Guard interlocking and locking switches — four types per ISO 14119

The guide on ISO 13849-1 performance levels covers how a safety function is assessed for reliability. This article covers a concrete safety component that assessment is often tied to: the interlocking switch (position switch) on a movable guard (door, flap, gate), as classified in ISO 14119.

Four types of interlocking switches

ISO 14119 distinguishes four main types, increasing in resistance to deliberate defeat:

Type	Operating principle	Example	Vulnerability
Type 1 — mechanical, uncoded	A position switch operated by a cam or arm on the guard	Roller-plunger switch, hinge switch	Any object that depresses the plunger (even a screwdriver) simulates a closed guard
Type 2 — mechanical, coded	A specifically shaped key/actuator must fit into the switch head	Tongue switch with profiled key	Harder to defeat than type 1, but an identically shaped key from another unit often also works
Type 3 — non-contact, uncoded	Detects the presence of a simple object (magnet, metal) without coding	Reed switch with a plain magnet, inductive sensor	Tolerant of dirt and misalignment, but defeated by any suitable magnet or metal plate
Type 4 — non-contact, coded	Responds only to a unique coded actuator signal	RFID or coded-magnet sensor	In practice usually only responds to the specific, taught-in actuator

Coding levels

For coded actuators (type 2 and 4), ISO 14119 defines three coding levels, based on the number of possible code variations:

Low — 1 to 9 code variations.
Medium — 10 to 1000 code variations.
High — more than 1000 code variations.

The higher the coding level, the smaller the chance that a random replacement actuator (for example from another unit of the same brand) accidentally or deliberately triggers the interlock. The machine's risk assessment determines which coding level is required: where the risk of defeat is high (for example when defeat is attractive to avoid lost production), a low coding level is insufficient and at least a medium or high level is needed.

Interlock versus guard locking

A plain interlocking switch only detects whether the guard is open or closed and interrupts the hazardous function as soon as the guard is opened. That is insufficient when the machine has an overrun time (coast-down of a rotating part, residual vibration) after the stop command that is longer than the time someone needs to actually reach the hazard after opening the guard. In that case an additional guard locking function is needed: the guard stays mechanically locked closed until the hazard has actually ceased (for example only after the coast- down time has elapsed), not merely until the stop command has been given.

Practical relevance

When assessing or replacing an interlocking switch on a machine guard, check which type (1 through 4) and which coding level is present, whether this matches the outcome of the risk assessment (see also the ISO 13849-1 guide for the corresponding performance level), and — where a relevant overrun time exists — whether a separate guard locking function is present alongside plain interlock detection.

Common mistakes

Using a type 1 (uncoded mechanical) interlock on a guard for which the risk assessment requires a higher coding level — a simple object can then already simulate the switch.
Assuming every interlock also locks — a plain interlock without a guard-locking function does not prevent the guard from being opened before an overrun time has elapsed.
Using type 3 (non-contact, uncoded) where the risk assessment identifies defeat with a simple magnet as a real risk — this type offers no protection against that.
Not checking the coding when replacing a faulty interlocking switch with "an equivalent" unit from another brand — at low or non- matching coding, the replacement actuator can have a different effective code than the original.