Circuit breaker coordination means that when a fault occurs, only the breaker closest to the fault trips. Every other breaker stays closed. The factory keeps running except for the faulted circuit.
This guide explains time-current curves, the 4 types of selectivity, how to do a coordination study in 5 steps, and includes a live coordination check calculator.
Poor circuit breaker coordination is one of the most common findings in electrical audits. A fault that should trip a 160 A feeder breaker ends up tripping the 2500 A main breaker.
The fix is a proper coordination study.
Why Circuit Breaker Coordination Matters
Imagine a factory: a 2500 A main breaker feeds four 400 A feeder breakers. A short circuit occurs on one feeder.
Without proper circuit breaker coordination, both breakers trip simultaneously and the entire factory loses power.
With good coordination, only the 400 A feeder breaker trips. The other three feeders and the main incomer stay live. Production on three out of four lines continues.
The short-circuit current at each bus is the starting point for any coordination study. Without it, you cannot confirm whether the breaker sequence holds selective at high fault currents.
Manufacturers publish selectivity tables with tested values for specific breaker model pairs. A coordination study that relies only on TCC overlap without checking the manufacturer's selectivity table can incorrectly conclude total selectivity is achieved.
Reading a Time-Current Curve
A time-current curve (TCC) is a log-log graph. Current on the x-axis, time in seconds on the y-axis. Both axes use logarithmic scales.
A TCC has two regions. The thermal region is the sloping part. Higher current means faster tripping. The instantaneous region is the vertical band on the right. Above the instantaneous pickup threshold, the breaker trips with no intentional delay.
For coordination to hold, the downstream breaker's curve must lie entirely below the upstream breaker's curve up to the maximum available fault current at that bus.
The lower edge of the upstream breaker's band must lie entirely above the upper edge of the downstream breaker's band throughout the full range of fault current. If the bands overlap anywhere, coordination is lost at that current level. Tools like ETAP and SKM display both edges automatically.
4 Types of Selectivity in Circuit Breaker Coordination
There are four ways to achieve selectivity between breakers. Click each tab to see how it works and when to use it.
Upstream breaker has a deliberate short-time delay before tripping
The upstream breaker has a short-time delay (typically 0.1 to 0.4 seconds). When a downstream fault occurs, the downstream breaker clears it first. The upstream breaker delay expires without needing to trip.
Best for: Main to feeder MCCB or ACB coordination in industrial distribution panels. Used between MCCBs and air circuit breakers where a large current ratio exists.
Key check: The upstream breaker must have an Icw (short-time withstand) rating that exceeds the available fault current at its bus for the full delay period.
Upstream instantaneous pickup set above the downstream bus fault current
The upstream breaker's instantaneous pickup is set higher than the maximum fault current the downstream breaker can see. The upstream instantaneous region never activates for a fault on the downstream circuit.
Best for: Final circuit breakers vs sub-distribution MCCBs with a large breaker rating difference. Works well when cable impedance limits the downstream fault current.
Key check: Requires a significant current ratio between devices. Often not achievable at low-impedance buses with very high available fault current.
Electronic trip units communicate directly with each other
When the downstream breaker detects a fault, it restrains the upstream breaker. The downstream device then clears the fault at its own instantaneous speed.
Best for: Critical facilities, hospitals, and data centres where both speed and full selectivity are needed. Eliminates the time delay requirement and the Icw concern.
Key check: Requires compatible electronic trip units in both breakers and interlocking wiring between them. Higher cost than time selectivity but gives the best combination of speed and selectivity.
Downstream current-limiting device clears the fault before upstream can respond
The downstream device clears the fault so fast and limits the let-through energy so strongly that the upstream breaker never receives enough energy to unlatch its trip mechanism.
Best for: A current-limiting fuse downstream of a larger MCCB. Also used when a current-limiting MCCB is upstream of standard fuses.
Key check: Evaluated using let-through energy (I squared t) curves, not time-current curves alone. Requires the manufacturer's I squared t data for both devices. Cannot be verified by TCC overlap analysis.
