Friday, April 29, 2011

ELECTRICAL FITTINGS

AR-461: BUILDING SCIENCE
By:
RAVINDAR KUMAR
Assistant Professor
Department of Architecture and Planning
NED University of Engineering and Technology
Karachi
LECTURE NO. 22
TOPIC:                                                    ELECTRICAL FITTINGS

INTRODUCTION:
There are various categories of electrical fittings about which an architect must be knowledgeable. In the following these shall be discussed in details.

SWITCH:[1]
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts. Each set of contacts can be in one of two states: either 'closed' meaning the contacts are touching and electricity can flow between them, or 'open', meaning the contacts are separated and non-conducting. A switch may be directly manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a light switch.
Automatically operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another work piece. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is a temperature-operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism. Some switches are used to isolate electric power from a system, providing a visible point of isolation that can be pad-locked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock.

ELECTRICAL SUBSTATION:[2]
A substation is a part of an electrical generation, transmission, and distribution system, where voltage is transformed from high to low, or the reverse, or many other important functions. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps.

A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution. The word substation comes from the days before the distribution system became a grid. The first substations were connected to only one power station where the generators were housed, and were subsidiaries of that power station.
DISTRIBUTION SUBSTATION:
A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a value suitable for local distribution.

The input for a distribution substation is typically at least two transmission or subtransmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.

The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.

Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations are typically the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

ELECTRICAL ROOM:[3]
An electrical room is a room or space in a building dedicated to electrical equipment. The size of the electrical room is usually proportional to the size of the building. In large buildings there may be a main and subsidiary electrical rooms. Electrical equipment may be for power distribution equipment, or for communications equipment. Electrical rooms typically house the following equipment:
  1. Electric switchboards
  2. Distribution boards
  3. Circuit breakers and disconnects
  4. Electricity meter
  5. Transformers
  6. Busbars
  7. Backup batteries in a Battery room
  8. Fire alarm control panels
  9. Distribution frames
In large building complexes, the primary electrical room may even house an indoor electrical substation.

LAYOUT AND CONSTRUCTION OF ELECTRICAL ROOMS:
The details of layout and construction of electrical rooms will be controlled by local building code and electrical code regulations. Requirements for an electrical room relate to fire safety and electrical hazards. An electrical room is usually required to be secured from access by unauthorized persons; these rules are especially strict where equipment within the room has exposed live terminals.
Regulations may require two separate means of exit from a room where the power rating of circuits exceeds some threshold, to allow for quick exit in an emergency. Rooms containing oil-filled equipment may be required to have fire-resistant construction or active fire suppression equipment in the room. Since power distribution often requires large numbers of electrical cables, special measures for fire resistance of cables and cable trays may be also specified by regulations.

In industrial buildings that handle flammable gases or liquids, or combustible powders, special electrical rooms may be prepared that have ventilation and other measures to prevent an explosion hazard that would otherwise exist with electrical equipment in hazardous areas. For large installations, it may be less costly overall to use a special room than to install a large number of devices that are resistant to the hazardous conditions. Similarly, in wet or corrosive environments, electrical equipment may be separated in a room that can be protected from the atmospheric conditions.

Building code and electrical code regulations will dictate minimal working space around equipment to allow for safe access for maintenance. Practical design of an electrical room will consider layout of the initial equipment and allow for additions over the economic life of the facility.

ELECTRICAL EQUIPMENT:[4]
Electrical equipment includes any machine powered by electricity. They usually consist of an enclosure, a variety of electrical components, and often a power switch. Examples of these include:
  1. Major appliance
  2. Microcontroller
  3. Power tool
  4. Small appliances

More specifically, often electrical equipment refers only to components part of the electrical distribution system such as:
  1. Electric switchboards
  2. Distribution boards
  3. Circuit breakers and disconnects
  4. Electricity meter
  5. Transformers

ELECTRICITY METER:[5]
An electricity meter or energy meter is a device that measures the amount of electric energy consumed by a residence, business, or an electrically powered device. Electricity meters are typically calibrated in billing units, the most common one being the kilowatt hour. Periodic readings of electric meters establish billing cycles and energy used during a cycle.

