of the cell. The actual voltage appearing at the terminal needs to be sufficient for the intended application.

Discharge Curve:

The discharge curve is a plot of voltage against percentage of capacity discharged. A flat discharge curve is desirable as this means that the voltage remains constant as the battery is used.

Capacity

The capacity of the battery is the quantity of electricity involved in the electro-chemical reaction. It is denoted by  Q given by

Q = x n F

Where x = number of moles of reaction, n = number of electrons transferred per mole of reaction and F = Faraday's constant

The capacity is usually given in terms of mass, not the number of moles:

Q=nFMr

Where Mr = Molecular Mass. This gives the capacity in units of Ampere-hours per gram (Ah/g).

Energy density

The energy density is the energy that can be derived peer unit volume of the weight of the cell.

9. Explain the elementary energy consumption ?

To calculate the consumption of an electrical appliance in kWh, you have to take into account three factors:

1. The capacity of your electrical appliance, expressed in watt.
2. The number of hours that the appliance is in use in one day.
3. 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).

10. Explain power factor improvement and battery backup?

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.

Battery backup:

Uninterruptible power source, Battery backup and Flywheel back up are the other names often used for UPS.

In a UPS, the energy is generally stored in flywheels, batteries , or supercapacitors . When compared to other immediate power supply system, UPS have the advantage of immediate protection against the input power interruptions.

It has very short on-battery run time; however this time is enough to safely shut down the connected apparatus (computers, telecommunication equipment etc) or to switch on a standby power source.

When there is any failure in main power source, the UPS will supply the power for a short time. This is the prime role of UPS. In addition to that, it also is able to correct some general power problems related to utility services in varying degrees.

The problems that can be corrected are voltage spike (sustained over voltage), Noise, Quick reduction in input voltage, Harmonic distortion and the instability of frequency in mains.