An IDMT (Inverse Definite Minimum Time) relay is a type of protective relay that is used to provide overcurrent protection for electrical systems. IDMT relays are commonly used in low-voltage and medium-voltage electrical systems to protect against overcurrents caused by fault conditions such as short circuits.
IDMT relays work by monitoring the current flowing through the electrical system and tripping (disconnecting) the circuit when the current exceeds a predetermined level for a certain amount of time. The time delay of the relay is inversely proportional to the current flowing through the circuit, which means that the relay will trip more quickly for higher levels of current. This allows the relay to provide protection against both high and low levels of overcurrent, depending on the specific IDMT curve selected for the application.
IEC and ANSI / IEEE follow standardized inverse-time characteristics for inverse-time operation. ANSI and IEC IDMT curve operating times are specified with coefficients A, B, and C. The coefficient values can be calculated by formula: