You can only connect PSUs in series if there is no connection between the PSU output and mains ground. Some common power supplies (e.g. ATX PSUs and laptop PSUs) do have a connection between the 0V output connection and mains ground.
You can only connect PSUs in series if there is no connection between the PSU output and mains ground. Some common power supplies (e.g. ATX PSUs and laptop PSUs) do have a connection between the 0V output connection and mains ground.