ISO 20816 Vibration Severity Zones: A, B, C and D Explained

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.

From ISO 10816 to ISO 20816

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; only the document number changed.

What the Four Zones Mean

The standard splits the vibration severity scale into four consecutive zones, each a judgement about condition rather than just a magnitude band:

• Zone A: newly commissioned machines normally fall here. Baseline, as-new condition.
• Zone B: acceptable for unrestricted long-term operation.
• Zone C: unsatisfactory for continuous operation; the machine may run for a limited period until remedial action can be scheduled.
• Zone D: severe enough to be considered likely to cause damage.

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.

RMS Velocity as the Measured Quantity

ISO 20816 uses broadband RMS vibration velocity, expressed in mm/s, measured on the bearing housing or other accessible non-rotating structure, typically over 10 Hz to 1,000 Hz. 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 three directions at each bearing, and the assigned zone is set by the highest value recorded.

Machine Groups and Support Type

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.

• Group 1: machines rated above 300 kW (up to 50 MW), and electrical machines with shaft height H ≥ 315 mm.
• Group 2: machines rated 15 kW to 300 kW, and electrical machines with shaft height 160 mm ≤ H < 315 mm.

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.

Typical Zone Boundaries (mm/s RMS)

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.

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.

Using Zones for Acceptance Testing and Alarm Setpoints

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 differs from, and complements, the fixed protection limits in API 670 machinery protection systems on critical turbomachinery, which use absolute trip values rather than a relative zone framework.

Teams increasingly load these boundaries as thresholds so a bearing reading crossing from Zone B into Zone C raises a work order automatically. Fabrico's CMMS attaches ISO 20816 zone thresholds to asset vibration readings and escalates a Zone D crossing immediately; 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. 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.

Frequently Asked Questions

Does ISO 20816 apply to all rotating machines?

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.

Is a higher zone always dangerous?

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.

Can I compare readings between ISO 10816 and ISO 20816 reports?

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.

Why does support stiffness change the limits?

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.