B757-200
Maintenance Airframe; Maintenance Powerplant
Communication Breakdown; Confusion; Situational Awareness; Training / Qualification
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action; General Flight Cancelled / Delayed
Aircraft; Human Factors; Incorrect / Not Installed / Unavailable Part; Procedure
Right-Hand (RH) Aft Overwing Slide Squib would not test. While following MEL Procedure 25-66R; slide deployed. Test box was not available at Terminal-ZZZ. Alternate MEL Procedure 25-66R didn't specifically state to pin slide bottle.
We were tring to Defer (DF) the Aft Right Overwing Slide Squid; using an MEL 25-66X Deferral (DF) procedure. We were on Step-6; trying to get the 22-34V Volts from the [Squib electrical] connector. The other Mechanic was opening and closing the door and I was tring to get the correct volts when the slide blew.  We did not read the entire Aircraft Maintenance Manual (AMM) 25-65-00/501; and we did not pin the slide. Not ETOPS.
Two Line Mechanics report a right-hand Aft Overwing Escape Slide inadvertently deployed on a B757-200 aircraft while they were performing an MEL deferral procedure involving a Voltage Test of the Slide Squib connectors. The slide safety pin was not installed.