If the socket strip is switched off via the built-in switch, the surge protection is not required as the socket strip and the connected devices are disconnected from the mains. This means there is no risk of overvoltage, as no current can flow to your devices.
In rare cases, the current may still flow past the switch. In this case, however, the overvoltage protection module kicks in again. It is therefore not dependent on being supplied with power.
As a rule, however, the following applies:
The surge protection is only required when the power strip is switched on. You can tell that it is working by the control indicator (Protection on). If this lamp is on, you can be sure that the surge protection is working.