Hotel Engineering Notes- Electrcity-2

ELECTRICAL CIRCUITS

An electrical circuit is a complete path through which electricity can flow.                                                                                         

In an electrical circuit, an open circuit is caused intentionally when a user opens a switch or unintentionally when vibration or mechanical damage cuts a wire. In electrical circuits, closing a switch creates a closed loop for the electrons to flow through.

OPEN CIRCUIT: When there is a break in the electrical circuit it is called open circuit.

CLOSE CIRCUIT: When the path of the current flow is complete, it is called close circuit.

SHORT CIRCUIT: When the path of the current flow is completed through not the intended path but through some leakage / damaged path to earth, it is called short circuit.

	Bulb  Supply               Switch 1-f, 230V,AC Open Circuit—when switch is open Closed Circuit – when switch is closed

 

Table of Electrical Symbols

Symbol	Component name	Meaning
	Electrical Wire	Conductor of electrical current
	Connected Wires	Connected crossing
	Not connected Wires	Wires are not connected
	SPST Toggle Switch (Single Pole Single Throw)	Disconnects current when open
	SPDT Toggle Switch (Single Pole Double Throw)	Selects between two connections
	Pushbutton Switch (N.O) (Normally Open)	Momentary switch - normally open
	Pushbutton Switch (N.C) (Normally Closed)	Momentary switch - normally closed
	Earth Ground	Used for zero potential reference and electrical shock protection.
	Chassis Ground	Connected to the chassis of the circuit
	Resistor	Resistor reduces the current flow.
	Variable Resistor / Rheostat	Adjustable resistor - has 2 terminals.
	Capacitor	Capacitor is used to store electric charge. It acts as short circuit with AC and open circuit with DC.
	Inductor	Coil / solenoid that generates magnetic field
	AC Voltage Source	AC voltage source
	Generator	Electrical voltage is generated by mechanical rotation of the generator
	Battery	Generates constant voltage
	Voltmeter	Measures voltage. Has very high resistance. Connected in parallel.
	Ammeter	Measures electric current. Has near zero resistance. Connected serially.
	Ohmmeter	Measures resistance
	Wattmeter	Measures electric power
	Lamp / light bulb	Generates light when current flows through
	Diode	Diode allows current flow in one direction only (left to right).
	Motor	Electric motor
	Transformer	Change AC voltage from high to low or low to high.
	Electric bell	Rings when activated
	Buzzer	Produces buzzing sound
	Fuse	The fuse disconnects when current above threshold. Used to protect circuit from high currents.
	Loudspeaker	Converts electrical signal to sound waves

Series and Parallel connections

		Here, we have three resistors (labeled R1, R2, and R3), connected in a long chain from one terminal of the battery to the other. The defining characteristic of a series circuit is that there is only one path for electrons to flow. In this circuit the electrons flow in a counter-clockwise direction, from point 4 to point 3 to point 2 to point 1 and back around to 4.
	We have three resistors, but this time they form more than one continuous path for electrons to flow. There's one path from 8 to 7 to 2 to 1 and back to 8 again. There's another from 8 to 7 to 6 to 3 to 2 to 1 and back to 8 again. And then there's a third path from 8 to 7 to 6 to 5 to 4 to 3 to 2 to 1 and back to 8 again. Each individual path (through R1, R2, and R3) is called a branch. The defining characteristic of a parallel circuit is that all components are connected between the same set of electrically common points. Looking at the schematic diagram, we see that points 1, 2, 3, and 4 are all electrically common. So are points 8, 7, 6, and 5. Note that all resistors as well as the battery are connected between these two sets of points.

 

SHORT CIRCUIT:

A short circuit is simply a low resistance connection between the two conductors supplying electrical power to any circuit. This results in excessive current flow in the power source through the 'short,' and may even cause the power source to be destroyed. If a fuse is in the supply circuit, it will do its job and blow out, opening the circuit and stopping the current flow

The two basic types of Over-current Protective Devices

(OCPDs):

Kit-Kat fuse or Porcelain re-wireable fuse

Fuse—An over-current protective device with a circuit-opening fusible part that is heated and severed by the passage of over-current through it. A fuse interrupts excessive current (blows) so that further damage by overheating or fire is prevented. Overcurrent protection devices are essential in electrical systems to limit threats to human life and property damage. Fuses are selected to allow passage of normal current and of excessive current only for short periods. A fuse (Kit-Kat fuse)consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-conducting and non-combustible housing. The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the

current flow. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit. Fuses act as a weak link in a circuit. They reliably rupture and isolate the faulty circuit so that equipment and personnel are protected. Following fault clearance they must be manually replaced before that circuit may be put back into operation.

 High Rupturing Capacity (HRC) Fuse

A high rupturing capacity (HRC) fuse is a fuse that has a high breaking capacity (higher kA Rating). The minimum fault value for an HRC fuse is 80kA. A fuse should be selected with a rating just above the normal operating current of the device to be protected. A general approach is that it should operate at 1.2 times the rated current. A typical fuse is made of silver-coated copper strips and granular quartz.

