Two-device Coordination — Decisive Engineering

A fuse downstream, a breaker upstream. Both protect the same feeder. You want the fuse to clear any fault on its load side before the breaker upstream of it picks up.

The plot below is miscoordinated on purpose. There’s overlap between the two curves — and the tutorial has flagged it for you. Your job is to make the overlap disappear by adjusting the upstream LVPCB’s settings.

What you’re seeing

The blue staircase is the upstream LVPCB. The pink band is the downstream fuse. The little red ✕ markers show where the two curves cross — at those currents, the upstream breaker would open before the downstream fuse finishes clearing.

That’s a coordination failure. The downstream fuse should win every race.

Try this — raise the short-time delay first

The first overlap appears just above the feeder LVPCB’s short-time pickup (3× sensor = 3,600 A). Above that current the upstream’s S shelf sits at 0.05 s while the downstream fuse’s max-clear curve is still up around several seconds. Upstream beats downstream.

Drag S delay up. Try 0.3 s. Watch the upstream’s middle shelf rise off the bottom of the plot. The right cluster of markers will move or disappear as the S shelf lifts above the fuse band.

Then push the short-time pickup right

There’s still a stripe of overlap because the LVPCB enters its S region too early — below the fuse’s current-limiting threshold, the fuse is still slow enough that the S shelf (even at 0.3 s) can beat it on some currents.

Push S pickup up. Try 8× sensor. Now the upstream LVPCB stays in its long-time region until 9,600 A. Below that current, only the long-time band is active, and it’s much slower than the fuse band.

And the instantaneous

If your I pickup is set low enough that it overlaps the current-limiting drop of the fuse, the breaker might pick up before the fuse finishes opening. Drag I pickup all the way left to 0 to turn instantaneous OFF entirely.

You can do that because the upstream LVPCB has a short-time withstand rating designed exactly for this scenario — it survives the bolted fault current long enough for the downstream device to clear.

What “coordinated” looks like

When the red markers are gone, the two curves no longer touch. For every fault current on the x-axis, the downstream fuse’s max-clear time is below the upstream LVPCB’s L/S/I curve.

You’ve achieved the goal of Lesson 4 — selective coordination: for any overcurrent condition, only the device immediately upstream of the fault opens.

Three failure modes you just navigated:

Inverse-time overlap — happens when the upstream device’s long-time band runs into the downstream device’s trip times. Fix by raising the upstream device’s L delay or pickup.
Short-time-on-fast-clearing — happens when the upstream’s flat S shelf is faster than the downstream’s curve at the same current. Fix by raising S delay, raising S pickup, or both.
Instantaneous-on-instantaneous — happens when both devices can trip in the same cycle on the same fault. Fix by turning the upstream instantaneous OFF, or by pushing its pickup above the maximum fault available at the downstream device.

A note on minimum separation

Real coordination studies don’t just ask that the curves not cross. They ask for a minimum time margin between curves — typically 0.1 second across the current range of interest, sometimes more if the authority having jurisdiction is conservative.

This margin exists because the plotted curves are nominal. A real downstream device can clear a little slower than its published curve (manufacturing tolerance, temperature, pre-loading), and a real upstream device can pick up a little faster than its published curve. Either drift eats into the separation you actually get, so the 0.1 s buffer is there to keep the two devices coordinated even after both have moved the wrong way.

For this exercise, getting the red markers to disappear is enough — but for a real study you’d be checking margin numerically against the curves’ published tolerances.

What’s next

Two devices in series is the simplest case. The interesting case is three devices — service main, switchgear feeder, motor branch — each one feeding the next, each one needing to coordinate with both its upstream and downstream neighbors. That’s Lesson 6.