17.Root Mean Square

Root Mean Square (RMS) Values

The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. Clearly for most of the time it is less than the peak voltage, so this is not a good measure of its real effect.

Instead we use the root mean square voltage (V_RMS) which is 0.7 of the peak voltage (V_peak):

                                         V_RMS = 0.7 × V_peak   and   V_peak = 1.4 × V_RMS

These equations also apply to current.
They are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 are different values for other shapes. 

The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.

For example a '6V AC supply' means 6V RMS, the peak voltage is 8.6V. 

The BANGLADESH ‘S mains supply is 220V AC, this means 220V RMS so the peak voltage of the mains is about 300V.