206Fundamental Electrical and Electronic PrinciplesConsidering Fig. 6.5 , the ordinates would be selected and measured in the same way as before. The value of each ordinate is then squared. The resulting values are then summed, and the average found. Finally, the square root of this average (or mean) value is determined. This is illustrated below: Iiiiinrmsn1222322…and, for a sinewave only, IIIrmsmm120 707.(6.9) Other waveforms will have a different ratio between r.m.s. and peak values.Note: The r.m.s. value of an a.c. is the value normally used and quoted. For example, if reference is made to a 240 V a.c. supply, then 240 V is the r.m.s. value. In general therefore, if an unqualiﬁ ed value for an a.c. is given, then the assumption is made that this is the r.m.s. value. Since r.m.s. values are those commonly used, the subscript letters r.m.s. are not normally included. Irms has been used above, simply for emphasis. The following convention is used: i, v, e, represent instantaneous valuesIav, V av, E av, represent average values Im, V m, E m, represent maximum or peak values, or amplitudeI, V, E, represent r.m.s. valuespeak factormaximum valuer.m.s. value or VVmm0 70721 414.. Worked Example 6.5 Q Calculate the amplitude of the household 240 V supply. A Since this supply is sinusoidal, then the peak factor will be 2 , so VVVmm22240339 4 voltso, V .Ans 6.6 Peak Factor This is deﬁ ned as the ratio of the peak or maximum value, to the r.m.s. value, of a waveform. Thus, for a sinewave only