Circuit breaker—A device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined over-current without damage to itself when properly applied within its rating. Miniature circuit breakers (MCBs) or moulded case circuit breakers (MCCBs) are also over-current protection devices often with thermal and magnetic elements for overload and short circuit fault protection. As a switch they allow isolation of the supply from the load. Normally the MCB requires manual resetting after a trip situation but solenoid or motor driven closing is also possible for remote control. The MCB is an automatic, electrically operated switching device that was designed to automatically protect an electric circuit from overload currents and short circuit currents. It is a complicated construction made up of almost 100 individual parts. It has the ability to respond within milliseconds when a fault has been detected. Westinghouse Electric introduced the world’s first MCB and it initially had a porcelain base and cover mounted in a metal housing.

		Fuse Advantages over MCBs The advantages of the fuse can be summarized as follows: # Cheaper when compared to MCBs # It is easy to identify where the fault is due to the open air gap # It can cut-off fault current long before it reaches its first peak # Hence, very little energy “let through” (I2t) # Perfect discrimination easily achievable due to the low cut-off value
	Fuse Disadvantages The disadvantages of the fuse can be summarized as follows: * The abrupt introduction of high resistance in the circuit by a badly designed and assembled fuse can create unwanted effects while clearing the fault * Although this is very rare, fuses are likely to produce high peak voltage which is much higher than the system voltage and can puncture the insulation of the rest of the circuit. * A lot of maintenance and replacement costs. Maintenance in the form of continuously monitoring the state of the fuse; and replacement after each and every fault * The cut-off current increases with the fuse rating * Fuse of incorrect ratings can easily be installed in the fuse holders * In a three phase power circuit, if one fuse blows, all the fuses must be replaced at the same time
	MCB Disadvantages The disadvantages of the MCB can be summarized as follows: * More expensive than the fuse * Difficult to identify where the fault occurred * Fault can be cleared in any time up to 10 cycles of the current waveform * Large amount of energy “let through” (10 times that released by the fuse)

 

MCB Advantages over fuses The advantages of the MCB can be summarized as follows: # Closed overload protection compared to HRC fuses # Stable tripping characteristics # Common tripping of all the phases of a motor # Instant re-closing of the circuit after a fault has been cleared # Safety disconnect features for circuit isolation # Terminal insulation for operator safety # Ampere ratings that can be fixed and modified compared to the possibility of introducing overrated fuses # It is reusable, hence very little maintenance and replacement costs # Lower power losses # Simplicity of mounting and wiring # Lower space requirements # Provision of accessories e.g. auxiliary switch # Stable arc interruption # Discrimination can be achieved either based on current or based on time

 

Earthing: The earthing  is made up of materials that is electrically conductive. A fault current will flow to 'earth' through the live conductor, provided it is earthed .  This is to prevent a potentially live conductor from rising above the safe level .  All exposed metal parts of an electrical installation or electrical appliance must be earthed . The main objectives of the earthing are to :

1) Provide an alternative path for the fault current to flow so that it will not endanger the user

2) Ensure that all exposed conductive parts do not reach a dangerous potential

3) Maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the  appliances or equipment . 

The qualities of a good earthing system are :

1) Must be of low electrical resistance

2) Must be of  good corrosion resistance

3) Must be able to dissipate high fault current repeatedly 

Switch on live wire

A switch or fuse is always placed on the live wire because if you break the neutral with the switch instead of the hot i.e. live, then you can not de-energize the circuit by turning off the switch. And as you can not see those electrons, it will hurt when you try to work on such a system. To eliminate the risk of electrocution (i.e. the flow of electrons from live wire to the earth via a human body) switches and fuses are always be placed in the live wire.

SAFETY

SAFETY TIPS 1. Do not touch an electric switch or appliance when hands are wet. 2. Be alert while replacing fuse/inserting plugs. 3. Do not use copper wire as a substitute for fuse wire. 4. Do not use wires with poor insulation. 5. Do not replace fuse unless cause is detected. 6. Do not hang wet clothes on electrical fittings/conducts. 7. Use 3 pin plugs and ensure that earth connection is proper. 8. Take help of qualified electrician for any alterations / modifications in wiring. 9. Do not replace bulb or any appliance with switch in "ON" condition. 10. Do not shift any appliance with switch in "ON" condition. 11. Check earthing frequently for physical damage, if any. 12. In case minor shock is felt anywhere, do not neglect it - contact licensed electrician. 13. Check the electrical specifications of equipment before it is switched "ON" viz. 110V, 230V, 110V, 440V etc.

Before removing any fuse from a circuit, be sure the switch for the circuit is open or disconnected. When replacing fuses, install the fuse first into the load side of the fuse clip, then into the line side.

Electricity Boards or Supply Companies generally give single phase supply upto 5 KW connected load and 3 phase supply to consumers having load more than 5KW.

1. Always use copper wires/cables of adequate size or one size above the load requirement. Increase in size of conductor reduces resistance thereby heat generation and fire hazard is minimised.

2. Electrical power circuits and communication circuits e.g. telephone should run in separate conduits/casing capings.

3. The wiring for high power equipment viz. air-conditioner, geyser etc. should be run separately with separate neutral brought from supply terminal. This reduces voltage fluctuation in other loads.

4. Normally, in wiring single pole switches are used which ensure that switches are in the "live" or "phase" wire of the circuit appliance. If the switch is connected in "neutral" wire, the equipment/circuit can give shock even though switch is in "off" position.

5. Proper earthing is must in house wiring. Only proper earthing will guarantee safety to you and your family (hence, earthing must be checked periodically).

6. All appliances are provided with 3 pin plugs. Please ensure that earthing wire is connected to all such plugs.

7. Always ensure that connections to sockets are made by 3 pin plugs and not by inserting loose wires or 2 pin plugs.

8. To prevent shocks & protection against fault in appliances, Earth Leakage Circuit Breaker (ELCB) needs to be installed.

9. In case of repeated tripping of ELCB, locate the fault and eliminate the same instead of by-passing ELCB.

10. Before adding heavy power consuming equipment viz. geyser, window AC etc. ensure that wiring is of adequate capacity to take this additional load.

11. Earthing wire should never be used as a return wire of any electrical circuit and it's use should be restricted only for the purpose of equipment body earthing.

12. (a) Standard practice of connecting wires for 3-pin sockets & plug top should be followed.

(b) While connecting 3 core cable to plug top, earthing wire should be little longer than P&N wires so that mechanical forces act equally on all the 3 wires.

Electrical Shock: Here are some electromedical facts that should make you think twice before taking chances.  It's not the voltage but the current that kills. People have been killed by 100 volts AC in the home and with as little as 42 volts DC. The real measure of a shock's intensity is in the amount of current (milliamperes) forced through the body. Any electrical device on a house wiring circuit can, under certain conditions, transmit a fatal amount of current. Currents between 100 and 200 milliamperes (0.1 ampere and 0.2 ampere) are fatal. Anything in the neighborhood of 10 milliamperes (0.01) is capable of producing painful to severe shock.

Clothing and Personal Protective Equipment:        See Figure below

Clothing should fit snugly to avoid danger of becoming entangled in moving machinery or creating a tripping or stumbling hazard. Thick-soled work shoes for protection against sharp objects such as nails. Make sure the soles are oil resistant if the shoes are subject to oils and grease. Wear rubber boots for damp locations Wear an approved safety helmet, if the job requires. Confine long hair or keep hair trimmed and avoid placing the head near to rotating machinery. Do not wear jewelry. Gold and silver are excellent conductors of electricity.

 

FIRE SAFETY:

The chance of fire is greatly decreased by good housekeeping. Keep rags containing oil, gasoline, alcohol, paint, varnish etc. in a covered metal container. Keep debris in a designated area away from the building. Sound an alarm if a fire occurs. Alert all workers on the job and then call the fire department. Make a reasonable effort to contain the fire.
Fire Extinguishers:

Always read instructions before using a fire extinguisher. Always use the correct fire extinguisher for the class of fire. See Figure. Fire extinguishers are normally red. Fire extinguishers may be placed on a red background so they can be easily located. Always use the correct fire extinguisher for the class of fire. Be ready to direct firefighters to the fire Inform them of any special problems or conditions that exist, such as downed electrical wires or leaks in gas lines. Report any accumulations of rubbish or unsafe conditions that could be fire hazards. Also, if a portable tool bin is used on the job, a good practice is to store a C02 extinguisher in it.

Lighting Schemes and Lighting Lamps:      (i) Direct lighting : In this, maximum light is thrown towards the ceiling from where it is diverted to the room through diffused reflection. This is suitable for drawing offices, workshops. (ii) Semi-direct lighting: Here, 50% of the light is sent from the source directly on the reading plane and rest is sent upwards. This provides uniform distributed lighting. (iii) Semi-indirect lighting: Here 40% of the light is sent upwards and 40% is sent directly to the surface.

Lighting Lamps: The six varieties of lamps are incandescent quartz, Fluorescent, Mercury vapour, Metal halide, High pressure sodium and low pressure sodium.

Lamp	Colour	Efficiency	Intensity	Life	Cost
					Initial	Operating
Incansescent Quartz	High	Low	High	Short	Low	High
Fluorescent cool white, warm white	Moderate	High	Moderate	Moderate	Moderate	Low
Mercury vapour	High	Moderate	Moderate	Moderate	High	Moderate
High pressure sodium	Low	High	High	Long	Low	Low