Means Equipment grounding(bonding) conductors well tie together to reference ground.An ungrounded system must be grounded the same as a grounded system even thought theres no reference from phase conductors to ground. Reason if one phase goes to ground(ground dector lights) nothing but an alarm or Ground dection light going out. BUT if one phase goes out then another the bonding conductors Must be able to carry fault.Why are theses systems used. Water treament plants, sewage plants areas where going dn at the ist fault is not a option. Gives the operator time when ist fault is noticed to schedule a shutdn and take care of the problem.Thats the way I see it. I;ve worked on several of theses systems as our power company well no longer supply a high leg delta but well supply an ungrounded or one leg grounded system.For better info see Electrical Grounding 6th edition (Ronald P.O;Riley page (90) or Soares grounding book
