RMS, root mean square, is found by squaring the instantaneous values over a cycle, averaging them and taking the square root. For a pure sine wave it equals peak divided by the square root of 2, about 0.707 times peak, so 230 V RMS mains peaks near 325 V. AC voltage, current and power ratings are RMS unless stated otherwise, because heating and power transfer follow it.
Scope shows 340 volts peak, that's about 240 RMS, so the supply's fine and the panel meter is lying.
Also heard as
- root mean square
- effective value
Related terms
True RMS
Describes a meter or reading that computes the real RMS of any waveform, not one scaled from a sine-wave average.
Harmonics
Voltage or current components at whole multiples of the supply frequency, produced mostly by nonlinear loads.
Clamp meter
Handheld meter whose jaws close around a conductor to measure current without breaking the circuit.