Downstream Ir
100 A
Protection Coordination & Selectivity Calculator
Check whether an upstream and downstream breaker achieve selectivity at a given fault current, see the grading margin and view an illustrative time-current characteristic (TCC) curve.
Result
Enter your load → get a full design & BOM in seconds
Sample outputSelectivity at 15.0 kA
PARTIAL ⚠
Downstream trips at 0.01 s · upstream at 0.10 s · margin 0.09 s
Upstream Ir
250 A
Grading margin
0.09 s
Requirement
≥ 0.2 s
TCC curves
Trip-time calculation
| Downstream thermal pick-up | Ir_d = 100 A × 1 = 100 A |
| Downstream instantaneous | Im_d = 100 A × 10 = 1000 A (Isc ≥ Im, instantaneous trip) |
| Upstream thermal pick-up | Ir_u = 250 A × 1 = 250 A |
| Trip time @ Isc | t_down = 0.01 s · t_up = 0.10 s · margin = 0.09 s |
Recommendation
Increase the upstream short-time delay to ≥ 0.21 s (or use an energy-selective / current-limiting combination).
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| Downstream MCCB | 100 A · Ir 100 A · Im 1000 A (10×) | 1 | POA |
| Upstream ACB/MCCB | 250 A · Ir 250 A · Im 2500 A (10×) · Tsd 0.1 s | 1 | POA |
| Total equipment (switchgear) | POA |
For protection engineers and switchgear designers grading low-voltage breakers and relays so only the breaker closest to the fault trips.
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Worked example
A pre-computed reference example (crawlable — no JavaScript required). Enter your own parameters above for a live result.
Example 1 — 100 A downstream, 250 A upstream, 15 kA fault
Selectivity at 15.0 kA
PARTIAL ⚠
Downstream trips at 0.01 s · upstream at 0.10 s · margin 0.09 s
Downstream Ir
100 A
Upstream Ir
250 A
Grading margin
0.09 s
Requirement
≥ 0.2 s
TCC curves
Trip-time calculation
| Downstream thermal pick-up | Ir_d = 100 A × 1 = 100 A |
| Downstream instantaneous | Im_d = 100 A × 10 = 1000 A (Isc ≥ Im, instantaneous trip) |
| Upstream thermal pick-up | Ir_u = 250 A × 1 = 250 A |
| Trip time @ Isc | t_down = 0.01 s · t_up = 0.10 s · margin = 0.09 s |
Recommendation
Increase the upstream short-time delay to ≥ 0.21 s (or use an energy-selective / current-limiting combination).
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| Downstream MCCB | 100 A · Ir 100 A · Im 1000 A (10×) | 1 | POA |
| Upstream ACB/MCCB | 250 A · Ir 250 A · Im 2500 A (10×) · Tsd 0.1 s | 1 | POA |
| Total equipment (switchgear) | POA |
Example 2 — magnetic overlap, 30 kA fault
Selectivity at 30.0 kA
PARTIAL ⚠
Downstream trips at 0.01 s · upstream at 0.10 s · margin 0.09 s
Downstream Ir
100 A
Upstream Ir
250 A
Grading margin
0.09 s
Requirement
≥ 0.2 s
TCC curves
Trip-time calculation
| Downstream thermal pick-up | Ir_d = 100 A × 1 = 100 A |
| Downstream instantaneous | Im_d = 100 A × 10 = 1000 A (Isc ≥ Im, instantaneous trip) |
| Upstream thermal pick-up | Ir_u = 250 A × 1 = 250 A |
| Trip time @ Isc | t_down = 0.01 s · t_up = 0.10 s · margin = 0.09 s |
Recommendation
Increase the upstream short-time delay to ≥ 0.21 s (or use an energy-selective / current-limiting combination).
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| Downstream MCCB | 100 A · Ir 100 A · Im 1000 A (10×) | 1 | POA |
| Upstream ACB/MCCB | 250 A · Ir 250 A · Im 1250 A (5×) · Tsd 0.1 s | 1 | POA |
| Total equipment (switchgear) | POA |
How it was calculated
- · Thermal pickup: Ir = In × tr.
- · Instantaneous pickup: Im = In × im.
- · Inverse-time trip (thermal region): t = 36 · T6 ÷ (I ÷ Ir)².
- · Downstream trip at Isc: instantaneous 0.01 s if Isc ≥ Im, else the thermal time.
- · Selectivity: grading margin = t_up − t_down ≥ 0.2 s (full selectivity).
Referenced standards
| Standard | Scope |
|---|---|
| IEC 60947-2 | Low-voltage switchgear and controlgear — circuit-breakers |
| IEEE 242 | Recommended practice for protection and coordination of industrial and commercial power systems (Buff Book) |
| IEC 60255-151 | Measuring relays and protection equipment — over/under current protection |
| GB/T 14048.2 | Low-voltage switchgear and controlgear — circuit-breakers |
Frequently asked questions
What is protection coordination and selectivity?
Coordination ensures the breaker closest to a fault trips first, so a downstream fault does not de-energize the whole feeder. Selectivity is achieved by time grading (upstream breaker delays) or current grading. The calculator checks the grading margin between series breakers and plots TCC curves.
What grading margin should I maintain between breakers?
Keep a time grading margin of 0.2-0.4 s between upstream and downstream breakers so the downstream clears first. The upstream trip time must exceed the downstream clearing time plus a safety margin covering breaker tolerance and relay error. The calculator checks this margin on the TCC curves.
What does a TCC curve show?
A time-current characteristic (TCC) curve plots trip time versus Fault Current for a protection device. Plotting upstream and downstream TCCs on one graph shows whether they are selective — the downstream curve must stay left of (below) the upstream curve across the fault range. The calculator renders the curves for the configured devices.
How do I coordinate a fuse with a breaker?
The fuse must clear faster than the breaker upstream for faults above the crossover current, and the breaker must protect for lower currents. Coordination is checked on the TCC plot — the fuse curve stays below the breaker curve up to the maximum fault level. The calculator compares the two characteristics.
What protection does a transformer feeder need?
A Transformer feeder needs overcurrent (50/51), earth fault (50N/51N), and for MV transformers, differential (87) and restricted earth fault for larger units, plus overload (49). The calculator focuses on the overcurrent and earth-fault grading between the feeder and downstream breakers.
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Results are engineering estimates for preliminary design and reference only. Always verify with the applicable standards, the equipment nameplate and a licensed engineer before procurement or installation.
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