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Safety·7 min read·

Stop categories 0, 1 and 2, and why an E-stop is not a guard

Three stop categories, what each removes, and the reason a normally closed E-stop reads true when everything is healthy.

Short answer

A category 0 stop removes power to the actuators immediately and lets the machine coast. A category 1 stop keeps power long enough to bring the machine to a controlled halt, then removes it. A category 2 stop halts the machine but leaves power on. These are IEC 60204-1 stop categories and have nothing to do with ISO 13849 architecture categories, despite sharing the word.

The safety chain, and where the rating appliesSensorguard switchLogicrelay or safety PLCActuatorcontactorTHE RATING APPLIES TO ALL OF THIS, NOT TO ONE BOXONE SAFETY FUNCTION

Two words in machinery safety mean two different things depending on which standard is on the desk, and the second most confusing pair is stop category.

The three stop categories

IEC 60204-1 defines them by what happens to the power.

Category 0. Power to the machine actuators is removed immediately. The machine coasts. Nothing controlled about it: a spindle keeps turning until friction stops it.

Category 1. Power is kept on long enough for the drives to bring the machine to a controlled stop, then removed. The removal is not optional and not conditional; it happens after a monitored delay or on confirmed zero speed.

Category 2. The machine stops but power stays on. Used where removing power would create a worse hazard, or where the machine must hold position.

Category 2 is not a valid emergency stop on its own for most machinery. An emergency stop must be category 0 or 1.

Which one to use

It sounds like category 0 is the safest, and often it is not.

Dropping power to a servo axis holding a heavy load means the load falls. Dropping power to a spindle in a cut means the tool stays in the workpiece, turning, until it stops. Dropping power to a hydraulic press mid-stroke can leave stored energy exactly where nobody wants it.

Category 1 exists for those cases: decelerate under control, then remove power. The controlled part must itself be monitored, because a drive that has failed will not decelerate anything, which is why category 1 stops are usually built around a safety relay with a timed or zero speed confirmed contact.

The normally closed rule

An E-stop button contains a normally closed contact. The circuit is held closed by an unpressed button, and the machine runs because the circuit is closed.

Press it and the circuit opens. Break the wire and the circuit opens. Corrode the terminal and the circuit opens. Every failure that matters looks the same as the button being pressed, and every one of them stops the machine.

This is why in the logic the E-stop tag is examined with a normally open contact, and reads true when everything is healthy. It looks backwards on the screen and it is correct on the machine.

Getting this the wrong way round in a program is a safety defect, not a style choice. It is also the single thing a language model most often gets wrong when asked for a stop rung, which is why LADX checks it rather than trusting it.

The mechanical requirements people forget

An E-stop device has requirements of its own, and they are as binding as anything in the logic.

  • Direct opening action: the contact is forced open mechanically, not pulled by a spring that could fail.
  • Latching: it stays pressed until deliberately released.
  • Releasing it must not restart the machine. Reset is a separate deliberate action.
  • Red on yellow, and reachable from every position where somebody might need it.

An E-stop is not a guard

The last point is the one worth ending on, because it is a design attitude rather than a detail.

An emergency stop is a complementary protective measure. It exists for the situation the guarding failed to prevent. It is not a substitute for guarding, and it is not the normal way to stop the machine at the end of a shift.

If operators are using the E-stop routinely, that is a finding about the machine's normal stop, not a habit to be trained out.

Common questions

Are stop categories the same as ISO 13849 categories?
No, and the shared word causes real confusion. Stop categories 0, 1 and 2 come from IEC 60204-1 and describe how the machine is brought to rest. ISO 13849 categories B, 1, 2, 3 and 4 describe the architecture of the safety function. A category 1 stop can be implemented at Category 3.
Why is an E-stop wired normally closed?
So that a broken wire, a loose terminal or a failed contact looks the same as the button being pressed. The circuit is held closed by a healthy system; anything that interrupts it stops the machine. Wiring it normally open means a cut cable disables the E-stop silently.
Can an emergency stop be a guard interlock?
No. An E-stop is a complementary protective measure for a situation the guarding did not prevent. It is not a substitute for guarding, it is not part of the normal stopping means, and using it to stop the machine at the end of every cycle wears out the mechanism it is meant to preserve.

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