Under Ground Cables
Introduction
Electric power can be transmitted or distributed either by overhead system or by underground system. The underground cables are preferable than overhead in some cases for due to some advantage as follows:--they are less liable to damage through storms or lightning,
-they have low maintenance cost,
-they have less chances of faults,
-they have smaller voltage drop,
-They have better general appearance,
However, there is some advantage 0ge also there is some elimination as follows:-
-they have greater installation cost,
-They introduce insulation problems at high voltages compared with the equivalent overhead system.
-The chief use of underground cables for many years has been for distribution of electric power in congested urban areas at comparatively low or moderate voltages. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltages. This has made it possible to employ underground cables for transmission of electric power for short or moderate distances.
The type of cable used will depend up on the voltage will applied to it, and service require. According to the recent improvement in design and manufacture cables become suitable for high voltage associated with modern transmission system.
In any installation, the importance of calculating & installing the correct size of cable for each circuit is paramount. This entails using the correct factors given in the IEE wiring regulations. Voltage drop in the final circuit must not be over loaded as this can have a bearing on the conductor size chosen. It is therefore essential for the designer or installer to ascertain the condition under which cables are to be installed. The external influences that are going to affect the cables, such as the ambient temperature that will be encountered along the cable route or whether the cables will be in contact with thermal insulation. Contact with other cables along the route must be ignored. Since this can have an effect not only on the current carrying capacity of the cables to be installed. It is important when considering voltage drop to remember that some electrical equipment such as the direct on line start squirrel cage induction motor, can take a six to seven times the equipment normal running current, when it is started. This means that for a short period of time, the voltage drop in the cables feeding the equipment may be higher than 4% permitted by the IEE wiring regulation, an excessive reduction in voltage, even for short period of time.
Basic definition
Electrical cables:
Is defined as '' single conductor insulated through it full length'' or ''two or more conductor provided with its own insulation and lay up together under one outer protective covering.
Skin effect:
Is occurs at alternating current which the current taken the outer surface of the conductor as a current pass and this affected on the cross sectional area of the cable when some cables grouped together.
Stranding effect:
is necessary to provide flexibility to cable it have a slandered number as (3, 7, 19, 37) and so on all number accept (3) having a centrally conductor with all the other around it.
The spiralling of the cable cause the increase in resistance since the length of each spiralled conductor is greater than the central conductor, and the follow of current is along various conductors.
Ambient temperature:
It is the temperature of air or other medium where the equipment is to be used.
Current carrying capacity:
It is the maximum current which can be carried by a conductor under specified condition without its steady state temperature exceeding a specified value.
Over current:
A current exceeding the rated value. For conductor the rated value is the current carrying capacity.
Short circuit current:
An over current resulting from a fault of negligible impedance between live conductors having a difference of potential under normal operating condition.
Design current:
The magnitude of the current intended to be carried by a circuit in normal service.
Overload:
Where a protection device is being used for overload protection, the regulations call for the protective device to operate when the current reaches a value of 1.45 times the rating of protective device.
Hand hole Enclosure:
An enclosure identified for use in underground systems, provided with an open or closed bottom, and sized to allow personnel to reach into, but not enter, for the purpose of installing, operating, or maintaining equipment or wiring or both as shown in the following figure.
Hand hole enclosure
Main Requirements OF Cable
(1) Conductor used in cable should be standard and its cross sectional area should be selected such that it may carry full load current without overheating and causing voltage drop.
(2) Insulation material: it should give high degree of safety and reliability at operation Condition.
(3) We should make a mechanical protection to the cable to withstand the rough usage in laying it.
(4) Material which used in manufacture of cable should give complete chemical and Physical stability through it.
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