A Job Shows a Zero-Length Bar on the Gantt

A job that shows a zero-length or backwards bar on the Gantt has an operation with no duration: its scheduled end equals or precedes its start. The overwhelming cause is a routing step whose per-unit hours and setup time are both zero, which produces a zero-duration operation. If the step is genuinely meant to be instant, it belongs as a material step rather than a timed work-center step. The fix is quick once you see which case you have.

EDGEBIC by User Solutions flags a schedule row whose end date is at or before its start date as a critical plan inversion, because such a row cannot draw a real bar and breaks the date math for every step that depends on it. This post, part of the EDGEBIC troubleshooting guide, walks the causes and the correct model for instant steps.

What a Zero-Length Bar Means

A bar's width is the elapsed time from an operation's start to its end. When the end equals the start, there is nothing to draw, so the bar collapses to a sliver or vanishes. When the end precedes the start, the bar would run backwards. Either way the schedule row is invalid, and the plan-inversion checks flag it:

• Schedule end at or before start flags an operation whose window has zero or negative length.
• Resource end at or before start flags the underlying allocation with the same problem.

This is a single-operation problem, distinct from the whole Gantt showing nothing after a run, where no bars appear at all. Here the job is scheduled; one of its operations just has no length. The opposite symptom, a bar spanning far more calendar time than its hours account for, is covered in a schedule bar wider than the hours suggest.

Cause 1: A Routing Step With No Hours and No Setup

The most common cause is a work-center step whose per-unit hours and setup time are both zero. The engine multiplies per-unit hours by quantity and adds setup to get the operation's duration; if both are zero, the duration is zero, and the operation ends the instant it starts.

How to tell: open the routing and read the flagged step. Per-unit hours zero and setup zero together produce this exactly.

Fix: decide what the step really is. If it takes real machine time, even a few minutes, give it a non-zero per-unit hours value so it has a duration to schedule. If it takes no time at all, it should not be a work-center step; convert it to a material step (next section).

Cause 2: An Instant Step Modeled as a Work-Center Step

Some routing events are genuinely instant: a stock issue, a paperwork gate, a logical marker. Modeled as a work-center step with zero hours, they trip the plan-inversion check every time. The correct shape for an instant event is a material step, which has no work center and no hours and is handled as an instant assignment rather than a timed operation.

How to tell: the step represents something that consumes no machine capacity and no clock time, yet it is set up as a work-center step.

Fix: model it as a material step. See how to add a material step to a routing. A material step is placed instantly at the right point in the sequence without trying to draw a timed bar, so the plan-inversion check never fires on it.

Cause 3: An Allocation That Computed Zero Effective Hours

Occasionally a fresh routing produces a zero-length operation after a reschedule even though the step looks fine, because the allocation computed zero or negative effective hours and stamped the end equal to the start. A minimum-allocation threshold that nothing met can leave the operation empty.

How to tell: the routing hours look correct, but the flag appears after a reschedule.

Fix: re-run scheduling for the job. A clean pass rebuilds the allocation from the routing. If the operation still shows zero length, fall back to cause 1 and confirm the step's hours are genuinely non-zero.

How to Confirm the Fix

After correcting the step and re-running scheduling, re-open the anomaly report and confirm the plan-inversion chips return zero rows for the job. On the Gantt, the operation should now draw a visible bar with a real width (or, if you converted it to a material step, appear as an instant marker in sequence rather than a timed bar). The end date must sit after the start date on every operation.

Prevention

• Never leave a work-center step at zero hours. A timed step is meant to consume time; give it real per-unit hours or setup, or it will collapse.
• Use material steps for instant events. Stock issues, gates, and markers belong as material steps, which are built to be instant and never trip the plan-inversion check.
• Audit routings after import. If routings arrive from your ERP with blank hour columns, a scheduling pass and an anomaly-report read surface any zero-duration steps before they reach a plan.
• Treat any plan inversion as critical. These checks are critical for a reason: a zero or backwards operation corrupts the date chain for everything downstream of it, so clear them before trusting the plan.