group 3 electrical system and material

7/28/2019 Group 3 Electrical System and Material

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ELECTRICAL SYSTEM AND MATERIAL: SERVICE & UTILIZATION

ELECTRIC SERVICE:

Service is tapped onto the utility lines at a mutually agreeable point at or beyond theproperty line.

Overhead Service:

- low cost, easily maintained and repaired, and faults easily located

Types of overhead service:a. bare copper cable supported on porcelain or glass insulators on crossarmsnormally used for high voltage (2.4 kV and higher) lines.

b. Weatherproof secondary circuits at 600V and below run on porcelain spoolsecondary racks with 1/c weatherproof cable as the conductor.

c. preassembled aerial cable consists of three or four insulated cables wrappedtogether with a metallic tape and suspended by hooks from the poles.

Underground Service:

- Preferred in areas where there are extreme weather conditions, where combination of snow, wind and ice increase the possibility of outages.

- Attractiveness (lack of overhead visual clutter); service reliability, and long life.

- Disadvantage is high cost.

Types of underground wiring:

a. Direct burial low cost and ease of installation

b. Direct burial duct medium but little strength

c. Concrete encased duct offers highest strength but expensive


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- Cable used is the basic service entrance cable or type SE.

- When provided with moisture proofing, designation is SE type U or USE.

- Underground cable other than service runs is classified type UF (underground feeder).

SERVICE EQUIPMENT:

TRANSFORMERS

- A device that changes or transforms alternating current of one voltage to alternating

current of another voltage.- Transformer is used to step down an incoming 4160V service to 480V for distributionwithin a building. (from primary voltages-2400V and up, to secondary voltages-480Vand below)

- Another transformer would be used in a local electric closet to step down the 480V to240V or 120V.

- Specified by type, phase, kVA rating, sound level and insulation class.

- Transformers are available in single phase or three-phase construction and rated inkVA or kilovolt-amperes.

Types of transformers:

a) Dry (air-cooled) units in the 600V class usually installed indoors intended for general puspose light and power circuits.

b) Liquid filled units rated above 5000V installed in substations mounted on concretepad.

Cheapest cooling medium used is mineral oil but application is limited because of itsflammability. New coolants are being developed but more expensive.

- Insulation used is either organic, inorganic, asbestos or silicone.


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Transformer Outdoors:

- Service transformer bank is necessary when the facility utilization voltage is different

from the utility voltage.- Advantages are: no building space required; reduce noise problem within building;lower cost; ease of maintenance and replacement; no interior heat problem.

Transformers Indoors:

- Subject to stringent NEC regulations for safety.

Types:

a. oil-filled transformers small size, low weight, low first cost, long life, excellentelectrical characteristics but flammable and must be installed in a fire-resistant vaultwhich involves heavy cost

b. non-flammable liquid-filled units have most of the advantages of the above and donot require a vault unless voltage is very high. Requires a sump or catch basin for all of the contained liquid. Relatively high first cost.

c. dry-type units shorter life, high noise level, greater weight and large size but is themajority choice. Advantage is ease of installation and almost unrestricted choice of location.

Transformer Vaults:

- basically a fire-rated enclosure, provided because of the possibility of transformer caserupture and an oil fire.

- Should be located where they can be ventilated to the outside air without use of ducts.

SERVICE EQUIPMENT ARRANGEMENTS AND METERING:


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Metering:

- Provided at either the utility or facility voltage, and at either the service point or insidethe building.

- Must be available for inspection and service- Furnished and installed by the utility company.

Service Switch:

- The purpose is to disconnect all of the electric service in the building exceptemergency equipment.

- Located at a readily accessible spot near the point at which the service conductors

enter the building.

Switches:

- Traditional electrical switching devices which close and open an electric current byphysically moving tow electrical conductors into contact with each other to close thecircuit, and physically separating them to open the circuit.

- Rated by current and voltage, duty, poles and throw, fusibility, and enclosure.

- Current rating of the switch is the amount of current that the switch can carrycontinuously and interrupt safely.

- General duty safety switches are intended for normal use in lighting and power circuits.

- HD or heavy duty switches are intended for frequent interrupting, high fault currentsand ease of maintenance.

- Switch may be constructed with or without provision for fusing. Fusible switch if

provided and non-fusible if otherwise.

Contactors:

- A switch that uses contact blocks of silver-coated copper, which are forced together toclose the circuit or are separated to break the circuit.


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- The common wall light switch is a small mechanically operated contactor

- A relay is a small electrically operated contactor.

- Most contactors are operated by means of an electromagnet that causes the contacts

to close.- They open by spring action or by gravity.

- Advantage of contactor over switches is their facility for remote control; switches aremanually thrown.

- The magnetic contactor is inherently a remote-controlled device.

Special Switches:

- Remote-control switches mechanically held, electrically operated contactor

- Automatic transfer switch a double throw switch arranged so that on failure of normalservice it automatically transfer to the emergency service. Control devices are voltagesensors that sense the condition of the service and operate the switch accordingly.

- Time-controlled switches operation is time based.

- Solid state switches; programmable switches

CIRCUIT-PROTECTIVE DEVICES:

- To protect insulation, wiring, switches, and other apparatus from overload and short-circuit.

Fuses:

- Fuses contain a narrow strip of fusible link or metal which is designed to melt (safely)when the current exceeds the rated value, thereby interrupting the power to the circuit.

- Cartridge fuse when enclosed in an insulating fiber tube. Made up to 600A.

- Plug fuse when in a porcelain cup used normally in homes; rated 5A to 30A

- Fuses trip relatively fast which can sometimes be a problem with motors which havelarge startup current surges. For motor circuits, you can use a "time-delay" fuse (one


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Panelboards:

- Same function as switchboard but in a smaller scale.

- Accepts a relatively large block of power and distributes it in smaller blocks.

- Comprises main buses to which are connected circuit-protective devices (breaker or fuses), which feed smaller circuits.

group 3 electrical system and material

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