How to Do a Circuit Breaker Coordination Study in 5 Steps
Gather System Data
Collect the utility short-circuit MVA, transformer impedances, cable impedances, and all existing breaker ratings and settings. The short-circuit calculation gives you the maximum and minimum fault current at each bus.
Maximum is for coordination checks; minimum is to verify each breaker will trip on a bolted fault at the end of its protected cable.
Work Top-Down from the Main Incomer
Start at the main incomer and work toward the final circuits. At each bus, the reference current is the maximum available fault current at that location.
A bus close to the transformer will have higher fault current and is where coordination is hardest to achieve.
Overlay the Time-Current Curves
Plot the upstream and downstream breaker TCCs on the same log-log graph. The downstream breaker curve must lie entirely below the upstream breaker curve up to the maximum available fault current.
Where the instantaneous bands overlap, coordination is lost and both breakers will trip simultaneously instead of selectively.
Check the Manufacturer Selectivity Table
For the specific upstream and downstream breaker model pair, check the manufacturer's published selectivity table. This table states the maximum fault current up to which total selectivity is guaranteed by test.
If the available fault current exceeds this value, only partial selectivity is achieved. Add a time delay, use zone selective interlocking, or choose a breaker pair with a higher verified selectivity current.
Verify That Each Breaker Protects Its Own Cable
A common error is optimising circuit breaker coordination while forgetting cable protection. The minimum fault current at the cable end must trip the breaker within the required time for earthing system compliance.
A perfectly coordinated system that fails to protect a downstream cable against a high-impedance fault is still a failed design.
Worked Example: Coordination Check at 12 kA
A distribution board has a 630 A MCCB upstream feeding a 160 A MCCB downstream. Available fault current at the downstream bus is 12 kA. We check whether the pair is selective.
Long-time pickup (Ir): 630 A
Short-time pickup (Isd): 6 x Ir = 3780 A at 0.3 s delay
Instantaneous pickup (Ii): 10 x Ir = 6300 A
Downstream 160 A MCCB:
Long-time pickup (Ir): 160 A
Short-time pickup (Isd): 5 x Ir = 800 A at 0.1 s delay
Instantaneous pickup (Ii): 8 x Ir = 1280 A
Available fault current: 12 000 A (12 kA)
Problem: At 12 kA, both instantaneous regions are exceeded.
Downstream Ii: 1280 A < 12 000 A ... trips instantly
Upstream Ii: 6300 A < 12 000 A ... also trips instantly
Result: Both trip. Coordination LOST.
Fix: Disable upstream Ii (set to OFF). Rely on Isd at 0.3 s delay.
Icw check: 12 kA vs Icw 50 kA for 1 s. Well within rating.
Downstream trips instantly (under 50 ms) at 12 kA.
Upstream 0.3 s delay expires. Fault already cleared by downstream.
Result: Selective coordination achieved.
With the instantaneous trip disabled, the upstream breaker must carry the full fault current for the entire short-time delay period without damage. Check the Icw rating on the nameplate and IEC 60947-2 test data before making this change.
If the available fault current exceeds the Icw rating, use zone selective interlocking instead.
Coordination Selectivity Check Calculator
Watch: Selectivity and Time-Current Curves Explained
Circuit Breaker Coordination Questions Engineers Ask
Related Articles on This Site
- How to Calculate Short-Circuit Fault Current
- Protective Relays Explained
- What Is Arc Flash? 4 Critical PPE Category Facts
- Thermal Overload Relay Explained
- Earthing Resistance Calculation
External References
- Time-Current Characteristic Curves for Selective Coordination | Allumiax Engineering
- Selectivity Between Circuit Breakers: Technical Guide | ABB
What We Learn Today
- Circuit breaker coordination means only the breaker nearest to a fault trips, keeping all upstream breakers closed.
- The downstream breaker TCC must lie entirely below the upstream TCC up to the maximum available fault current at that bus.
- When disabling the upstream instantaneous trip for time selectivity, always verify the upstream Icw rating against the available fault current first.