Low Over Current Setting (I>)

High Over Current Setting (I>>)

What is IDMT Relay?

Inverse Definite Minimum Time (IDMT) relays are protective relays that operate with an inverse time characteristic. The operating time decreases as the fault current increases.

Key Formulas

Example 1: Industrial Feeder

Feeder: 384A Load, 11-22kA Fault Current

CT: 600/1A, Relay Error 7.5%, CT Error 10%

Settings: I> at 125%, 0.8A plug, 0.125 TMS, Normal Inverse

Example 2: Transformer Protection

Feeder: 250A Load, 8-15kA Fault Current

CT: 300/1A, Relay Error 5%, CT Error 7.5%

Settings: I> at 120%, 1.0A plug, 0.1 TMS, Very Inverse

Example 3: Motor Protection

Feeder: 150A Load, 4-8kA Fault Current

CT: 200/1A, Relay Error 10%, CT Error 15%

Settings: I> at 150%, 1.125A plug, 0.15 TMS, Extreme Inverse

IEC 60255 Standards

IEC 60255-151: Measuring relays and protection equipment

IEC 60255-3: Insulation coordination

IEC 60255-8: Thermal electrical relays

IEEE Standards

IEEE C37.90: Relays and relay systems

IEEE C37.112: IDMT characteristic equations

IEEE C57.13: CT requirements

Safety Factors

Relay Error: Typically 5-10%

CT Error: Typically 5-10%

Safety Margin: Minimum 0.3 seconds

CB Time: Typically 0.1-0.2 seconds