The next important parameter when choosing the right SPD is to pick an SPD with an MCOV 15 to 25% above the reference to ground. It is important to size the MCOV accurately because if too low of an MCOV is chosen, the device will chop off more of the surge, providing better protection. However, it will activate more often and will cause premature SPD failure. If the MCOV is too high, the SPD will chop off less of the surge, meaning the device will be subject to more voltage and therefore inadequate protection.