Normal electrical appliances in houses and commercial establishments, in line with relevant technical standards, are expected to operate smoothly within a broad voltage range of 205 to 270 V. Hence, this voltage range is marked as ‘Normal’. Voltage from 204 V and upto 131 V is shown as ‘Low’ voltage and voltage above 271 V is marked as ‘High’ voltage. Voltage below 131 (i.e. 130 and less) is considered as no supply (or zero voltage), as at such low voltage many appliances, tube lights etc. may not work and it might adversely affect safety of equipment and people. Agricultural consumers are typically supplied 3 phase supply, and in this case ESMI monitors and reports, 1 phase, phase to neutral voltage at these locations, with same classification as above.
A further technical Explanation - Electricity suppliers are expected to ensure supply voltage at consumer end within a band. This ‘declared or rated’ voltage band is specified by respective state electricity regulatory commissions (SERC) in their Standards of Performance or other similar regulations. Typically, SERC’s have specified voltage range of 230 V + /- 6 % for single phase supply and 400 V phase to phase, + /- 6 % for three phase supply (this implies 230 V between phase and neutral). This translates into specified voltage level of 216 V to 244 V. (e.g. DERC, APERC) Some SERC’s also specify declared voltage level of 230 V or 240 V, with variation band of + /- 6 %. (MERC), implying approved voltage range of 226 V to 254 V. Current, BIS specified ‘rated’ / ‘nominal’ voltage for electrical installations is 240 V /415 V (IS 12360). But there is a move to align this standard to international IEC standard, which is 230 V / 400 V with allowed variation of + /- 10%, implying allowed voltage range of 207 V to 253 V for single phase supply or phase to neutral for a three phase supply.
Considering these regulations, accuracy of electricity supply monitoring devices and ESMI objective of primarily tracking supply quality for ordinary household, commercial and industrial establishments, we consider ‘normal’ voltage range as 205 V to 270 V, ‘low’ range from 204 V to 131 V, and ‘high’ range of above 271 V. Any voltage below 131 (i.e. 130 and below) is considered as no supply, as such low voltage is practically not usable and it might adversely affect safety of equipment and people.