ISO 20816-3 vibration severity zones explained
What zones A to D mean, how machine group and foundation set the limits, and how to turn the standard into alarm levels for your machines.
What the standard covers
ISO 20816-3 gives guidance for judging the vibration of industrial machines from measurements on non-rotating parts, usually the bearing housings. It replaced ISO 10816-3 and applies to machines with a power above 15 kW and operating speeds between 120 and 30,000 rpm: pumps, fans, compressors, motors, generators and similar equipment.
The measure is broadband vibration velocity in mm/s RMS, typically over 10 to 1000 Hz, or 2 to 1000 Hz for machines running below 600 rpm. One number per measurement point, compared against zone limits.
The four zones
| Zone | Meaning |
|---|---|
| A | Vibration typical of newly commissioned machines. |
| B | Acceptable for unrestricted long-term operation. |
| C | Not suitable for long-term continuous operation. The machine can run for a limited time until a suitable opportunity for remedial action. |
| D | Severe enough to cause damage to the machine. |
Machine groups and foundations
The limits depend on two things:
- Machine group. Group 1 covers large machines, rated above 300 kW, or electrical machines with a shaft height of 315 mm or more. Group 2 covers medium machines, 15 to 300 kW, or electrical machines with a shaft height from 160 mm up to 315 mm.
- Foundation. A support is rigid in a given direction when the lowest natural frequency of the machine on its support is at least 25 % above the main excitation frequency, usually the running speed. Otherwise it is flexible. Large machines on steel frames are often flexible; machines bolted to massive concrete blocks are usually rigid.
Zone boundaries, velocity in mm/s RMS
| Machine group | Support | A / B | B / C | C / D |
|---|---|---|---|---|
| Group 1, large | Rigid | 2.3 | 4.5 | 7.1 |
| Group 1, large | Flexible | 3.5 | 7.1 | 11.0 |
| Group 2, medium | Rigid | 1.4 | 2.8 | 4.5 |
| Group 2, medium | Flexible | 2.3 | 4.5 | 7.1 |
These are the boundaries published in ISO 10816-3 and carried into ISO 20816-3 for these machine groups. Check the current edition of the standard, and any machine-specific standard such as ISO 10816-7 for rotodynamic pumps, before using them for acceptance testing.
From zones to alarm levels
The standard suggests two operational limits:
- Alarm, a warning that vibration has changed and needs attention. A common starting point is the machine's normal baseline plus 25 % of the upper limit of zone B, and normally no higher than 1.25 times the B/C boundary.
- Trip, the level where continued running risks damage. It normally does not exceed 1.25 times the C/D boundary.
A baseline matters as much as the table. A machine that has always run at 1.0 mm/s and jumps to 2.5 mm/s has changed, even though 2.5 mm/s is still in zone B for many machines. Trending each point against its own history catches that change; a fixed zone limit alone does not.
What a severity number can't tell you
Overall velocity says how much the machine vibrates, not why. Unbalance, misalignment, looseness, a bearing defect and a resonance can all produce the same zone C reading. Finding the cause needs the spectrum, the time waveform and, for bearings, the envelope: see bearing fault frequencies.
How Misoki uses this
Misoki condition monitoring shows ISO-zone severity on every asset, with alarm limits you set per point and axis. Prism AI then looks past the number to name the fault behind it and grade how serious it is.
See your machines against the zones.
Send us a week of vibration data and we walk you through severity and the faults behind it.