Technician reported inadvertently activating an engine fire bottle during a routine test of the Engine Driven Pump Shut Off Valve.

Narrative

777-200; left engine-drive pump (EDP) supply shut off valve repetitive operational test.Prior to the event occurring; I was aware that you had to do this test by just pulling up on the fire handle and pushing it straight back down and I told my partner I do not remember them pulling breakers when I last participated in this task. My partner and I opened up my tablet to the AMM manual per task card to ensure we were using the correct instructions. We pulled and collared the 5 breakers listed under AMM subtask XX-XX-XX-XXX-XXX. While we agreed that I would use the flatbed lift truck to go up on each of the engine pylons to watch the EDP shut off valves. The RH EDP OSP checked good but the LH side one was not. We were confused as to why it was not working and so my partner went to consult a fellow Mechanic familiar with the job. During our agreed upon break; I read through the AMM and found that we pulled CB B5) and we probably needed that breaker in per AMM subtask XX-XX-XX-XXX-XXY. My partner came back with the other Mechanic and we discussed that the LH EDP shutoff valve was obviously not going to work with the breaker pulled out. So; I said we should push back in that one breaker and do the test again. When I was up on the lift at the LH pylon I called my partner over the phone that I was ready and she could pull the handle. She said ok and then all I heard was what sounded like a loud boom and air; then another quick succession boom and heavy blast of air flow from underneath me from the engine. I said to her over the phone it sounded like she blew a bottle; but at least the shutoff valve worked.