In settings when energy savings during certain periods are desired, meters may measure demand, the maximum use of power in some interval. In some areas, the electric rates are higher during certain times of day, to encourage reduction in use. Also, in some areas meters have relays to turn off nonessential equipment.

UNIT OF MEASUREMENT:
The most common unit of measurement on the electricity meter is the kilowatt hour, which is equal to the amount of energy used by a load of one kilowatt over a period of one hour, or 3,600,000 joules. Some electricity companies use the SI mega joule instead.

Demand is normally measured in watts, but averaged over a period, most often a quarter or half hour. Reactive power is measured in "Volt-amperes reactive", (varh) in kilovar-hours. By convention, a "lagging" or inductive load, such as a motor, will have positive reactive power. A "leading", or capacitive load, will have negative reactive power. Volt-amperes measures all power passed through a distribution network, including reactive and actual. This is equal to the product of root-mean-square volts and amperes.

Distortion of the electric current by loads is measured in several ways. Power factor is the ratio of resistive (or real power) to volt-amperes. A capacitive load has a leading power factor, and an inductive load has a lagging power factor. A purely resistive load (such as a filament lamp, heater or kettle) exhibits a power factor of 1. Current harmonics are a measure of distortion of the wave form. For example, electronic loads such as computer power supplies draw their current at the voltage peak to fill their internal storage elements. This can lead to a significant voltage drop near the supply voltage peak which shows as a flattening of the voltage waveform. This flattening causes odd harmonics which are not permissible if they exceed specific limits, as they are not only wasteful, but may interfere with the operation of other equipment. Harmonic emissions are mandated by law in EU and other countries to fall within specified limits.

OTHER UNITS OF MEASUREMENT:
In addition to metering based on the amount of energy used, other types of metering are available. Meters which measured the amount of charge (coulombs) used, known as ampere-hour meters, were used in the early days of electrification. These were dependent upon the supply voltage remaining constant for accurate measurement of energy usage, which was not a likely circumstance with most supplies. Some meters measured only the length of time for which charge flowed, with no measurement of the magnitude of voltage or current is being made. These were only suited for constant-load applications. Neither type is likely to be used today.

TYPES OF METERS:
Electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) and finding the product of these to give instantaneous electrical power (watts) which is then integrated against time to give energy used (joules, kilowatt-hours etc.). Meters for smaller services (such as small residential customers) can be connected directly in-line between source and customer. For larger loads, more than about 200 ampere of load, current transformers are used, so that the meter can be located other than in line with the service conductors. The meters fall into two basic categories, electromechanical and electronic.
ELECTROMECHANICAL METERS:
The most common type of electricity meter is the electromechanical induction watt-hour meter. The electromechanical induction meter operates by counting the revolutions of an aluminium disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. It consumes a small amount of power, typically around 2 watts. The metallic disc is acted upon by two coils.
One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil. This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power being used. The disc drives a register mechanism which integrates the speed of the disc over time by counting revolutions, much like the odometer in a car, in order to render a measurement of the total energy used over a period of time. The type of meter described above is used on a single-phase AC supply. Different phase configurations use additional voltage and current coils.

ELECTRONIC METERS:
Electronic meters display the energy used on an LCD or LED display, and can also transmit readings to remote places. In addition to measuring energy used, electronic meters can also record other parameters of the load and supply such as maximum demand, power factor and reactive power used etc. They can also support time-of-day billing, for example, recording the amount of energy used during on-peak and off-peak hours.

ELECTRIC SWITCHBOARD:[6]
An electric switchboard is a device that directs electricity from one source to another. It is an assembly of panels, each of which contains switches that allow electricity to be redirected. The U.S. National Electrical Code (NEC) defines a switchboard as a large single panel, frame, or assembly of panels on which are mounted, on the face, back, or both, switches, over current and other protective devices, buses, and usually instruments. The role of a switchboard is to divide the main current provided to the switchboard into smaller currents for further distribution and to provide switching, current protection and metering for these various currents. In general, switchboards safely distribute power to transformers, panel boards, control equipment, and ultimately to system loads.

The operator is protected from electrocution by safety switches and fuses. There can also be controls for the supply of electricity to the switchboard, coming from a generator or bank of electrical generators, especially frequency control of AC power and load sharing controls, plus gauges showing frequency and perhaps a synchroscope. The amount of power going into a switchboard must always equal to the power going out to the loads. Inside the switchboard there is a bank of bus bars - generally wide strips of copper to which the switchgear is connected. These act to allow the flow of large currents through the switchboard, and are generally bare and supported by insulators.

