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Protection Coordination Concepts Guide

Educational Overview · Not a TCC-Plotting Calculator

What this page is: A conceptual primer on protective device coordination — what it means, how time-current curves (TCCs) express it, and why a real coordination study needs manufacturer trip-curve data and dedicated software. It is not a numeric TCC-plotting tool. For a hands-on, interactive TCC simulator built around illustrative sample devices, see the Breaker Coordination Simulator.

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What is protective device coordination?

Coordination (also called selectivity) means arranging protective devices — fuses, circuit breakers, and relays — so that when a fault occurs, the device closest to the fault clears it before any upstream device operates. The goal is to isolate only the smallest necessary portion of the electrical system, keeping the rest of the plant or building energized. A well-coordinated system might trip a single branch breaker for a fault on that branch, while an uncoordinated system could trip the main breaker for the same fault — blacking out the entire building over a problem confined to one circuit.

Time-current curves (TCC): the language of coordination

Every overcurrent protective device has a time-current characteristic — a curve (traditionally plotted on log-log paper) showing how long the device takes to operate at a given current. Three regions matter most:
  • Pickup (minimum trip current) — the current above which the device begins timing toward a trip; below pickup, it does not operate.
  • Time-delay (long-time / short-time) band — a region where the device intentionally waits before tripping, giving downstream devices a chance to clear the fault first.
  • Instantaneous trip — a current level above which the device trips with no intentional delay, usually reserved for very high fault currents close to the device.
Coordination studies overlay the TCCs of every device in a series (radial) path on the same log-log plot. If the curves are stacked so each downstream curve sits fully below and to the left of the next upstream curve — with adequate separation — the devices are coordinated across the full range of possible fault currents.

The coordination margin (interval)

Between two adjacent device curves, engineers maintain a time margin — commonly cited as roughly 0.2 to 0.4 seconds — to account for breaker interrupting time, relay overtravel, and curve tolerance, so the downstream device reliably finishes clearing before the upstream device begins to operate.
This 0.2–0.4 sec figure is a widely-cited rule of thumb, not a fixed code requirement — actual required margin depends on the specific devices, their published tolerance bands, and interrupting times, and is verified device-by-device in a real study.

Selective vs. non-selective coordination

  • Selective (fully coordinated): every device in the series path clears faults within its own zone without causing any upstream device to operate, across the entire range of available fault current. This is the design goal for most distribution systems.
  • Non-selective (partially coordinated): coordination holds only over part of the fault-current range — often at high fault currents, where instantaneous trip elements on both devices may operate together (a compromise sometimes accepted intentionally for arc-flash energy reduction, or accepted as a documented limitation where full selectivity isn't achievable).

Why this needs real device data and dedicated software

Meaningful coordination requires the actual published trip-curve data for the specific breaker/relay/fuse models being installed — pickup settings, time-current bands, and instantaneous trip points are unique to each manufacturer and model, and are frequently adjustable in the field (relay settings, breaker trip units). A generic calculator cannot substitute for this device-specific data. Real coordination studies are performed with:
  • Dedicated power-system analysis software (SKM PowerTools/CAPTOR, EasyPower, ETAP, and similar) that stores manufacturer TCC libraries and overlays them automatically.
  • Manual TCC log-log paper plots (or their digital equivalent) for simpler radial systems, tracing each device's published curve by hand.
  • Coordination combined with the short-circuit study — the available fault current at each point (see the Short-Circuit Study Calculator) sets the current range over which coordination must actually hold.

About the Protection Coordination Concepts Guide

This page is a conceptual overview of protective device coordination (selectivity) for electrical distribution systems — what it means, how time-current curves express it, the margin engineers target between adjacent devices, and why a real coordination study depends on manufacturer-specific trip-curve data and dedicated software rather than a generic calculator.

Where coordination fits in a power system study

Protection coordination is typically performed alongside a short-circuit study: the short-circuit study establishes the available fault current at every point in the system, and the coordination study then verifies that protective devices along each radial path clear faults selectively across that full current range. IEEE 242 (the "Buff Book") is the primary industry reference for protection and coordination of industrial and commercial power systems.

Common coordination scenarios

Fuse-to-fuse coordination compares published total-clearing and minimum-melt curves with a manufacturer-recommended ratio between upstream and downstream fuse ampere ratings. Breaker-to-breaker coordination compares long-time, short-time, and instantaneous trip settings on adjustable trip units. Fuse-to-breaker and relay-to-relay coordination each have their own established techniques, all built around comparing published or configured time-current characteristics on a common log-log plot.

What this guide intentionally does not do

This page does not plot device-specific TCC curves or determine actual coordination for real equipment, because that requires the exact published (or field-configured) trip-curve data for the devices actually being installed. For a hands-on, illustrative TCC plot built around sample breaker settings, see the Breaker Coordination Simulator in this studio's Simulators tab — but for an actual installation, coordination must be verified with the real device curves in dedicated software or by a qualified protection engineer.

Frequently asked questions

What does "coordination" mean in electrical protection?

Coordination (selectivity) means protective devices are set so the device closest to a fault clears it first, isolating only the smallest necessary part of the system and leaving the rest of the installation energized.

What is a time-current curve (TCC)?

A TCC is a plot (traditionally on log-log paper) of how long a protective device takes to operate at a given fault current. It shows the pickup point, any intentional time-delay bands, and the instantaneous trip level, and is the basic tool used to compare devices for coordination.

How much time margin is needed between two devices?

A commonly cited rule of thumb is roughly 0.2–0.4 seconds between adjacent device curves, to allow for breaker interrupting time, relay overtravel, and curve tolerances. This is a general guideline, not a fixed code value — the actual required margin depends on the specific devices involved and is confirmed in the coordination study.

Can I use a simple calculator to check coordination for my building?

Not reliably. Real coordination depends on the exact trip-curve data (and field-configured settings) of the specific breakers, fuses, or relays installed. That data has to come from the manufacturer and be overlaid in dedicated software (SKM, EasyPower, ETAP) or plotted manually — a generic calculator without real device curves cannot verify actual coordination.

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