I'm a maintenance planner at a hydroelectric dam. Each of our 11 generating unit main step up transformers are protected by a deluge system (1 deluge valve per transformer). We cannot perform a full flow test without de-energization of the transformer, so we clearly fall under NFPA 25 section 184.108.40.206.3.2, which states that the full flow test shall be conducted at the next scheduled shut down, and the next bullet states that the frequency shall not exceed 3 years. To me, that sounds like the end of the story. Full flow test at least every 3 years, whenever we're scheduled to de-energize the transformer. Don't worry about annual.
Our fire insurance provider, however, has stated that we also need to perform an annual "partial" test in between. I assume this comes from the combination of sections 220.127.116.11.3.1 and 18.104.22.168.6, which apply when "the nature of the property is such that water cannot be discharged for test purposes". But we CAN discharge water for test purposes...we just can't do it without de-energizing the transformer. We currently do not have a means of routing the water outside of the protected area for the "partial" test. Insurance provider has stated that the partial can be accomplished by having the main isolation valve partially open and then close it right after the deluge trips and alarm sounds, to avoid sending water to the nozzles. This process relies on a person standing by to shut a multi-turn gate valve at the right time. There's no way anybody in our group would be confident in saying that water would never make it to the nozzles using this method. So we would still require that the transformer be de-energized even to do a partial test. "Partial" would appear to gain us nothing.
We have no AHJ over us to clarify requirements or to tell us "thou shalt do such and such", but we try to make sure we are complying with standards and best practices. Should we A.) Do full flow test at least every 3 years and just don't sweat the annual, B.) Install bypass piping and valves at each transformer to be able to divert water to a drain for annual partial test, C.) Something else ? Insurance has advised against installing diversion plumbing to keep things simple and reliable, but has stated that it is neither condoned nor prohibited by NFPA standards. Thanks for any opinions and suggestions.