Answer / suneetha

this is due to form factor
rms value of induced emf= form factor * average value

form factor is calculated as 1.11
so the ratings are usually expressed in multiples of 11

Answer / chandru_williams

1. When an alternator generates voltage, its rms value is
given by 4.4*f*$*t, where $ is value of total flux per
pole. So we can see all the voltages are made inevitably
multiple of this value (1.1, which is the form factor of
ac wave). Also it provides us the best economic
construction of step up and step down transformers.



2. We always use a multiple of 1.11 bcoz for a pure sine
wave the FORM FACTOR is the ratio of rms value of voltage
or

current with the avg. value of voltage or current and for
pure sine wave rms value of current is Imax/root '2' and

avg. value is 2Imax/pie. and which comes out to be 1.11 so
we cant have a combination of other then a multiple of 1.11.

Answer / shankar rao

Transformer rating depending up on Emf induced on Trafo.
Emf. induced Eqs. is depending up on Farm factor : 1.11

Answer / manolito aguilar

bcoz for a pure sine wave to form factor is the ratio of
rms value of voltage.

Answer / shakir hussain

They aren't always - here are a few examples - 13.8, 34.5, 69, 115, 230, 345. In reality, voltage is a range, perhaps .96 - 1.04 times this value, so equipment is sized according to this range (e.g. 69kV may have equipment rated for 72.5kV).

The reason is probably because someone started using that voltage, and everyone else joined in so systems fit together nicely - you can have a backup source from your neighbor if you're using similar stuff.

Insulation coordination can also a factor for this standard.

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