Выражение встречается в следующем контексте:

3. Turbine HP Hydraulic system
HP hydraulic system consists of:
HP hydraulic aggregate
HP hydraulic actuators
Stop valves are controlled_ through the safety block “2 out of 3” for trip functions_. The turbine is tripped when at least two of three DC magnets are deenergized.
Control valves are controlled by using Rexroth type servo valve (2 windings at the control magnet – electro-hydraulic element). The servo valves are used for continuous position control, so very high control quality and position accuracy is achieved. All other necessary parts such as position sensors etc. are also included.

вот эта часть непонятна
Stop valves are controlled_through the safety block “2 out of 3” for trip functions. The turbine is tripped when at least two of three DC magnets are deenergized.

Заранее спасибо

Запорные клапаны срабатывают на отключение через предохранительную блокировку «2 из 3». Турбина отключается при отключении питания минимум двух из трех электромагнитов постоянного тока.