ELECTRICAL CONDUIT:[7]
An electrical conduit is an electrical piping system used for protection and routing of electrical wiring. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Flexible conduit is available for special purposes. Conduit is generally installed by electricians at the site of installation of electrical equipment. Its use, form, and installation details are often specified by wiring regulations, such as the U.S. NEC or other national or local code. The term "conduit" is commonly used by electricians to describe any system that contains electrical conductors, but the term has a more restrictive definition when used in wiring regulations.
Early electric lighting installations made use of existing gas pipe to gas light fixtures (converted to electric lamps). Since this technique provided very good protection for interior wiring, it was extended to all types of interior wiring and by the early 20th century purpose-built couplings and fittings were manufactured for electrical use.

COMPARISON WITH OTHER WIRING METHODS:
Electrical conduit provides very good protection to enclosed conductors from impact, moisture, and chemical vapors. Varying numbers, sizes, and types of conductors can be pulled into a conduit, which simplifies design and construction compared to multiple runs of cables or the expense of customised composite cable. Wiring systems in buildings are subject to frequent alterations. Frequent wiring changes are made simpler and safer through the use of electrical conduit, as existing conductors can be withdrawn and new conductors installed, with little disruption along the path of the conduit. A conduit system can be made waterproof or submersible. Metal conduit can be used to shield sensitive circuits from electromagnetic interference, and also can prevent emission of such interference from enclosed power cables. When installed with proper sealing fittings, a conduit will not permit the flow of flammable gases and vapors, which provides protection from fire and explosion hazard in areas handling volatile substances. Some types of conduit are approved for direct encasement in concrete. This is commonly used in commercial buildings to allow electrical and communication outlets to be installed in the middle of large open areas. For example, retail display cases and open-office areas use floor-mounted conduit boxes to connect power and communications cables. Both metal and plastic conduit can be bent at the job site to allow a neat installation without excessive numbers of manufactured fittings. This is particularly advantageous when following irregular or curved building profiles.

The cost of conduit installation is higher than other wiring methods due to the cost of materials and labor. In applications such as residential construction, the high degree of physical damage protection is not required so the expense of conduit is not warranted. Conductors installed within conduit cannot dissipate heat as readily as those installed in open wiring, so the current capacity of each conductor must be reduced if many are installed in one conduit. It is impractical, and prohibited by wiring regulations, to have more than 360 degrees of total bends in a run of conduit, so special outlet fittings must be provided to allow conductors to be installed without damage in such runs. While metal conduit can be used as a grounding conductor, the circuit length is limited. A long run of conduit as grounding conductor will not allow proper operation of over current devices on a fault.

DISTRIBUTION FRAME:[8]
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made. For example, the Main Distribution Frame (MDF) located at a telephone central office terminates the cables leading to subscribers on the one hand, and cables leading to active equipment (such as DSLAMs and telephone switches) on the other. Service is provided to a subscriber by manually wiring a twisted pair (called a jumper wire) between the telephone line and the relevant DSL or POTS line circuit. In broadcast engineering, a distribution frame is a location within an apparatus room through which all signals (audio, video, or data) pass, with the ability to arbitrarily route and connect sources and destinations between studios and other internal and external points. Connections can either be soldered, or made using terminal blocks. Because the frame may carry live broadcast signals, it may be considered part of the air chain.

FIRE ALARM CONTROL PANEL:[9]
A fire alarm control panel (FACP), or fire alarm control unit (FACU), is an electric panel that is the controlling component of a fire alarm system. The panel receives information from environmental sensors designed to detect changes associated with fire, monitors their operational integrity and provides for automatic control of equipment, and transmission of information necessary to prepare the facility for fire based on a predetermined sequence. The panel may also supply electrical energy to operate any associated sensor, control, transmitter, or relay. There are four basic types of panels: coded panels, conventional panels, addressable panels, and multiplex systems. A fire alarm control panel is required under the building code for a majority of new commercial building construction in most countries.

