ISO 10816 does not judge a machine on a single pass or fail line. It divides the range of possible vibration into four evaluation zones, A through D, separated by three boundaries that each carry a different operational meaning. The boundary you cross tells you whether a machine is running like new, is fit for the long haul, needs a repair planned, or should be stopped. Because those boundary values move with the machine group and how the machine is mounted, getting the classification right is what makes a zone verdict trustworthy.
ISO 10816 Zone Boundary in one line: An ISO 10816 zone boundary is one of the three dividing lines between the standard's four evaluation zones: the A/B boundary, the B/C boundary, and the C/D boundary. Each boundary is expressed as an overall RMS velocity in millimeters per second, and its value depends on the machine group and whether the support is rigid or flexible. Crossing the B/C boundary means plan a repair, and crossing the C/D boundary means the vibration can cause damage.
ISO 10816 grades a machine using its overall RMS velocity and drops that single number into one of four evaluation zones. Zone A is the range typical of a machine that has just been commissioned and is in excellent condition. Zone B covers vibration considered acceptable for long-term unrestricted operation, so a machine anywhere in A or B is fit to keep running without special concern. Zone C is the restricted range, where the machine is regarded as unsatisfactory for continuous long-term running and should be treated as usable only until a repair opportunity arises. Zone D is severe enough that the vibration is considered capable of causing damage.
What matters operationally is not the zones themselves but the boundaries that separate them, because a boundary is the exact point at which the verdict changes. The A/B boundary is the line between new-machine condition and normal acceptable operation, and it is mostly of interest at commissioning and for setting a baseline. The B/C boundary is the important one for day-to-day monitoring: crossing it means the machine has left the range that is acceptable for the long term and someone should schedule a repair. The C/D boundary is the most serious: crossing it means the vibration has reached a level that can damage the machine, and continued running is a genuine risk.
The zones give a plain-language answer to the acceptance question without an operator having to interpret a raw number in isolation. A reading sitting comfortably inside zone B needs no action, a reading that has drifted across the B/C boundary calls for planning, and a reading past the C/D boundary calls for a stop. What the boundaries deliberately do not tell you is the cause. A machine sitting in zone C is vibrating too much, but whether the culprit is imbalance, misalignment, or a bearing fault is a question for spectral analysis, not for the zone chart.
The single feature that trips up newcomers is that the boundaries are not fixed millimeter-per-second numbers. The same overall velocity that spells trouble on a small machine can be perfectly normal on a large one, so ISO 10816 sorts machines into groups by characteristics such as size, power, and type. Each group gets its own set of boundary values, and a larger, more robust machine tolerates a higher velocity before reaching the B/C and C/D boundaries than a small one does. Applying a large turbine's boundaries to a small pump, or the reverse, produces alarms that either never fire when they should or fire constantly when nothing is wrong.
Foundation flexibility shifts the boundaries as well. ISO 10816 distinguishes between machines on rigid supports and machines on flexible supports, because the way vibration at the bearing housing relates to the forces and stress inside the machine changes with the stiffness of the base. A flexibly mounted set generally has higher boundary velocities than a rigidly mounted one of the same group, since a flexible foundation moves more freely for the same internal force. The classification of rigid versus flexible is a judgment about the whole machine-and-foundation system near its running speed, not just about how the bolts look.
Getting both dimensions right, the machine group and the support condition, is what makes the boundary values meaningful for a given machine. In practice an engineer picks the correct table for the machine group, then reads the rigid or flexible column to find the actual A/B, B/C, and C/D velocities. Only after that classification is the raw mm/s reading comparable to a boundary. This is also why two identical readings on two different machines can land in different zones: the numbers are the same but the boundaries they are measured against are not.
The zone boundaries translate almost one for one into the alert and danger setpoints that a monitoring system uses. The B/C boundary is the natural alert or warning level, because crossing it is the moment the machine leaves acceptable long-term operation and a repair should be planned. The C/D boundary is the natural danger or trip level, because crossing it means the vibration can now do damage. Since the boundaries are defined per machine group and support condition, a monitoring platform that knows each machine's classification can pre-populate sensible setpoints from the standard rather than leaving an operator to guess a number out of the air.
A cloud SCADA historian is a good home for this because it continuously logs the overall RMS velocity from each measurement point and compares it against the configured boundaries. Merobix trends the broadband value against the group-appropriate zone boundaries so that a machine drifting across the B/C boundary raises a warning automatically, and a machine reaching the C/D boundary raises a danger alarm, without anyone having to watch every reading. For a fleet of pumps, motors, and fans spread across remote sites, that automatic comparison to a recognized standard is what makes wide vibration coverage practical rather than aspirational.
Trending the value against the boundaries over time adds what a single comparison cannot: the rate of approach. A machine can sit safely inside zone B for years, but a value climbing steadily toward the B/C boundary is a developing problem worth investigating before it ever crosses. Historized data lets an operator establish each machine's normal position within its zone and watch for the upward drift toward a boundary, which turns the standard from a one-time acceptance gate into a running early-warning tool. When a boundary alert does fire, the same history shows how fast the machine got there, which helps decide whether to intervene now or at the next planned outage.
The B/C boundary is the line between acceptable long-term operation and restricted operation, so crossing it means the machine should have a repair planned but can usually keep running for a limited period. The C/D boundary is the line into the damage-capable range, so crossing it means the vibration is severe enough to harm the machine and continued running is risky. In monitoring terms, the B/C boundary maps to a warning setpoint and the C/D boundary maps to a danger or trip setpoint.
No. The boundary values in millimeters per second change with the machine group, which is based on size, power, and type, and they also change depending on whether the machine sits on a rigid or a flexible support. A large machine on a flexible foundation has higher boundary velocities than a small rigidly mounted one. That is why the same reading can land in zone B on one machine and zone C on another, and why correct classification has to come before any comparison to a boundary.
ISO 20816 is the current standard that consolidates and supersedes the ISO 10816 series, and it keeps the same four-zone A through D scheme with boundaries that depend on machine category and mounting. The evaluation philosophy carries across, so many operators still refer to the limits by the ISO 10816 name even when the governing document is now ISO 20816. For setting broadband warning and danger setpoints, the intent of the two is aligned.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.