86.RMS value of AC

  The root mean square value of a quantity is the square root of the mean value of the            squared values of the quantity taken over an interval.

  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.

                                         Vrms = 0.7xVpeak
  
  It is the equivalent steady DC constant) value which gives the same effect.

  For example, a lamp connected to a 6V RMS AC supply will shine with the same                  brightness when connected to a steady 6V DC supply.
  AC voltmeters and ammeters show the RMS value of the voltage or current