BUSBAR:[10]
In electrical power distribution, a bus bar is a thick strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus. Bus bars are used to carry very large currents, or to distribute current to multiple devices within switchgear or equipment. For example, a household circuit breaker panel board will have bus bars at the back, arranged for the connection of multiple branch circuit breakers. An aluminum smelter will have very large bus bars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminum from molten salts. The size of the bus bar is important in determining the maximum amount of current that can be safely carried. Bus bars can have a cross-sectional area of as little as 10 mm² but electrical substations may use metal tubes of 50 mm in diameter (1,963 mm²) or more as bus bars.

TRANSFORMER:[11]
A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.

ELECTRICAL FUSE:[12]
In electronics and electrical engineering a fuse is a type of sacrificial over current protection device. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overload or device failure is often the reason for excessive current. A fuse interrupts excessive current (blows) so that further damage by overheating or fire is prevented. Wiring regulations often define a maximum fuse current rating for particular circuits. Over current 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. In 1847, Breguet recommended use of reduced-section conductors to protect telegraph stations from lightning strikes; by melting, the smaller wires would protect apparatus and wiring inside the building. A variety of wire or foil fusible elements were in use to protect telegraph cables and lighting installations as early as 1864. A fuse was patented by Thomas Edison in 1890 as part of his successful electric distribution system.

CONSUMER UNIT:[13]
A consumer unit is a box of fuses or breakers, usually arranged in a single row. This is unlike a distribution board which has multiple rows of fuses or breakers and usually serves two or more locations, which may be split phase, two phase two phases taken from three phase, or three phases. A consumer unit fitted with just fuses is often referred to as a "fuse box".

CIRCUIT BREAKER:[14]
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.
DISTRIBUTION BOARD:[15]
A distribution board (or panel board) is a component of an electricity supply system which divides an electrical power feed into subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit, in a common enclosure. Normally, a main switch, and in recent boards, one or more Residual-current devices (RCD) or Residual Current Breakers with Over current protection (RCBO), will also be incorporated. Distribution boards are also referred to as a:
  1. breaker panel
  2. circuit breaker panel
  3. consumer unit, or CU
  4. electrical panel
  5. fusebox
  6. fuseboard
  7. load centre/center
  8. panelboard
  9. power breaker
  10. service panel
  11. DB board (South Africa)
REFERENCES:


[1] Switch; From: http://en.wikipedia.org/wiki/Switch#Power_switching (Retrieved April 29, 2011)
[2] Electrical Substation; From: http://en.wikipedia.org/wiki/Electrical_substation (Retrieved April 29, 2011)
[3] Electrical Room; From: http://en.wikipedia.org/wiki/Electrical_room (Retrieved April 29, 2011)
[4] Electrical Equipment; From: http://en.wikipedia.org/wiki/Electrical_equipment (Retrieved April 29, 2011)
[5] Electricity Meter; From: http://en.wikipedia.org/wiki/Electricity_meter (Retrieved April 29, 2011)
[6] Electric Switch Board; From: http://en.wikipedia.org/wiki/Electric_switchboard (Retrieved April 29, 2011)
[7] Electrical Conduit; From: http://en.wikipedia.org/wiki/Electrical_conduit#Fittings (Retrieved April 29, 2011)
[8] Distribution Frame; From: http://en.wikipedia.org/wiki/Distribution_frame (Retrieved April 29, 2011)
[9] Fire Alarm Panel; From: http://en.wikipedia.org/wiki/Fire_alarm_control_panel (Retrieved April 29, 2011)
[10] Bus Bar; From: http://en.wikipedia.org/wiki/Busbar (Retrieved April 29, 2011)
[11] Transformer; From: http://en.wikipedia.org/wiki/Transformer (Retrieved April 29, 2011)
[12] Electrical Fuse; From: http://en.wikipedia.org/wiki/Fuse_(electrical) (Retrieved April 29, 2011)
[13] Consumer Unit; From: http://en.wikipedia.org/wiki/Consumer_unit (Retrieved April 29, 2011)
[14]Circuit Breaker; From: http://en.wikipedia.org/wiki/Circuit_breaker (Retrieved April 29, 2011)
[15] Distribution Board; From: http://en.wikipedia.org/wiki/Distribution_board (Retrieved April 29, 2011)

1 comment:

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