Bolt guard locking and tongue-actuator guard locking both fall under ISO 14119, but they solve different door problems: a bolt unit combines the interlock with a mechanical door bolt and external handle, while a tongue switch uses a coded metal blade (the "tongue" or "key") that enters the switch head and is trapped by a lock pin [S2]. The decision turns on door size, alignment tolerance, coding level, and whether a person trapped inside needs an escape release.
For the broader safety discussion, ISO 14119:2024 treats both as type 1/type 2 mechanical interlocking devices when the actuator is part of the guard itself, and the standard explicitly allows bolt locks on disconnector switches, valves, and hinged access doors [S1]. The selection grid below is built around the four quantitative anchors in the research: ready-position gap 1.0–3.5 mm, hinge radius typically >300 mm, M5 mounting at 4.0 Nm, and the type 2 coding limit of 1–9 (low), 10–1,000 (medium), or >1,000 (high) possible codes [S2][S3][S4].
Operating principle and where each device sits in ISO 14119
Both devices answer the same safety question: "Can the guard be opened before the hazard has stopped?" ISO 14119 splits the answer into two halves. The first half is the interlock function: guard open stops the machine and prevents a restart. The second half is guard locking: the guard is held physically closed until run-down, a time delay, or a standstill monitor releases it [S3]. Bolt units and tongue switches are two different mechanical implementations of that second half.
A tongue switch is a type 2 mechanical coded device: a shaped metal key fixed to the moving guard enters a slot in the switch head, operates the contact block, and is then trapped by a spring-loaded or solenoid-driven locking bolt [S2]. Because the actuator is a separate coded part, the spare actuator in the maintenance drawer is the type 2 family's known weakness, which is why ISO 14119 grades coding from low (1–9 codes) to high (>1,000) and demands non-detachable mounting, one-way screws, or welding for high-defeat-risk guards [S3].
A bolt unit is essentially a tongue switch with a door bolt, external handle, and often a rear escape release added on. Euchner's bolt catalogue describes the package as a safety switch plus handle assembly where an escape release lets a person inside the enclosure disable the lock from within, with the release lever positioned so it is reachable from the hazard zone [S8]. The bolt provides a real mechanical latch, so the door is not relying on the interlock pin alone to keep it shut during operation [S2].
Mechanical envelope: ready-position gap, hinge radius, and mounting
The first hard number to lock in is the ready-position gap between the actuator face and the switch head: 1.0–3.5 mm, with alignment held within ±1 mm of the slot centre [S2]. Outside this band you get intermittent contacts, safety relays that refuse to reset, and accelerated wear of the entry slot. IDEM's tongue-switch datasheet reinforces the figure indirectly: it specifies M5 mounting bolts torqued to 4.0 Nm, a minimum 100 mm spacing between adjacent switches or actuators, and a dedicated mechanical stop on the guard to absorb slam energy so the actuator enters without striking the housing [S4].
For hinged doors the next constraint is the hinge radius. Vox In Tech's buyer guidance sets the practical minimum at a hinge radius greater than 300 mm; below that, the actuator swings through too tight an arc and either binds in the slot or shears off [S2]. When geometry forces a tighter hinge, the workaround is an adjustable or multi-axis actuator head rather than a bolt unit; bolt units assume a relatively conventional swing or slide geometry because the handle and bolt are rigid extensions of the switch body.
The same datasheet adds two environmental exclusions that hit both device families: do not mount in direct sunlight, under excess vibration or mechanical shock, where static discharge or other electrical noise is present, near possible radioactive exposure, or close to power supply lines [S4]. For high-vibration cells (presses, hammers, vibratory feeders) the cushioned actuator variant, usually designated with a "D" suffix, carries rubber bushes that absorb door impact and protect both the actuator and the switch head from the six-month premature-failure mode [S2].
Selection criteria: door type, coding level, escape release, holding force
The first decision is door type and approach direction. Sliding doors with a straight approach take a T-shaped straight actuator; hinged doors with a right-angle approach take an L-shaped blade that turns 90° into the slot; deep frames or offset mounts need long-T or long-L reach; awkward geometry not yet known at design stage calls for an adjustable actuator trimmed on site; and compound or multi-axis approaches use a rotating head [S2]. Bolt units sit on top of this same matrix: they are specified once the door has been classified as a personnel access door, which is the threshold at which a bare tongue switch is no longer enough on its own.
The second decision is coding level, driven by foreseeable motivation to defeat. ISO 14119 ranks the countermeasures in a fixed order: remove the motivation first, then specify high coding, then mount out of reach behind obstruction, then fit the actuator non-detachably with one-way screws, security fasteners, or welding [S3]. For a tongue switch this maps to RFID-coded actuators, which IDEM offers as the MLZ/MLZ-M family and which the datasheet says must be recorded against the risk assessment because the risk of spare actuators is treated as a design input, not a user footnote [S4].
The third decision is the escape release, which is the dividing line between a tongue switch and a bolt unit. Euchner bolts for safety guards specify that the escape release must be accessible from inside the enclosure and that it must disable the guard-locking device without defeating the safety switch [S8]. Where personnel can be trapped inside the hazard zone (large robot cells, walk-in enclosures, fenced process areas), the bolt unit with rear release is effectively mandatory; a tongue switch on a small hatch with no enclosed hazard has no need for it.
