Once you run the shut down command it will shut the system off in less than 20 seconds. I would suggest to run this from a physical server for this reason. There is no way to run the command and have it wait for 240 seconds. 
Attempting to script a remote shutdown of an SCv3020 running 7.3+.  I believe I need Dell EMC Storage Manager Data Collector running, load the Powershell module, and initiate the command. I checked Powershell SDK cookbook and see no mention of shutdown command. I see this: [URL] So command would be : Stop-DellScController - shuts down a single controller Stop-DellStorageCenter - shuts down an entire system   Are these commands correct, and if so, I would like to initiate the shutdown command with a delay of about 240 seconds.  Is this possible?       
Dell SCv3020 - Powershell remote shutdown