▁loss ▁of ▁equipment ▁which ▁is ▁online , ▁without ▁losing ▁position ▁and / or ▁heading . ▁A ▁single ▁failure ▁can ▁be , ▁amongst ▁others : ▁Th r uster ▁failure ▁Gener ator ▁failure ▁Power bus ▁failure ▁( when ▁gener ators ▁are ▁combined ▁on ▁one ▁power bus ) ▁Control ▁computer ▁failure ▁Position ▁reference ▁system ▁failure ▁Reference ▁system ▁failure ▁ ▁For ▁certain ▁operations ▁redund ancy ▁is ▁not ▁required . ▁For ▁instance , ▁if ▁a ▁survey ▁ship ▁los es ▁its ▁D P ▁cap ability , ▁there ▁is ▁normally ▁no ▁risk ▁of ▁damage ▁or ▁inj uries . ▁These ▁operations ▁will ▁normally ▁be ▁done ▁in ▁Class ▁ 1 . ▁ ▁For ▁other ▁operations , ▁such ▁as ▁div ing ▁and ▁heavy ▁lif ting , ▁there ▁is ▁a ▁risk ▁of ▁damage ▁or ▁inj uries . ▁Depending ▁on ▁the ▁risk , ▁the ▁operation ▁is ▁done ▁in ▁Class ▁ 2 ▁or ▁ 3 . ▁This ▁means ▁at ▁least ▁three ▁Position ▁reference ▁systems ▁should ▁be ▁selected . ▁This ▁allows ▁the ▁principle ▁of ▁voting ▁logic , ▁so ▁the ▁failing ▁PR S ▁can ▁be ▁found . ▁For ▁this ▁reason , ▁there ▁are ▁also ▁three ▁D P ▁control ▁computers , ▁three ▁gy ro comp ass es , ▁three ▁M RU ' s ▁and ▁three ▁wind ▁sens ors ▁on ▁Class ▁ 3 ▁ships . ▁If ▁a ▁single ▁fault ▁occurs ▁that ▁je op ard izes ▁the ▁redund ancy , ▁i . e ., ▁failing ▁of ▁a ▁thr uster , ▁generator ▁or ▁a ▁PR S , ▁and ▁this ▁cannot ▁be ▁resolved ▁immediately , ▁the ▁operation ▁should ▁be ▁abandoned ▁as ▁quickly ▁as