RMS Sags Variations in the RMS value are often used to trigger capturing of PQ data. The most common type of RMS variation is the sag, (or dip in European lingo). Some studies show over 60% of the PQ disturbances are sags, which is when the RMS value goes below 90% of the nominal value. On a typical office or residential outlet, that would be dropping from 120Vrms to 108Vrms. If the voltage goes down below 10% of nominal, we call that an interruption. Conversely, if it increases above 110% of nominal, that is a swell.