Electrical Systems PDF

# MECHANICAL AND ELECTRICAL SYSTEMS ## Electrical Systems ### 1. GENERAL #### 1.1 DEFINITION OF ELECTRICITY - Electricity is a form of energy generated by friction, induction or chemical change, having magnetic, chemical and radiant effect. - It is the motion of free electrons through a solid c...

# MECHANICAL AND ELECTRICAL SYSTEMS ## Electrical Systems ### 1. GENERAL #### 1.1 DEFINITION OF ELECTRICITY - Electricity is a form of energy generated by friction, induction or chemical change, having magnetic, chemical and radiant effect. - It is the motion of free electrons through a solid conductor. - An electric current travels at the speed of light and therefore the rate of conduction of an electric current is considered as instantaneous. **Notes:** - William Gilbert, an English Physicist is considered as the "Father of Electricity" for his studies on "Electric Attraction" and "The Electric Force". - Electricity is a property of the basic particles of matter, which like an atom, consists of the negative charge, electron; the positive charge, proton; and the neutron, which is not electrically charged. - There are two (2) general classes of electricity: dynamic electricity (electro dynamic) which flows through a substance in the form of an electric current, and static electricity (electrostatic). #### 1.2 SOURCES OF ELECTRICITY ##### 1.2.1 BATTERY - a combination of two or more electric cells capable of storing and supplying direct current by electro-chemical means. 1. **Primary Batteries** - delivers electricity as soon as its parts are assembled, or put together, provided that it is connected to a circuit. 2. **Secondary/Storage Batteries** - electricity from some external source must be passed through it before it can deliver an electric current (charging the battery). Main uses are to supply emergency lighting circuits for hallways, stairways, exits and to energize police and fire alarm systems and certain types of signal systems. ##### 1.2.2 GENERATOR - a machine that converts mechanical energy into electrical energy. 1. **Alternating Current Generators or Alternators** - The bulk of electrical energy utilized today is in the form of alternating current, including energy for power and lighting. 2. **Direct Current Generators** - These furnish electrical energy for elevators, escalators, intercommunicating telephone systems, control of signal systems, and clock systems. ### 1.3 OTHER DYΝΑΜΟ ELECTRIC MACHINES #### 1.3.1 MOTORS - a machine that converts electrical energy to mechanical energy. #### 1.3.2 TRANSFORMERS - for converting one voltage to another, from lower to higher (step-up transformer) from higher to lower (step-down transformer). - The two major types of transformers are the a) Oil-insulated Transformers and the b) Dry-Type Transformers. 1. **Outdoor Transformers:** - H-frame transformer bank - capacity as high as 1,000 KVa - Cluster mounted on a single pole - 225 Kva capacity - Pad mounted 2. **Indoor Transformers:** - Dry type - Askarel Insulated ("Askarel" - synthetic non-flammable liquid) - Oil Insulated - installed in a fire rated transformer vault except when capacity not exceeding 112.5 Kva. **TRANSFORMER RATING** 1. Step-up transformer 2. Step down transformer #### 1.3.3 ROTARY CONVERTERS - for changing alternating current to direct current and vice versa. ### 1.4 TYPES OF CURRENT #### 1.4.1 DIRECT CURRENT - a current which flows at a constant time rate and in the same direction. #### 1.4.2 ALTERNATING CURRENT - a current which is periodically varying in time rate and in direction. It rises from zero to maximum, falls to aero, reverses its direction and again returns to zero. - A complete set of these changes is called as cycle. - The number of times the current goes through the above cycles per second is called the frequency of the alternating current and is referred to as the "hertz". - The frequency commonly used for lighting is 60 cycles per second and for motors 25 cycles per second. **Note:** - The hertz is named after H.R. Hertz. ### 1.5 UNITS OF ELECTRICITY #### 1.5.1 UNIT OF QUANTITY - **COULOMB** - a coulomb of electricity comprises approximately 6.25 x 10^18 electrons. - **AMPERE** - An ampere of current represents a rate of flow of one coulomb'or 6.25 x 10^18 electrons/second through a given cross section. **Note:** - AMPACITY is the current carrying capacity of a wire or cable expressed in Amperes, without undue heating. - The ampere is named after Andre M. Ampere. #### 1.5.2 UNIT OF ELECTRIC POTENTIAL - **VOLT** - is the electromotive force or potential difference between two points in an electric field which will move a charge of one coulomb between these points. **Note:** - The volt is named after Alessandro Volta, an Italian scientist who discovered that electrons flow when two different metals are connected by a wire and then dipped into a liquid that conduct of carry electrons. #### 1.5.3 UNIT OF RESISTANCE - **OHM** - The resistance which will allow one ampere of current to flow when one volt is impressed upon it. - In direct current circuit (d-c) this unit is called resistance and is abbreviated R; in an alternating-current (a-c) circuit it is called impedance and is abbreviated Z. #### 1.5.4 UNIT OF ELECTRIC POWER - Electric power is the rate of doing electrical work and the unit is WATT or the KILOWATT. - One kilowatt is equal to 