Can you please tell me how can I put the nRF51822 in System Off mode?
If you're using softdevice the call sd_power_system_off(). If you're not using softdevice then do it like this NRF_POWER->SYSTEMOFF = 1;.
thank you Nikita,
can you please tell me what sd_power_system_off() do more than NRF_POWER->SYSTEMOFF=1 !?
It has the same functionality but when softdevice is enabled you have restricted access to the NRF_POWER register and you can access to it only with softdevice calls, for example sd_power_system_off(). P.S. misclicked and converted your comment to answer by mistake.
When using a SoftDevice, ALWAYS use the sd_power_system_off. When the SoftDevice is running, it takes control over numerous interfaces including the internal Flash read and write. If you were using this module, I can imagine a situation where the Flash write is pending or even running and you will put the system into power off directly which may lead to Flash corruption. Using the preprogrammed "sd_" function takes care over all those issues to ensure safe system off.