Protection coordination is critical for ensuring electrical system reliability and safety. It involves calculating settings to prevent equipment damage and outages.
This article explores Protection Coordination Calculators based on IEEE and NEC standards, covering formulas, tables, and practical examples.
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- Calculate time-current characteristic for a 100 A breaker with 10 kA fault current.
- Determine coordination time interval between upstream and downstream relays for 5 kA fault.
- Find minimum pickup current for a relay protecting a 200 A feeder per NEC guidelines.
- Compute fuse sizing for a motor circuit using IEEE coordination curves.
Common Values for Protection Coordination Calculator – IEEE, NEC
| Parameter | Typical Values | Units | Notes |
|---|---|---|---|
| Nominal System Voltage | 120, 208, 480, 600 | Volts (V) | Common low and medium voltage levels |
| Breaker Interrupting Rating | 10, 25, 65, 100 | kA | Maximum fault current breaker can safely interrupt |
| Relay Pickup Current (I_pickup) | 1.0 to 2.0 × I_load | Amps (A) | Typically set above load current to avoid nuisance trips |
| Time Dial Setting (TDS) | 0.05 to 1.0 | Unitless | Adjusts relay operating time on inverse time curves |
| Coordination Time Interval (CTI) | 0.3 to 0.5 | Seconds (s) | Minimum time difference between upstream and downstream device operation |
| Fuse Current Rating | 125% to 175% of motor full load current | Amps (A) | Per NEC Article 430 for motor protection |
| Protection Device | Typical Pickup Current | Time Dial Range | Typical Application |
|---|---|---|---|
| Instantaneous Overcurrent Relay | 5 to 10 × I_pickup | N/A | Fast clearing of high magnitude faults |
| Inverse Time Overcurrent Relay (IEEE C37.112) | 1.0 to 2.0 × I_load | 0.05 to 1.0 | General feeder and transformer protection |
| Time-Current Fuse | 125% to 175% of motor FLC | N/A | Motor branch circuit protection per NEC 430 |
| Thermal Magnetic Breaker | 1.0 to 1.25 × I_load (thermal) | Instantaneous magnetic trip at 5 to 10 × I_load | General branch circuit protection |
Fundamental Formulas for Protection Coordination Calculator – IEEE, NEC
1. Time-Current Characteristic (TCC) for Inverse Time Overcurrent Relays
The IEEE C37.112 standard defines several inverse time curves. The general formula for the operating time (t) is:
- t: Operating time of the relay (seconds)
- TDS: Time Dial Setting (unitless, typically 0.05 to 1.0)
- K, α, C: Constants depending on the curve type (see table below)
- I: Fault current magnitude (Amps)
- Ipickup: Relay pickup current setting (Amps)
| Curve Type | K | α | C (seconds) | Typical Application |
|---|---|---|---|---|
| Standard Inverse (SI) | 0.14 | 0.02 | 0 | General feeder protection |
| Very Inverse (VI) | 13.5 | 1.0 | 0 | Transformer protection |
| Extremely Inverse (EI) | 80 | 2.0 | 1 | Motor protection |
2. Coordination Time Interval (CTI)
CTI is the minimum time difference between the operation of downstream and upstream protective devices to ensure selectivity:
- tupstream: Operating time of the upstream device (seconds)
- tdownstream: Operating time of the downstream device (seconds)
- Minimum CTI ensures proper discrimination and avoids simultaneous tripping
3. Fuse Sizing for Motor Protection (NEC Article 430)
Fuses protecting motor circuits are sized based on the motor full load current (FLC):
- Ifuse: Fuse current rating (Amps)
- FLC: Motor full load current (Amps) from manufacturer data or NEC tables
- Kfuse: Multiplier, typically 125% to 175% depending on motor type and application
4. Breaker Interrupting Rating Check
To ensure the breaker can safely interrupt fault current:
- Ifault: Calculated prospective fault current at breaker location (Amps or kA)
- Iinterrupting rating: Breaker interrupting capacity (kA)
- Breaker must have interrupting rating equal or greater than maximum fault current
Real-World Application Examples
Example 1: Coordination Between Two Overcurrent Relays on a Feeder
A 480 V feeder is protected by two inverse time overcurrent relays: Relay A (upstream) and Relay B (downstream). Relay B protects a 100 A load and Relay A protects the upstream feeder section. The fault current at Relay B location is 5,000 A, and at Relay A location is 10,000 A. Determine suitable relay settings to ensure coordination with a minimum CTI of 0.3 seconds.
- Relay B pickup current (Ipickup_B) = 1.2 × 100 A = 120 A
- Relay A pickup current (Ipickup_A) = 1.5 × 100 A = 150 A (higher to avoid nuisance trips)
- Time Dial Setting (TDS) for Relay B = 0.2 (chosen for faster operation)
- Time Dial Setting (TDS) for Relay A = to be calculated
Using the Standard Inverse curve constants (K=0.14, α=0.02, C=0):
Calculate operating time for Relay B:
(41.67)0.02 ≈ 1.15
tB = 0.2 × 0.14 / (1.15 – 1) = 0.028 / 0.15 ≈ 0.187 seconds
To maintain CTI ≥ 0.3 seconds:
Calculate TDS for Relay A to achieve tA = 0.487 seconds:
(66.67)0.02 ≈ 1.17
TDS = t × (1.17 – 1) / 0.14 = 0.487 × 0.17 / 0.14 ≈ 0.592
Therefore, set Relay A TDS to approximately 0.6 to ensure coordination.
Example 2: Fuse Sizing for a 15 HP Motor per NEC 430
A 15 HP, 460 V, 3-phase motor has a full load current (FLC) of 21 A (from NEC Table 430.250). Determine the appropriate fuse size for motor branch circuit protection.
- NEC recommends fuse sizing between 125% and 175% of FLC.
- Calculate minimum fuse rating:
- Calculate maximum fuse rating:
Select the nearest standard fuse rating between 30 A and 35 A. A 30 A fuse is typically chosen to balance protection and motor starting current.
Additional Technical Considerations
- Relay Coordination Curves: IEEE C37.112 provides multiple curve types to tailor protection based on system characteristics.
- NEC Compliance: NEC Article 240 and 430 provide mandatory guidelines for overcurrent protection device sizing and coordination.
- Breaker Interrupting Capacity: Always verify that the breaker interrupting rating exceeds the maximum available fault current to prevent catastrophic failure.
- Coordination Time Interval: Industry practice typically uses 0.3 to 0.5 seconds as the minimum CTI to ensure selectivity.
- Motor Starting Currents: Consider inrush currents when setting pickup currents and fuse sizes to avoid nuisance tripping.
- System Voltage Levels: Protection settings vary significantly with system voltage; always use voltage-appropriate data.
Authoritative References and Standards
- IEEE Standard C37.112-2016 – Inverse Time Overcurrent Relays
- National Electrical Code (NEC) – NFPA 70
- Schneider Electric Protection Relay Technical Guides
- Eaton Overcurrent Protection Solutions
Protection coordination calculators based on IEEE and NEC standards are indispensable tools for electrical engineers. They ensure system safety, reliability, and compliance with regulatory requirements.