Calculator Inputs
Example Data Table
| Device | Curve | Pickup (A) | Fault (A) | TMS | Approx Trip (s) |
|---|---|---|---|---|---|
| Panel Main | IEC Standard Inverse | 400 | 3200 | 0.10 | 0.98 |
| Motor Feeder | IEC Very Inverse | 160 | 1200 | 0.08 | 0.17 |
| Utility Relay | IEEE Very Inverse | 600 | 6000 | 0.20 | 0.29 |
Formula Used
Current multiple: M = I / Ip
IEC inverse curve: t = TMS × k / (Mα − 1) + delay
IEEE inverse curve: t = TD × [A / (Mp − 1) + B] + delay
Three phase fault power: kVA = √3 × V × I / 1000
Single phase fault power: kVA = V × I / 1000
Thermal energy estimate: I²t = I² × t
If the instantaneous element is enabled, its time is used when current reaches the instantaneous pickup.
How To Use This Calculator
- Enter the device name for your report record.
- Select a standard curve or a custom curve model.
- Enter pickup current and the expected fault current.
- Set time dial, delay, and optional trip time clamps.
- Enable instantaneous action only when the device includes it.
- Add voltage and phase type for fault power estimation.
- Enter upstream trip time when checking coordination margin.
- Press calculate, then export the result as CSV or PDF.
Time Current Curve Guide
What This Calculator Does
A time current curve shows how fast a protective device may trip at a given current. It helps compare overload, short circuit, and coordination behavior. This calculator estimates one operating point and builds a small curve table. It supports IEC inverse curves, IEEE inverse curves, custom constants, definite delay, and instantaneous action. The result is an engineering estimate, not a replacement for the device sheet.
Why Time Current Curves Matter
Electrical protection should clear faults fast, yet avoid nuisance trips. A motor start, transformer inrush, or temporary overload can produce high current for a short time. The chosen curve must ride through normal events and still isolate dangerous faults. Good settings also keep upstream devices slower than downstream devices, where selective coordination is required.
Inputs You Should Review
Start with the pickup current. This is the current where the inverse curve begins. Then enter the expected fault current or test current. The calculator divides fault current by pickup current to find the current multiple. Next, choose the curve family and time dial. A higher time dial gives a longer trip time. Add intentional delay when the relay has a definite short time setting. Enable instantaneous action only when the device has that function.
Understanding The Output
The main result gives the estimated trip time. The tool also reports the current multiple, apparent fault power, I²t energy, load margin, and optional coordination margin. The generated points help sketch a curve or compare several settings. Export options let you keep the result for a study file.
Practical Use Notes
Always compare estimates with manufacturer curves. Real breakers and relays have tolerances. CT ratio, relay firmware, ambient temperature, and maintenance condition can change performance. Use this page for screening, education, and early setting checks. Final protection choices should follow local codes, utility rules, and a qualified electrical engineer.
Common Study Workflow
Many users first enter a downstream breaker setting. They record its trip time at the likely fault level. Then they enter the upstream device with a higher pickup or time dial. The upstream time should normally exceed the downstream time by the selected margin. This simple check supports cleaner reports and safer adjustments during early design review phases.
FAQs
What is a time current curve?
It is a curve showing how long a protective device may take to trip at different current levels. Higher current usually gives faster operation.
What is pickup current?
Pickup current is the threshold where the relay or breaker starts timing. Current below pickup normally should not trip the inverse element.
What does current multiple mean?
Current multiple is fault current divided by pickup current. A multiple of 5 means the device sees five times its pickup setting.
What is TMS or time dial?
TMS or time dial scales the trip time. Raising it usually slows the curve. Lowering it usually makes the device trip faster.
When should I use instantaneous pickup?
Use it only when the protective device has an instantaneous element. It represents very fast clearing above a selected current threshold.
Can this replace manufacturer curves?
No. This calculator gives estimates from common formulas. Always confirm settings with manufacturer data, standards, and qualified engineering review.
Why is trip time shown as N/A?
N/A appears when current is at or below pickup, or when the formula cannot produce a valid finite timing value.
What is coordination margin?
Coordination margin is the upstream trip time minus the downstream trip time. A positive margin helps selective protection planning.