IEEE Extremely Inverse TCC curves · Adjust trip settings · Verify selectivity
This simulator plots time-current curves (TCCs) for upstream and downstream overcurrent protective devices on a log-log chart, allowing engineers to verify selective coordination. Electrical engineers use it when designing power distribution systems where faults must be isolated without unnecessarily de-energizing unfaulted portions of the system.
Selective coordination requires that the downstream protective device clears a fault before the upstream device begins to operate. On a log-log TCC chart (current on the x-axis, time on the y-axis), the downstream device's curve must lie entirely to the left of and below the upstream device's curve at all fault current levels. A minimum time margin of 0.1–0.3 seconds is required between electromechanical devices to prevent simultaneous operation during the mechanical operating time.
The IEEE Extremely Inverse curve used here follows: t = TDM × [A / ((I/Ip)^p − 1) + B], where A = 28.2, B = 0.1217, p = 2, TDM is the time-dial multiplier, I is the fault current, and Ip is the pickup current (rated current). Adjusting the TDM shifts the entire curve up or down on the time axis, while adjusting the pickup current (rating) shifts the curve horizontally.
Instantaneous trip settings define the current level above which the breaker trips with no intentional time delay (typically within 0.02 seconds). For proper coordination, the downstream instantaneous trip must be set below the upstream instantaneous trip.
NEC 700.32 requires selective coordination for emergency systems in health care facilities and high-rise buildings. NEC 701.27 extends this requirement to legally required standby systems. NEC 708.54 requires selective coordination for critical operations power systems. IEEE 242 (Buff Book) is the standard reference for protection and coordination of industrial and commercial power systems, including TCC methodology. NFPA 99 requires selective coordination in health care essential electrical systems. The term "selective coordination" as used in NEC means that only the OCPD closest to a fault opens.
The available fault current at each point in the system is the key input — coordination must be achieved across the entire range from minimum fault current (single-phase line-to-ground) up to the maximum available bolted fault current. Fuses generally have better selectivity than circuit breakers due to steeper TCC curves, but cannot be reset after operation.
Current-limiting fuses can achieve coordination where circuit breakers cannot, especially at high fault current levels. Zone-selective interlocking (ZSI) is a modern alternative that allows breakers at different levels to communicate, enabling instantaneous clearing without time-graded delays. For critical systems, this can meet NEC coordination requirements while also minimizing equipment stress.
Adjust the upstream (Main) and downstream (Feeder) breaker ratings, instantaneous trip multipliers, and time-dial multipliers using the sliders. Set the available fault current at the downstream bus. The TCC chart updates in real time, and the coordination status banner indicates whether selective coordination is achieved. The trip times table shows how long each device takes to operate at the selected fault current level. Adjust settings until the downstream curve is fully below and to the left of the upstream curve at all current levels.
Per NEC 100, selective coordination means the localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings. In practice, it means only the breaker immediately upstream of a fault trips — not every breaker in the path to the source.
For electromechanical (thermal-magnetic) circuit breakers, IEEE 242 recommends a minimum 0.1–0.3 second time margin between device curves to account for breaker operating time tolerances. For electronic trip units with tighter tolerances, smaller margins may be acceptable, but 0.1 seconds is typical. Some jurisdictions require 0.5 seconds.
Both current (ranging from rated pickup to tens of thousands of amperes) and time (ranging from milliseconds to thousands of seconds) span multiple orders of magnitude. A log-log scale compresses this range into a readable chart and converts the inverse-time TCC equations into smooth curves. Coordination margins that appear tight on a linear scale are clearly visible on log-log.
Yes — selective coordination depends on the TCC settings (time-dial and instantaneous trip), not just the rating ratio. A 100A breaker with a TDM of 0.3 and a 400A main with a TDM of 1.0 will typically show good selectivity. The key is that at every fault current level, the downstream device must clear before the upstream device begins to operate.
If the downstream instantaneous trip current exceeds the upstream instantaneous trip setting, both devices trip simultaneously on high-level faults — loss of selectivity. To fix this, increase the upstream instantaneous setting, lower the downstream setting, or use a current-limiting fuse downstream to reduce the let-through current below the upstream instantaneous threshold.
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