A category 2 stop keeps power available to the machine actuators after motion has ceased, which is fundamentally incompatible with the ISO 13850 requirement that emergency stop functions remove drive energy as part of the stop [S1][S4].
Standards EN ISO 13850 and IEC 60204-1 define three stop categories, and only category 0 (uncontrolled, immediate power removal) and category 1 (controlled deceleration, then power removal) are listed as legally permissible emergency stop implementations [S4]. Category 2 is therefore excluded from the emergency stop function and is instead used for normal operational stops where the machine must remain poised in a powered, controlled state [S2][S3].
What Each Stop Category Actually Does
Category 0 is stopping by immediate removal of power to the machine actuators, an uncontrolled stop whose arrest depends on whatever mechanical friction or braking is already in the drivetrain [S4]. Category 1 is a controlled stop with power available to the machine actuators to achieve the stop, followed by removal of power when the stop is achieved [S4]. Category 2 is a controlled stop with power remaining available to the machine actuators, so the drive stays energised after motion has stopped [S3][S4].
The three categories are control-function definitions under EN 60204-1, not safety architecture categories, and stop categories should not be confused with EN 954-1 / ISO 13849-1 performance levels B, 1, 2, 3, 4 [S1]. A category 0 or 1 stop can be implemented at any required PL; the category describes what the stop does, not how reliably the safety chain has been engineered.
Why Category 2 Fails the Emergency Stop Test
Category 2 is not permitted for emergency stop functions because power remains on the drive elements after standstill, so a fault, a sticky pushbutton, or a wire break in the stop chain can re-energise motion while the operator still believes the machine is in a safe state [S1][S2].
The safety rationale is straightforward: an emergency stop must remove the energy that can cause the hazard, and a drive that stays powered is one firmware command or one welded contactor away from re-starting [S6]. ISO 13850 codifies this by listing only categories 0 and 1 as acceptable emergency stop implementations, with the choice between them driven by the risk assessment of the specific machine [S4][S9].
Risk Assessment Decides Between Category 0 and Category 1
For machines whose hazardous elements have negligible inertia and where stopping cannot create additional hazards, category 0 is the appropriate emergency stop category, because simply cutting power is the fastest and simplest way to arrest motion [S4].
For machines with high-inertia hazardous elements, or where unpowered stop creates secondary hazards (for example dropping a suspended load or letting a regenerative drive push back into the bus), category 1 is required so the drive can apply controlled deceleration before power is finally removed [S4][S6]. The selection rule is the same on both sides of the Atlantic: category 0 and category 1 are the only legal choices, and the risk assessment picks which one fits the machine [S5].
Drive-Level Mapping Under IEC 61800-5-2
IEC 61800-5-2 defines safety stop functions that map onto the three EN 60204-1 categories, with Safe Torque Off (STO) corresponding to a category 0 stop, Safe Stop 1 (SS1, with variants SS1-d, SS1-r, SS1-t) corresponding to a category 1 stop, and Safe Stop 2 (SS2, with variants SS2-d, SS2-r, SS2-t) corresponding to a category 2 stop [S4].
The SS1 and SS2 variants differ in how the transition into the final stop state is supervised: SS1-d controls the deceleration rate to set limits, SS1-r monitors the deceleration ramp, and SS1-t triggers STO after a fixed application-specific time delay, with the same d/r/t pattern applied to SS2 [S4]. Because SS2 ends in a Safe Operating Stop with power still available to the motor, it is the drive-level function that is explicitly excluded from use as an emergency stop. STO and SS1 are the only IEC 61800-5-2 functions that can back an ISO 13850 emergency stop [S4].
VFD-Only Emergency Stops Are Usually Not Compliant
A variable frequency drive that is not safety-rated cannot deliver an emergency stop on its own, because the stop command in a non-safety VFD does not meet the integrity requirements of the safety chain and the drive electronics themselves may not be relied upon to remove torque in a fault condition [S7].
Where a VFD is used, the machine builder normally pairs the drive with a safety module or a safety-rated VFD that supports STO or SS1 over a hardwired safety circuit, and the upstream contactor is wired to perform the final category 0 or category 1 power removal required by the risk assessment [S7]. The relevant emergency stop button actuator is then placed in series with the safety circuit, not used as a software stop bit in the drive.
Common Misuses and Selection Checklist
Misuse pattern one: a designer uses a category 2 stop because the machine needs to hold position or torque at standstill (for example, a vertical axis or a web tension loop) and treats the same hardware as the emergency stop. The fix is a separate, hardwired emergency stop circuit that removes power, leaving the category 2 hold under its own non-safety control loop. [S3]
Misuse pattern two: a plant tries to economise by sharing the emergency stop pushbutton between multiple operator stations, then argues that a "local" category 2 stop should suffice because the operator only needs to halt the line in front of them. The standard response is that an emergency stop has to be a category 0 or 1 function on the hazardous motion itself, regardless of how many stations share the button [S5]. A quick field checklist: confirm the chosen category is 0 or 1, confirm the drive function is STO or SS1 (not SS2 or plain STO-via-software), and confirm the emergency stop button breaks the safety circuit directly. Anything that leaves the drive energised after stop is a category 2 stop and is not a legal emergency stop [S1][S2][S4].
Testing, Maintenance, and What to Watch For
Functional tests of an emergency stop must verify that pressing the actuator results in actual removal of drive energy within the time assumed by the risk assessment, not just a software "stopped" flag on the HMI [S1]. A useful check is to measure the DC bus voltage or the output stage enable signal at the drive terminals during the test, because a category 2 implementation can show zero speed while the drive stage is still active.
For machine builders selecting components, the practical signals to track through 2026 are: whether the chosen VFD supports STO and SS1 at the required Performance Level under ISO 13849-1 or SIL under IEC 61508, and whether the emergency stop wiring is routed as a hardwired safety circuit independent of the standard control PLC. Reference points include the emergency stop summary page, the measurement and test overview for verification procedures, and the electronic test and measurement catalogue for the instruments used to capture stop-time and bus-voltage traces during commissioning.
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