SUBSTATION

Relay TCC Coordination Calculator — IEC 60255-151

IDMT upstream/downstream relay pair coordination, CTI check, and instantaneous element grading per IEC 60255-151.

IEC 60255-151 IEEE C37.112 IEC 60909
Presets
Upstream Relay
Downstream Relay
Fault Current Range
Time-Current Curve (TCC) — Log-Log Plot
CTI Coordination Checks
Recommendations
Export Report
Related Tools
Protection Coordination Curves Transformer Differential Protection Short Circuit (IEC 60909)
Methodology
IDMT Formula (IEC 60255-151)+
Operating time: t = TMS × k / ((I/Is)^a − 1). Curve coefficients from IEC 60255-151 Table 1: SI (k=0.14, a=0.02), VI (k=13.5, a=1.0), EI (k=80.0, a=2.0), LTI (k=120.0, a=1.0). Below pickup (I ≤ Is) the relay does not operate; at instantaneous pickup (I ≥ I_inst) the definite time t_inst applies.
CTI Coordination Check+
CTI = t_upstream − t_downstream evaluated at I_sc_min, I_sc_max, 50% of I_sc_max, and just below the downstream instantaneous pickup. The minimum CTI across all check points must be ≥ 0.2s per IEC 60255-151 §6.2. A CTI < 0.2s indicates a coordination failure requiring TMS adjustment.
Instantaneous Element Grading+
The instantaneous pickup I_inst must satisfy I_inst_up > I_inst_dn. The upstream instantaneous is set above the maximum fault current infeed to the downstream bus, typically 1.25–1.5× the downstream bus maximum fault current. Both instantaneous elements provide a definite-time override at their respective pickup levels.
Log-Log TCC Plot+
The TCC chart uses logarithmic scales on both axes (fault current in A, operating time in s) per standard protection engineering practice. Sixty logarithmically spaced points are computed from 1.01× max(Is_up, Is_dn) to 1.1× I_sc_max. The upstream curve is shown as a solid blue line; downstream as a green dashed line.
Frequently Asked Questions
What is IDMT protection?+
Inverse Definite Minimum Time (IDMT) relays operate faster at higher fault currents. The operating time follows a standard curve formula t = TMS × k / ((I/Is)^a − 1) per IEC 60255-151, where Is is the pickup current, TMS is the time multiplier, and k/a are curve-dependent constants. This provides both fast clearance at high fault levels and time-grading discrimination at lower levels.
What are the IEC 60255-151 curve types?+
IEC 60255-151 defines four IDMT curve types: Standard Inverse (SI, k=0.14, a=0.02) for general distribution; Very Inverse (VI, k=13.5, a=1.0) for fuse-like characteristics and cable protection; Extremely Inverse (EI, k=80.0, a=2.0) for motor protection and earth faults; and Long Time Inverse (LTI, k=120.0, a=1.0) per IEEE C37.112 §5.4 for very large machines.
What is CTI and why must it be ≥ 0.2s?+
Coordination Time Interval (CTI) is the operating time difference between the upstream and downstream relay at the same fault current: CTI = t_upstream − t_downstream. A minimum CTI of 0.2s (IEC 60255-151 §6.2) allows the downstream relay to trip and the circuit breaker to interrupt the fault before the upstream relay reaches its operating time, ensuring selectivity. If CTI < 0.2s, both relays may trip, causing unnecessary loss of supply.
How does the instantaneous element improve selectivity?+
The instantaneous (definite time) element provides very fast clearance (typically 50–80ms) at high fault currents above I_inst, independent of the IDMT curve. For correct selectivity, the upstream instantaneous pickup must be set higher than the downstream (I_inst_up > I_inst_dn) so that only the downstream relay responds to faults in its protection zone. The upstream instantaneous typically covers bus faults only.
When should I use EI vs SI curves?+
Standard Inverse (SI) is used for general overcurrent protection where the characteristic needs to coordinate with fuses. Extremely Inverse (EI) provides much faster operation at high multiples of Is and is preferred for motor protection, earth fault protection, and applications where the fault current ratio is high. Very Inverse (VI) is a good compromise and matches fuse characteristics closely. EI gives better discrimination when the fault current at the remote end of a feeder is only slightly above the pickup.
How do I choose TMS for grading?+
Start from the most downstream relay and set the smallest TMS that gives acceptable operating times (typically TMS_dn = 0.1–0.2). Then calculate the upstream TMS such that t_up = t_dn + CTI at the maximum through-fault current. Working upstream, each relay's TMS is set to give CTI ≥ 0.2s above the relay below. Typical TMS steps of 0.1–0.2 per grading level are common in distribution networks.