1.0 General
1.0.1 Standard values of voltages as mentioned in IS 12360 relating to a.c. transmission and distribution systems are reproduced in this section. However, it may be noted that for types of installations covered in subsequent parts of the Code, lower voltage values would be more relevant.
1.0.2 For medium and low voltage of distribution system, nominal voltages were 230 V for single-phase and 230/400 V for three-phase system were the recommended standard values. Later on, to align with IEC recommendations and in view of the economic advantages, nominal values of 240 V for single-phase and 240/415 V for three-phase were adopted with a tolerance of ± 6 percent. IS 12360 may be referred for the latest values.
1.0.3 In the case of voltages above 1 kV, product standards relate the voltage rating of equipment with respect to highest system voltages (generally, 10% above the nominal voltage).
1.1 Standard Declared Voltage
1.1.1 Single-phase, Two-wire System
The standard voltage shall be as per IS 12360.
1.1.2 Three-phase System
1.1.2.1 The standard voltages for three-phase system shall be as per IS 12360.
1.1.3 As per the international standards, the direct current (d.c.) of 60V is established as the upper limit for a Safety Extra Low Voltage circuit (SELV), that is, safe to touch directly under dry conditions. Considering both safety and distribution losses, 48V is chosen as the distribution voltage for d.c.
1.1.4 For ELVDC and LVDC, IS 16711 may be referred.
1.0 Voltage Limits for a.c. Systems
1.0.1 The voltage at any point of the system under normal conditions shall not depart from the declared voltage by more than the values given below:
a) 6 percent in the case of low or medium voltage;
NOTE — IS 12360 may be referred for the latest provisions.
b) 6 percent on the higher side or 9 percent on the lower side in the case of high voltage; or
c) 12.5 percent in the case of extra high voltage.
NOTE — The permissible variations given above are in accordance with CEA (Safety) Regulations, 2010, and are applicable to the supply authorities.
1.0.2 For installation design purposes, the limits of voltage between which the system and the equipment used in the system shall be capable of operating continuously are as follows:
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System Voltage (Un) |
Highest Voltage (Um) |
Lowest Voltage |
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(1) |
(2) |
(3) |
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240 V |
264 V |
216 V |
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415 V |
457 V |
374 V |
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3.3 kV |
3.6 kV |
3.0 kV |
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6.6 kV |
7.2 kV |
6.0 kV |
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11 kV |
12 kV |
10 kV |
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22 kV |
24 kV |
20 kV |
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33 kV |
36 kV |
30 kV |
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66 kV |
72.5 kV |
60 kV |
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132 kV |
145 kV |
120 kV |
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220 kV |
245 kV |
200 kV |
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400 kV |
420 kV |
380 kV |
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NOTES 1 This variation in voltage should not be confused with the permissible variation from the declared voltage as given in 3.2.1. 2 IS 12360 may be referred for the latest provisions |
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2 PREFERRED CURRENT RATINGS
The preferred current ratings shall be selected from the R5 series. If intermediate values are required, the same shall be selected from R10 series [see IS 1076 (Part 1)].
3 STANDARD SYSTEM FREQUENCY
5.1 The standard system frequency shall be 50 Hz.
5.2 The limits within which the frequency shall be maintained are governed by the CEA (Safety) Regulations, 2010.