LTMR08MFM has Earth fault protection with internal and external CT option? What is the necessity of external CT, when earth fault protection is available with internal option?
Find in attachment extract from LTMR manual. It explains the brief information's about ground fault using internal or external CT.
The ground CBCT is optional, since the ground current protection using internal current transformers is also possible based on calculation of ground current igr = i1 + i2 +i3 (sum of actual values in each phase).
The internal ground current fault threshold is 50% FLCmin For LTMR08 FLCmax = 8Amps and FLCmin=1/20*FLCmax (ie,0.4Amps) Hence the minimum ground fault threshold(50% FLCmin) = 0.2Amps and the response time is 500ms.
2. For the earth fault protection is with external CBCT is having minimum detected current 20mA and the response time is 100ms. Hence it indicates that the ground current protection with external CT is more accurate and faster than internal calculation.
Important : The ground fault protection in LTMR is meant only of equipment protection, for human protection the you need to use dedicated earth fault or earth leakage relays like Vigirex etc.