1,000 watts. **Note:** - The watt is named after James Watt, a Scottish inventor. #### 1.5.5 UNIT OF ENERGY - Energy is the capacity for doing work. In electrical units, energy is expressed in WATT-HOURS or KILOWATT-HOURS. ### 1.6 OHM'S LAW - The current, I, that will flow in a d-c circuit is directly proportional to the voltage, V, and inversely proportional to the resistance, R, of the circuit. **I (amp) = V (Volts) / R (Ohms)** **Notes:** - The higher the voltage, the larger the current. - The higher the resistance, the lower the current. - Other equations associated with the Ohm's Law are: - W (Watts) = I^2R or VI - Whr = I^2R t - KWhr = I^2Rt/1000 ### 1.7 ELECTRIC LOAD CONTROL - is the effective utilization of available energy by reducing peak loads and lowering demand charge. - The control devices and systems are referred to as load shedding control, peak demand control, peak load regulation, and power use control. #### 1.7.1 LOAD SCHEDULING AND DUTY-CYCLE CONTROL - the installation's electric loads are analyzed and scheduled to restrict demand by shifting large loads to off-peak hours and controlled to avoid coincident operation. #### 1.7.2 DEMAND METERING ALARM - in conjunction with a duty cycle controller, demand is continuously metered and an alarm is set on when a predetermined demand level is exceeded. #### 1.7.3 AUTOMATIC INSTANTANEOUS DEMAND CONTROL - also called "rate control", it is an automated version of the demand metering alarm system, where it automatically disconnects or reconnects loads as required. - In setting up the system, the controllable or "sheddable" loads are separated from those which must remain uninterrupted or the "unsheddable" loads. - The sheddable loads are: - Non-essential lighting - Ventilation fans - Space heating - Comfort cooling - Non-critical batch process equipment - Electric boilers - Sewage ejectors with appropriate level controls - Transfer pumps - The unsheddable ones are: - Essential lighting - Elevators - Refrigeration - Compressors - Process equipment - Office machinery #### 1.7.4 IDEALCURVE CONTROL - This controller operates by comparing the actual rate of energy usage to the ideal rate, and controls KW demand by controlling the total energy used within a metering interval. #### 1.7.5 FORECASTING SYSTEMS - are computerized systems which continuously forecast the amount of energy remaining in the demand interval, then examine the status and priority of each of the connected loads and decide on the proper course of action. **Demand Charge** - the levying of a charge for power (kw) in addition to the normal energy (kwh) charge. ### 1.8 MEASURING ELECTRIC CONSUMPTION #### 1.8.1 KWH METERS - To measure energy, the factor of time is introduced, such that: - energy = power x time. - A-C electric meters are basically small motors, whose speed is proportional to the power being used. - The number of rotations is counted on the dials which are calibrated directly in kilowatt-hours. ## 2. BUILDING ELECTRICAL SYSTEMS ### 2.1 BRANCH CIRCUITS #### 2.1.1 CIRCUIT AND CIRCUIT ARRANGEMENTS - An electrical circuit may be defined as a complete conducting path carrying current from a source of electricity to and through some electrical device or load and back to the source. - The two wire circuit, which is the most elementary of all wiring systems, consists of a live wire carrying the current to the various power consuming devices in the circuit and a neutral or grounded wire which is the return wire carrying the circuit back to the source of supply. - **OPEN CIRCUIT (Broken or Incomplete)**- conducting elements are disconnected as to prevent the flow of electricity. - **SHORT CIRCUIT** - a condition resulting from bridging any part of a circuit with a conductor of a very low resistance. **Branch Circuit** - the circuit conductors between the final over current protective device and the outlets. (NEC) #### 2.1.2 CLASSIFICATIONS OF BRANCH CIRCUITS 1. **General Purpose Branch Circuit** - supplies outlets for lighting and appliances, including convenience receptacles. 2. **Appliance Branch Circuit** - supplies outlets intended for feeding appliances. Fixed lighting is not supplied. 3. **Individual Branch Circuit** - is designed to supply a single specific item, such as a motor load or a unit air-conditioner. #### 2.1.3 GENERAL CIRCUITING GUIDELINES - The National Electrical Code requires the following rules and guidelines for the design of branch circuits for residences: - **General:** Branch circuits shall be sufficient to supply a load of 30 watts per square meter in buildings excluding porches, garages and basements. - The requirement of 30 watts per square meter is up to 80 sqm for a 20 amperes circuit (2,400 watts) or 60 sqm for a 15 amperes circuit (1,800 watts). - However, good practice suggests that the load should not exceed 1,600 watts for a 20 amperes circuit and 1,200 watts for a 15 amperes circuit. - **In all but the smallest installations, connect lighting, convenience receptacles, and appliances in separate circuits. The Code requires a minimum of 2 - 20 amperes appliance branch circuit to feed all small appliance outlets in the kitchen, panty, dining and family room.** - **Convenience receptacles in an area shall be wired to at least two different circuits so that in case of failure in any one of the circuits, the entire area will not be deprived of power.**