what is ELR and IDMT?

ELR is Earth Leakage Relay which is a protective relay for earth fault protection in power panels which operate when earth fault is detected in the lines. This works in the core balance principle by use of summation CT

IDMT (Inverse Definite Minimum Time) relay works on the principle of rotation of disc in the electromagnetic field due to eddy current generated in the disc. The movement depends on the CT output and is set, to operate under two parameters time and current. Depending up on the actual delay of operation and the max tripping current with the use of preset plugs. IDMT relays are essential for relay coordination in power distribution systems.

This is a very brief note of a big subject and need minimum two hours lecture to explain fully. It is essential knowledge for electrical engineer. If you want to read more search in some text books or Google where lot of information is available which you can read.