Overcurrent protection & TCC coordination

Reference notes

Use Next → on the narrator above to step through overcurrent protection and time-current-characteristic coordination — the most common protection topic on the PE Power exam.

ANSI device numbers

• 50 — instantaneous overcurrent. Trips with no intentional delay when current exceeds a high threshold.
• 51 — time-overcurrent. Trips after a delay that depends on the current level, with an inverse-time characteristic.
• 50N / 51N — residual ground-fault elements (driven by the sum of phase CTs through a neutral path).
• 67 / 67N — directional overcurrent (adds a polarizing voltage or sequence input to distinguish forward from reverse fault flow). Required on parallel-source or ring topologies.

TCC curve mathematics

The IEEE C37.112 standard inverse curve family is:

where:

• t = trip time, seconds
• TD = time-dial multiplier (selected by the engineer; typically 0.5 to 10)
• M = I / I_pickup (current as a multiple of pickup)
• A, B, p = curve-shape constants per the table below

Setting the pickup

Pickup is the current below which the time-overcurrent element will never trip. Standard practice:

• Feeder breaker: pickup ≈ 1.25 × maximum continuous load current (rides through cold-load inrush, harmonics, motor starting).
• Motor protection: pickup ≈ 1.5 × motor FLA (rides through normal start inrush of ~6× FLA for a few seconds).
• Transformer primary: pickup ≈ 1.0 × FFLA (full-load amps), with NEC 450.3 maximum at 250–600 % depending on configuration.
• Ground fault (51N): pickup ≈ 10–20 % of phase pickup, low enough to detect arcing ground faults that don't trip the phase elements.

Setting the time-dial

Once pickups are chosen, set the time-dial (TD) so the upstream device's curve sits the required CTI above the downstream device's curve at the maximum fault current. Procedure:

1. Pick the most-downstream device's curve shape and pickup. Set its TD to give minimum acceptable trip time at the maximum fault.
2. For the next device upstream: compute the downstream device's trip time at the max-fault current it would see. Add the CTI. Solve the TCC equation for the TD that gives this total trip time at the same current.
3. Repeat all the way to the utility interface.

Coordination time interval (CTI)

The minimum vertical gap between adjacent TCC curves at the maximum fault current. Typical values:

• 0.30 s with modern microprocessor relays and MV vacuum breakers.
• 0.40 s with older electromechanical relays.
• 0.20 s with very fast LV breakers (e.g., insulated-case or air circuit breakers).

The CTI budget breaks down roughly as: breaker opening time (5 cycles ≈ 0.083 s for MV vacuum), relay overtravel (~0.05 s for electromechanical, ~0 for digital), and a safety margin (~0.15 s).

Coordination example

Three devices in series: MCC feeder → plant main → utility incomer. At the maximum 3-φ fault current at the MCC bus:

• MCC feeder curve should trip in ~0.10 s.
• Plant main curve should be at ~0.40 s (0.30 s above).
• Utility curve should be at ~0.70 s (another 0.30 s above).

Each layer of protection provides backup for the layer below — if the MCC breaker fails, the plant main clears the fault 0.30 s later; if both fail, the utility breaker clears 0.30 s after that. This is "fully selective" coordination.

Keyboard shortcuts

• → next step  ·  ← previous step
• R replay narration  ·  M mute / unmute  ·  F fullscreen
• Click anywhere on the TCC plot to move the fault-current cursor and see which device trips first.