"RMS" stands for "Root-Mean-Squared".  It is often defined as the amount of AC power that produces the same heating effect as an equivalent DC power.

RMS is only relevant to varying voltages and is not used for DC analysis.

For example in the UK the typical RMS mains voltage is 240V (well it used to be before harmonisation with EU).  To match the equivalent power of 240V DC the mains AC voltage has to peek quite a bit higher than 240V.

For a good explanation on methods to calculate RMS see these resources:

http://www.electronics-tutorials.ws/accircuits/rms-voltage.html

 

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