ISO 20816 vibration severity zones A, B, C and D give rotating equipment engineers a way to judge whether a measured vibration level is healthy, tolerable, or a warning that damage is underway. The standard translates a single number, RMS vibration velocity in millimetres per second, into an operating verdict a shift supervisor can act on without a vibration analyst on call.
ISO 20816 is the current standard for evaluating mechanical vibration of machines from measurements on non-rotating parts. It supersedes the older ISO 10816 series, part by part, as each section is revised. Part 3 (ISO 20816-3) replaced ISO 10816-3 for industrial machines above 15 kW and speeds between 120 and 30,000 rpm.
The zone concept and A/B/C/D letters carry over unchanged. If your CMMS or acceptance reports still cite ISO 10816-3 vibration severity limits, the boundaries are the same ones now published under ISO 20816-3; the casing velocity values did not change, but ISO 20816-3 also absorbed the shaft vibration criteria of ISO 7919-3, so new reports should cite the new number.
The standard splits the vibration severity scale into four consecutive zones, each a judgement about condition rather than just a magnitude band:
Zones A and B are operate-freely territory. Zone C is a planning trigger, not an emergency. Zone D means stop the machine or accept accelerated wear and failure.
ISO 20816 uses broadband RMS vibration velocity, expressed in mm/s, measured on the bearing housing or other accessible non-rotating structure, over 10 Hz to 1,000 Hz, or 2 Hz to 1,000 Hz for machines running below 600 r/min. RMS velocity is preferred over peak displacement because it correlates well with fatigue-inducing energy in the mid-frequency range where unbalance, misalignment, and looseness signatures live.
Readings are usually taken in two perpendicular radial directions at each bearing housing, and the zone is set by the highest radial value. Axial readings are useful for diagnosis, but the zone limits apply to axial vibration only on thrust bearings.
A single severity table cannot fit every machine: a large turbogenerator and a small pump motor tolerate very different absolute vibration for the same relative condition. ISO 20816-3 handles this by dividing industrial machines into two groups by power and shaft centre height, then splitting each group again by support stiffness.
Within each group, rigid support foundations carry lower absolute limits than flexible ones, since a flexible foundation absorbs and redistributes energy differently than a stiff one. Getting the group and support type right is the most common source of misapplied vibration alarms.
The table below gives ISO 20816-3 zone boundary velocities for rigidly mounted machines. Flexibly mounted machines in the same groups carry higher boundary values, so confirm mounting type before applying a table from memory.
| Machine group | Zone A/B boundary | Zone B/C boundary | Zone C/D boundary |
|---|---|---|---|
| Group 1 (above 300 kW, rigid support) | 2.3 mm/s | 4.5 mm/s | 7.1 mm/s |
| Group 2 (15 to 300 kW, rigid support) | 1.4 mm/s | 2.8 mm/s | 4.5 mm/s |
A Group 2 pump motor on a rigid base measuring 2.0 mm/s RMS sits comfortably in Zone B; the same reading on a smaller, flexibly mounted unit might still be within Zone A. Context matters more than the raw number.
Zones serve two jobs: acceptance testing, where a new or overhauled machine should be commissioned in Zone A or within Zone B before sign-off, and ongoing monitoring, where alarm and trip setpoints are derived from the zone boundaries, adjusted for machine criticality.
This complements API 670, which specifies the hardware, installation and alarm and shutdown logic of permanent machinery protection systems on critical turbomachinery.
Teams increasingly load these boundaries as thresholds so a bearing reading crossing from Zone B into Zone C can trigger a follow-up task. Fabrico's CMMS attaches ISO 20816 zone thresholds to asset vibration readings and can send a notification the moment a reading crosses into Zone D; see it with a Fabrico demo .
A rising trend within a zone, say from 1.0 to 2.0 mm/s while still inside Zone B, is often a more useful early warning than the zone label itself. You only see that trend if readings are kept against the asset over time, which is what machine monitoring is for.
Persistent vibration in the upper range of a zone, especially on variable-speed equipment, warrants a look at rotor dynamics; see critical speed and rotordynamics for how resonance amplifies modest forces into Zone C or D readings.
Looseness signatures in the spectrum often trace back to the bearing itself: see our guide to bearing internal clearance classes (C2 to C5) for how the wrong clearance choice shows up in vibration data.
ISO 20816-3 gives a simple rule for alarms. Set the ALARM limit at your machine's own steady baseline plus 25 percent of the upper limit of Zone B, and do not normally let it exceed 1.25 times that Zone B limit. For a Group 2 motor on a rigid base, Zone B ends at 2.8 mm/s, so the step is 0.7 mm/s: a baseline of 1.2 mm/s gives an alarm at 1.9 mm/s, and no alarm should sit above 3.5 mm/s.
TRIP limits protect the machine's mechanical integrity and do not follow the baseline. The standard recommends keeping them no higher than 1.25 times the upper limit of Zone C, which is 5.6 mm/s for Group 2 rigid and 8.9 mm/s for Group 1 rigid. Revise the alarm whenever the baseline changes, for example after an overhaul.
For flexibly supported machines the boundaries are higher: Group 1 flexible uses 3.5, 7.1 and 11.0 mm/s, and Group 2 flexible uses 2.3, 4.5 and 7.1 mm/s. A support counts as rigid in a given direction when the lowest natural frequency of the machine and its support is at least 25 percent above the main excitation frequency, usually running speed. Otherwise it is flexible. A frame can be rigid vertically and flexible horizontally, and each direction is then judged against its own table.
A reading does not have to reach Zone C to need attention. ISO 20816-3 treats any step change, up or down, larger than 25 percent of the Zone B/C boundary as significant, which is 0.7 mm/s for Group 2 rigid and 1.1 mm/s for Group 1 rigid. When that happens, start a diagnostic check even if the zone has not changed. For new machines, acceptance values have historically been set in Zone A or Zone B, normally no higher than 1.25 times the A/B boundary: about 1.8 mm/s for Group 2 rigid and 2.9 mm/s for Group 1 rigid.
No single part covers everything. ISO 20816-1 sets out general principles; later parts address specific machine types, such as Part 3 for industrial machines, with others covering steam turbines, wind turbines, and reciprocating machines. Use the part written for the category being assessed.
Not immediately. Zone C means continuous operation is unsatisfactory, but the machine can typically keep running for a limited period while a repair is planned. Zone D is the point at which damage is considered likely, and continued operation should not be routine.
Yes. For parts that have been reissued, boundary velocity values are unchanged; only the standard number and some editorial content changed. A Zone B reading under ISO 10816-3 remains Zone B under ISO 20816-3.
A flexible foundation dissipates and transmits vibration energy differently than a rigid one, so the same fault produces a different measured velocity at the bearing housing. The standard raises boundary values for flexible supports to keep the severity judgement tied to actual condition, not foundation type.
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