Need Technical Detail of Energized Cable Sensing Device

Dear All, In our MCC (Motor control center ) we are having 15 number of Ring main units with Schneider 11kv,RE2c type. And we are getting supply through 2 feeders from FEWA (sub station ). this is the thing if any fault comes in RMU (Ring Main unit) corresponding RMU will trip and main feeder will trip due to fault type,if this happens we have to check each RMU which is tripped because we don't have any monitoring system for RMU. At the same time we feeling difficult to fix fault locator (due to shutdown ). So we need a detector which will detect from outside of cable, we are using trefoil 630 sqmm xmpl cable (so we can't use CT). Any sensor's available,just we need to know which RMU's power gone first (with in some millisecond ).
Our idea is by using Electromagnetic field, it is suitable or any other method is there ? What about MEMS (Micro Electro Mechanical System) can i use mems as a live cable sensor.