The System Protection Unit (SPU) protects memories from illegal erases, writes, and accesses.
An attempt to erase, write, or access a protected memory region leads to a secure fault. System protection can be turned off in the debug mode by configuring the relevant FLASHREGION[n].PERM for flash and RAMREGION[n].PERM for RAM.
To disable the SPU, the processor must be reset and stopped in the reset vector using the Flash Patch and Breakpoint unit (FPB). This disables any SPU protection and prevents any firmware from re-enabling the SPU.
See nRF9160 SPU - System protection unit for more information.