Siemens SIMATIC S7-1500 Getting Started page 18

Hardware section
2.2 Installing the assembly
3. Remove the 4-pole connection plug and screw the load current supply (PM) tight.
4. Insert the U-connector into the back of the CPU.
5. Mount the CPU on the mounting rail and screw tight.
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Getting Started, 05/2014, A5E03981761-AC
S7-1500