How to safely expose one wire of an AC mains circuit?
I'd even go further than what Graham Nye says, and look at mains-rated (UL-Listed, not RU-Recognized) relays, contactors etc. designed to attach directly to AC mains wiring methods and provide low voltage terminals with separation. 

An example is Aube relays, which are designed to mount in AC mains enclosures (e.g. a junction box and standard 1/2" knockout). They are designed to keep mains on the mains side, and low voltage on the low voltage side. They have a built in transformer, so operating the relay is as simple as shorting two terminals (which have 24VAC on them). That connects the transformer to the relay coil, which picks up the relay.  

Now it's not necessary to handle mains at all (except in Electrical Code approved enclosures), and all your work is being done on the low voltage side.   They make other relays, such as RiB, with coil voltages of your choice if you want to supply coil power.