Another important application for Safety Certified Capacitors is in AC/DC power supplies. The capacitors are used on the AC, or input side of the circuit for EMI filtering. Like phone lines, AC power lines carry a lot of noise as well as surges and transients. The capacitors bridge the isolation barrier, so they must withstand the surges and impulses in case of power surges. Most AC/DC power supplies use X1/Y2 rated capacitors. Some power supplies require Y1 rated capacitors which are not available on the market in a surface mount package. The only surface mount solution available is to use two Y2 capacitors in series which, in this case, is considered equivalent to a Y1 capacitor.