The fourth decision is holding force Fzh, the static force the locked actuator can withstand, which IDEM flags as the figure the specifier must check against both normal use and the dynamic overshoot from a slamming guard: "dynamic effects caused by bouncing of the guard shall not create an impact reaction force which exceeds the holding force" [S4]. If the expected impact forces exceed Fzh, the datasheet requires design measures to absorb the energy, typically the mechanical stop and cushioned actuator combination, not a heavier switch.
Decision matrix: bolt unit vs tongue switch on four criteria
The cleanest way to choose is to put the criteria side by side. On coding level, a tongue switch with RFID teach is a type 2 device reaching the high-coding band (>1,000 codes), while a standard bolt unit without RFID coding typically lands in the low band (1–9 codes) unless an RFID-coded tongue is fitted inside the bolt assembly [S3][S4]. On door size, the tongue switch is the right answer for small hatches and covers; the bolt unit takes over once the door is a personnel access door needing a proper mechanical latch and external grab handle [S2].
On alignment tolerance, both devices share the same envelope: ready position 1.0–3.5 mm, ±1 mm slot alignment, M5 fasteners at 4.0 Nm, and a 100 mm keep-out zone between adjacent devices [S2][S4]. The bolt unit does not relax these numbers; it just adds the external handle and the rear release on top. On escape release, only the bolt unit provides one in the form Euchner describes, namely a lever accessible from the hazard zone that releases the lock without defeating the switch [S8]. A bare tongue switch has no such feature because it sits on a small cover, not a personnel door.
The criteria-based call: specify a tongue switch when the guard is a small cover or hatch, the door is not a personnel access point, no one can be trapped inside, and high coding is needed to deter defeat. Specify a bolt unit when the guard is a personnel access door, a mechanical latch and external handle are required to keep the door shut during operation, an escape release is needed from inside, or the door is large enough that the interlock pin alone is an insufficient mechanical hold. For RFID-coded bolt units, IDEM's MLZ/MLZ-M datasheet confirms the family is rated for series connection to a PLe Category 4 controller per ISO 13849-1 while maintaining the type 2 coded actuator [S4].
Installation, testing, and the defeat problem
Both device families share the same installation discipline: torque M5 fasteners to 4.0 Nm, respect the 100 mm keep-out, fit a dedicated mechanical stop to the guard, use alignment guides so the actuator enters the slot without striking the housing, and verify the locking function plus all control circuits after installation [S4]. The IDEM datasheet also requires that, for PLe applications, a manual function test is performed at least once per month, and for PLd at least once per year, to detect fault accumulation per ISO 14119 [S4].
Defeat is the central in-service failure mode and the reason ISO 14119:2024 was rewritten. The standard treats it as a design problem and ranks the measures: remove the motivation to bypass first (make the task rare, fast, or possible without opening the guard), then specify high-coded devices, then mount out of reach, then fix the actuator non-detachably with one-way screws, security fasteners, or welding [S3]. A cable-tied spare actuator or a fridge magnet bridging a reed switch is the textbook defeat, and the standard's response is to make both mechanically and electronically improbable rather than to rely on operator discipline.
For bolt units specifically, the escape release is a known temptation because it is a manual mechanical override. Euchner's installation rule is that the escape release must be within the hazard zone and that operating it must disable the guard-locking device, not the safety switch, so the door can be opened from inside during a trapped-person event without the safety outputs being bypassed [S8]. Misusing the escape release as a routine external bypass is the bolt-unit equivalent of the tongue-switch spare-actuator problem.
For related actuator and mechanical-interlock content, see our write-up on ball screw vs lead screw efficiency and backlash decision matrix and our breakdown of ANSI B73.1 vs API 610 centrifugal pumps for the broader spec-by-spec selection discipline. For the wider safety-device taxonomy that bolt units and tongue switches sit inside, the API 608 vs API 6D ball valve standard selection map and the ATEX 2G vs EPL Gb vs Zone 1 equivalence decoded for specifiers pieces use the same standards-and-decision-criteria format applied to other equipment families.
Standards, sourcing, and what to track next
Two standards govern both devices. ISO 14119:2024 is the type-B2 standard for interlocking devices associated with guards, including guard locking; it defines the four device types (1 mechanical uncoded, 2 mechanical coded, 3 non-contact uncoded, 4 non-contact coded) and the low/medium/high coding bands [S1][S3]. ISO 13849-1 governs the safety-related control system performance level (PLe Cat 4 for series-wired RFID tongue switches per IDEM's MLZ family) and the diagnostic coverage that determines the monthly or yearly manual function test cadence [S4].
Watch for two signals over the next cycle. First, ISO 14119's defeat-mitigation hierarchy is being interpreted more strictly in European machinery-safety audits, with assessors asking for documented evidence of motivation removal, not just high-coded devices [S3]. Second, the escape-release rules for bolt units are being read alongside ISO 14119's anti-defeat clauses, with the audit question shifting from "is there a release?" to "can the release be operated only from inside, and does it disable the lock only, not the safety outputs?" [S8]. Both signals point in the same direction: spec the device family by door size and personnel access first, then layer coding and test cadence on top.
Detailed specification references: locking assembly, dry type transformer, and electric actuator.