Guard Locking Safety Interlock Switches: A Complete Buyer’s Guide for Indian Machine Builders
Every machine with a moving guard door faces the same question: what stops an operator from opening that door while the hazard is still live? A standard limit switch will tell the control system that the door has opened — but by then the operator’s hand is already inside. On a machine that takes eight seconds to coast down, that is eight seconds of exposure.
A guard locking safety interlock switch solves this. It does two jobs at once: it monitors whether the guard is closed, and it physically holds the guard shut with a spring-loaded bolt until the machine has reached a safe state. Only then does the safety controller release the lock.
This guide covers everything an Indian machine builder, panel builder or maintenance engineer needs to specify the right device — locking force, locking principle, contact configuration, standards, and the mistakes that cause the most site rejections.
What is a guard locking safety interlock switch?
A guard locking interlock is a solenoid-operated safety device fitted to the leading edge of a sliding, hinged or removable machine guard. It consists of two parts:
- The switch body, mounted on the fixed frame, containing the contact blocks, the lock bolt and the solenoid coil.
- A coded actuator key (a “tongue”), mounted on the moving door, which enters the switch head and is trapped by the lock bolt.
When the actuator is inserted and the bolt is extended, the normally-closed safety contacts are closed and the machine is allowed to run. The bolt cannot be withdrawn until the safety relay or safety PLC energises the solenoid — and it will only do that once the stop command has been issued and the run-down or standstill condition is satisfied.
Because the actuator is a separate coded part rather than a cam or roller, the device is classified as a Type 2 interlocking device under EN ISO 14119 (adopted in India as IS 16812).
The seven specifications that actually matter
1. Locking force (retention force)
This is the pull force the lock bolt will withstand before it fails. 1300 N is the practical industry standard for machine guards, fence gates and access doors, and covers the overwhelming majority of Indian applications — power presses, injection moulding, packaging lines, CNC enclosures and robot cells.
Higher forces (2000 N and above) exist for very large, heavy or wind-loaded doors. Do not over-specify: locking force is not a safety rating, it is a mechanical rating. What matters is whether it exceeds the foreseeable force a person can apply to the door.
2. Locking principle — spring lock or solenoid lock
This is the single most important selection decision, and it is covered in detail in our dedicated article on locking principles.
- Spring locking / solenoid unlocking (power-to-unlock): the guard stays locked when de-energised, including during a power cut. The correct choice for any machine with a run-down time.
- Power-to-lock: the solenoid must stay energised to hold the lock, so the guard releases on power failure. Only for applications where a risk assessment demands escape on loss of supply.
For Indian conditions, where supply interruptions are routine, spring locking is almost always the right answer.
3. Contact configuration
Count your circuits before you order. A typical guard-locking device provides two separate monitoring blocks:
- Door / actuator monitoring — tells you the guard is closed.
- Lock (bolt) monitoring — tells you the bolt is actually extended.
A four-contact device (1NC + 1NO on each block) gives you single-channel safety with status feedback to the PLC. A six-contact device gives you dual-channel (Category 3 / 4) architecture from a single unit. Get this wrong and you will be adding a second device on site.
4. Positive opening (forced disengagement)
The NC safety contacts must be forced disengagement type — mechanically driven open, not spring-returned. Look for a stated disengagement force and travel (typically ≥80 N and ≥10 mm). Without this, the contacts can weld closed and the machine will keep running with the guard open.
5. Body format and mounting
Two formats dominate:
| Format | Typical size | Best for |
|---|---|---|
| