ISO Vibration Severity In Plain English: Zones, Groups, And The Decisions They Should Change

What the standard says

ISO 20816-3 (the successor to the widely quoted ISO 10816-3) evaluates machine condition from broadband vibration velocity, in mm/s RMS, measured on non-rotating parts — in practice, the bearing housings. It applies to coupled industrial machines in the medium and large power range at typical industrial speeds.

The standard sorts a reading using three pieces of context:

1. Machine group. Group 1 covers large machines (above roughly 300 kW / 402 hp); Group 2 covers medium machines (roughly 15–300 kW / 20–402 hp). Larger machines, with more mass and wider stances, tolerate somewhat higher housing vibration.
2. Support class. A machine on a rigid foundation is judged more strictly than one on flexible mounts, because flexible supports cushion the same energy into larger but less damaging motion.
3. Evaluation zone. Zone A is where newly commissioned machines should sit. Zone B is acceptable for unrestricted long-term operation. Zone C is not acceptable for continuous long-term running — the machine may operate for a limited period while a remedy is planned. Zone D is vibration severe enough to cause damage.

For the most common case — a Group 2 machine on a rigid base — the published boundaries fall near 1.4 mm/s (A/B), 2.8 mm/s (B/C), and 4.5 mm/s (C/D). A 75 kW pump motor trending at 2.2 mm/s is a Zone B machine: fine to run, worth watching. The same reading on flexible mounts sits even more comfortably.

What it means on the plant floor

The zone tables are screening bands, not verdicts. They answer one question well: for a machine of this size on this foundation, is this level of overall vibration ordinary or not? They were never designed to say why a machine vibrates or which component is degrading.

Three field realities follow:

• Trend beats threshold. A machine that moved from 1.1 to 2.4 mm/s in six weeks deserves more attention than one that has sat at 3.0 mm/s for five years. The standard itself treats a significant change as an evaluation criterion, independent of zone.
• Context decides comparability. Speed, load, and operating state move the number. A reading captured during startup or low load is not comparable to one captured at steady production — which is why ISO 17359 puts operating-state context at the centre of any monitoring programme.
• Zones set starting bands where history is missing. On a newly instrumented asset with no baseline, the B/C boundary is a defensible first alert level and the C/D boundary a defensible first action level — until enough trend history exists to tighten them.

The three decisions this should change

1. Initial alarm bands on new monitoring points. Set the first alert at the B/C boundary for the machine’s group and support class, and the first action threshold at C/D — then re-derive both from the machine’s own history after a few months of trending. Do not ship a monitoring programme with one global threshold for every asset.
2. Acceptance criteria at commissioning and after overhaul. Zone A/B is a concrete, citable hand-back criterion for a rebuilt fan or a newly installed pump set. Writing it into work-order close-out turns a vague “runs smooth” into a measured acceptance.
3. Escalation discipline when a machine enters Zone C. Crossing into C starts a planning clock — inspect, diagnose, and schedule correction within a limited running period. It is neither “ignore it” nor “stop the line now.” Codifying that middle response is where most alarm-fatigue problems on vibration points get solved.

Where it stops being useful

• Early bearing defects hide below it. Broadband velocity dilutes small high-frequency impacts across the whole band; a bearing can progress well into a defect while the overall reading stays in Zone B. Bearing-specific coverage needs envelope or high-frequency methods — the zone table cannot carry that job.
• It is not a universal table. Reciprocating machinery, machines with special foundations, and several specific machine types are covered by other parts of the ISO 10816/20816 series with different criteria. Applying Part 3 boundaries to a diesel engine or a vertical pump without checking is a category error.
• Boundaries are not contractual physics. The values are consensus screening levels. A precision grinder may be unacceptable well inside Zone B; a rugged crusher may run for years brushing Zone C. The machine’s own history, criticality, and failure consequence always outrank the generic table.