NBC 2016
5. Distribution of supply & cabling –
Following topics are covered in this section
5.1 – General
5.2 System of supply
5.3 Substation equipment & Accessories
5.1 General:
5.1.1 In the planning and design of an electrical wiring installations, due consideration shall be made of all the prevailing conditions. It is recommended that advise of a competent electrical engineer be sought at the initial stage itself with a view to providing an installation that will prove adequate for its intended purpose, be reliable, safe & efficient.
5.1.2 A certain redundancy in the electrical system is necessary & has to be built in from the initial design stage itself. The extent of redundancy will depend on the type of load, its criticality, normal hours of use, quality of power supply in that area, coordination with the standby power supply, capacity to meet the starting current requirements of large motors etc.
5.1.3 In modern building technology, following high demands are made of the power distribution system and its individual components;
- a) Long life & good service quality,
- b) Safe protection in the event of fire,
- c) Low fire load,
- d) Flexibility in load location and connection, but critical in design,
- e) Low space requirement, and
- f) Minimum effort involved in carrying out retrofits.
5.1.4 The high load intensity in modern large buildings and high rise buildings demands compact & safe solution for the supply of power. The use of bus-bar trunking system is ideal for such applications. Bus-bar trunking can be installed in vertical risers shafts or horizontally in passages for transmission and distribution of power. They allow electrical installations to be planned in a simple and a neat manner. In the building complexes, additional safety demands with respect to fire barriers and fire load can also be met with use of bus-bar trunking. Bus-bar trunking system also reduces the combustible material near the area with high energy in comparison with other distribution system such as cables and makes the building safe from the aspect of vulnerability to fire of electrical origin. In addition, unlike cable systems the reliability of a bus-bar trunking system is very high. These systems also require very little periodic maintenance. Choice of bus-bar trunking for distribution in building can be made on the basis of,
a) reduced fire load (drastically reduced in comparison to the cable system);
(Note – Insulation material of cables are required to be fire resistant and an essential performance requirement is that the insulating material may burn or melt and flow when directly exposed to a temperature (or fire) higher than what it is class designated for, but should not continue to burn after the flame or the source of heat or fire is withdrawn. Even if the above fire resistance property is exhibited by the cable insulation, a large collection of cables will make the cable insulation fail to exhibit this retardant property. While specific guidelines for limiting number of cable and bunching is not available and in such cases the switch over to a bus trunking system is the proper alternatives).
b) Reduced maintenance over its entire lifetime,
c) Longer service lifetime in comparison with a cable distribution; and
d) enhanced reliability due to rigid bolted joints and terminations and extremely low possibility of insulation failure.
5.2 System of Supply:
5.2.1 All electrical apparatus shall be suitable for the voltage and frequency of supply.
5.2.2 In case of connected load of 100KVA and above, the relative advantage of high voltage three phase supply should be considered. Though the use of high voltage supply entails the provisions of space and the capital cost of providing suitable transformer substation at the consumer’s premises, following advantages are gained:
a) advantage in tarrif,
b) more effective earth fault protection for heavy current circuits,
c) elimination of interference with supplies to other consumers permitting the use of large size motors, welding plant, etc; and
d) better control of voltage regulation and more constant supply voltage.
(Note- Additional safety precautions required to be observed in HV installations shall also be kept in view).
In many cases there may be no choice available to the consumer, as most of the licensees have formulated their policy of correlating the supply voltage with the connected load or the contract demand.
Generally the supply is at 240V single phase up to 5KVA, 415/230V 3 phase from 5kVA to 100 KVA, 11KV (or 22KV ) for loads up to 5 MVA and 33KV or 66 KV for consumers of connected load or contract demand more than 5 MVA.
5.2.3 In very large industrial buildings where heavy electric demands occur at scattered locations, the economics of electrical distribution at high voltage from the main substation to other subsidiary transformer substations or to certain items of plant, such as large motors and furnaces, should be considered. The relative economy attainable by use of medium or high voltage distribution and high voltage plant is a matter of expert judgement and individual assessment in light of experience by a professionally qualified electrical engineer.
5.3 Substation equipment & Accessories:
Substations require an approval by the Electrical Inspectorate. Such approval is mandatory before energizing the substation. It is desirable to get the approval for the general layout, schematic layout, protection plan, before the start of the work form the Inspectorate.
All substation equipment and accessories & material etc shall confirm to relevant Indian Standards, whereever they exist, otherwise the consumer (or his consultant) shall specify the standards to which the equipment to be supplied confirms and that shall be approved by the authority. Manufacturers of the equipment have to furnish certificate of conformity as well as type test certificates for record, in addition to specified test certificates for acceptance tests and installation related tests for performance of protective gear.
5.3.1 Supply Company’s High Voltage Meter Board
In case of single point high voltage metering, energy meters shall be installed in building premises as per 4.2.2.1, at such a place which is readily accessible to the owner/operator of the building and the authority. The supplier or owner of the installation shall provide at the point of commencement of supply a suitable isolating device fixed in a conspicuous position at not more than 1.7 m above the ground so as to completely isolate the supply to the building in case of emergency. In this connection, Central Electricity Authority (installation and operation of meters) regulations, 2006, as amended from time to time shall be complied with.
5.3.2 High Voltage Switch Gear
5.3.2.1 The selection of the type of high voltage switch gear for any installation inter alia depends upon the following:
a) Voltage of the supply system;
b) Prospective short circuit current at the point of supply;
c) Size & layout of electrical installations;
d) Accommodation available and
e) Nature of industry.
Making and breaking capacity of switchgear shall be commensurate with short circuit potentialities of the supply system and the supply authority shall be consulted on this subject. HV switchgear and control gear shall confirm to he accepted standards (8-2(14)) and the other relevant Indian Standards.
5.3.2.2. Guidelines on various types of switchgears can be commensurate with short circuit potentialities of the supply system and the supply authority shall be consulted on this subject.
HV switchgears & control gear shall confirm to the accepted standards (8-2(14)) and the other relevant Indian Standards.
5.3.2.2 Guidelines on various types of switchgear equipment and their choice for a particular application shall be in accordance with good practices (8-2(12)).
5.3.2.3 In extensive installation of switch gear (having more than four incoming supply cables or having more than 12 circuit breakers), banks of switchgears shall be segregated from each other in order to prevent spreading of the risks of damage by fire or explosion arising from switch failure. Where a bus bar section is switch is installed, it shall also be segregated from adjoining banks in the same way (see good practice (8-2(13)).
5.3.2.4 It should be possible to isolate any section from the rest of the switch boards such that work might be undertaken on this section without the necessity of making the switch board dead.
Isolating switches used for the interconnection of sections or for the purpose of isolating circuit breakers of other apparatus, shall also be segregated within its compartment so that no live part is accessible when work in a neighboring section is in progress.
5.3.2.5 In the case of double or ring main supply switchgears with interlocking arrangement shall be provided to prevent simultaneous switching of two different supply sources. Electrical and/or mechanical interlocks may preferably be provided.
5.3.3 HV Cables
5.3.3.1 The sizing of the cable shall depend upon the method of laying cable, current to be carried, permissible maximum temperature it shall withstand, voltage drop over the length of the cable, the prospective short circuit current to which the cable may be subjected, the characteristics of the overload protection gear installed, load cycle, thermal resitivity of the soil and the operating voltage (see also good practice (8-2(15)).
5.3.3.2 All HV cables shall be installed in accordance with good practices (8-2(15)). The HV cable shall either be laid on the cable rack/ built up concrete trenches/tunnel/basement or directly buried in the ground depending upon the specific requirement.
When HV cable is hanging/running below the basement ceiling slab, the cable shall be laid in a fire rated enclosure/ cable tray. The advice of the cable manufacturer with regard to installation, jointing & sealing should also be followed.