The next significant characteristic point is the continuous operating voltage of the arrester.
For this, the formal symbol Uc is used in accordance with the IEC standards; in Anglo-American circles the term MCOV (Maximum Continuous Operating Voltage) is customary. This is the power-frequency voltage, which the arrester can be operated at, without any type of restrictions. All properties of the arrester, which have been demonstrated in the type tests, are valid, assuming that this arrester is energized at a voltage level equivalent to its continuous operating voltage. As is seen in Figure 2, the continuous operating voltage is greater than the highest continuously occurring phase-to-earth voltage. An allowance of at least 5% (IEC 60099-5) is recommended. With this, possible harmonics in the system voltage are taken into account. In the chosen example the arrester shows a continuous operating voltage of Uc = 268 kV, which is almost 11% above the highest continuous possible phase-to-earth voltage1.