Sometimes a system admin with a server without a UPS, or needing to change a UPS must be able to safely do so without turning off a machine, or causing risk to himself. What I am Proposing here is a system of maintaining power to a unit, while unplugging it.
It consists of a thin wedge device - which is clasped over the plug, easing it slightly off the socket, and engaging the two or three pins (I envisage this in all known mains plug formats). There is a risk if the wedge is too thin - it may not handle the current needed - versus the risk of it being too thick, and disengaging the plug terminals from the mains socket before engaging them to provide power. The device itself of course has a normal mains plug on the other end. The user should then be able to re-attach the plug to another socket using the device - all the while providing power to the plug.
It must be safe in a way - using a low voltage sensor circuit to see when it has actually engaged the plug, then enabling voltage to flow through it - touching it with finger should not mean electrocution.
It should have a handle and simple operation.
Idea licensed as Creative Commons by-sa 2.0.
2015 calling - If it matters, a server should have a redundant power supply and this is an unlikely scenario. It would also be very hard to make this safe - or workable. Don’t try this!