Emergency circuit or not, turn off the circuit breaker or the only emergency you will have is when the ambulance or hearse comes after you. Cutting into a hot wire will transmit electricity to the cutting tool and its holder. By the sound of your question, you don't have the electrical knowledge or experience to tackle this job.
Hire an electrician and don't mess with live wires. If this is the type that an alarm goes off if it loses power, then wait until after work hours and always have a helper available when working on electrical circuits to give cpr and call an ambulance if the need arises.