Friday, May 30, 2008

Utilities (Electrical Systems): Service & Utilization


Service is tapped onto the utility lines at a mutually agreeable point at or beyond the property 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 crossarms normally used for high voltage (2.4 kV and higher) lines.
b. Weatherproof – secondary circuits at 600V and below run on porcelain spool secondary racks with 1/c weatherproof cable as the conductor.
c. preassembled aerial cable – consists of three or four insulated cables wrapped together 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
- 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).



- 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 distribution within a building. (from primary voltages-2400V and up, to secondary voltages-480V and below)
- Another transformer would be used in a local electric closet to step down the 480V to 240V 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 in kVA 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 concrete pad.

Cheapest cooling medium used is mineral oil but application is limited because of its flammability. New coolants are being developed but more expensive.
- Insulation used is either organic, inorganic, asbestos or silicone.

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.

a. oil-filled transformers – small size, low weight, low first cost, long life, excellent electrical characteristics but flammable and must be installed in a fire-resistant vault which involves heavy cost
b. non-flammable liquid-filled units – have most of the advantages of the above and do not 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 the majority 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 case rupture and an oil fire.
- Should be located where they can be ventilated to the outside air without use of ducts.


- Provided at either the utility or facility voltage, and at either the service point or inside the 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 except emergency equipment.
- Located at a readily accessible spot near the point at which the service conductors enter the building.

- Traditional electrical switching devices which close and open an electric current by physically moving tow electrical conductors into contact with each other to close the circuit, 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 carry continuously 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 currents and ease of maintenance.
- Switch may be constructed with or without provision for fusing. Fusible switch if provided and non-fusible if otherwise.

- A switch that uses contact blocks of silver-coated copper, which are forced together to close the circuit or are separated to break the circuit.
- 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 are manually 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 normal service it automatically transfer to the emergency service. Control devices are voltage sensors that sense the condition of the service and operate the switch accordingly.
- Time-controlled switches – operation is time based.
- Solid state switches; programmable switches

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

- 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 have large startup current surges. For motor circuits, you can use a "time-delay" fuse (one brand is "fusetron") which will avoid tripping on momentary overloads. A fusetron looks like a spring-loaded fuse.

- A fuse can only trip once, then it must be replaced.

Circuit Breakers:
- An electromechanical device that performs the same protective function as a fuse and in addition, acts as a switch and is equipped with both thermal and magnetic trips.
- When the current flow through the device exceeds the rated value, a bimetallic strip heats up and bends, "trips" the latch, and the spring pulls the contacts apart
- The heavier the overload, the faster the trip action.
- Breakers can be reset a finite number of times - each time they trip, or are thrown when the circuit is in use, some arcing takes place, which damages the contacts. Thus, breakers should not be used in place of switches unless they are specially listed for the purpose.

Switchboards and Switchgear:
- Freestanding assemblies of switches, fuses, and/or circuit breakers, which normally provide switching and feeder protection to a number of circuits connected to a main source.
- Modern switchboards are enclosed in a metal structure.
- High-voltage equipment (above 600V) is referred to as switchgear.
- Main metal-clad switchgear is located in basements or housed in separate, well-ventilated, electrical switchgear rooms.

Unit Substations:
- An assembly of primary switch and fuse or breaker, step down transformer, meters, controls, buswork, and secondary switchgear is called a unit substation.

- 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.

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