In an executed REF protection, the neutral CT No.1 after many years of system operation is fronted to burning due to unknown accident (see below figure). Operator group decide to replace of failed CT. They chose four more expensive CTs with two times rated power for prevention of same accident; they think the current transformer with higher volt-ampere capacity is more stable against thermal stresses due to secondary over loading. But in practice they face to mysteries case, after an sever earth fault on power transformer not only new neutral current transformer but also related protective relay was burned due to that sever earth fault current.
How can you explain the reason of that protection system failure? They think, they can always have a more reliable and safe protection system with an over design equipments choosing!! What is your idea?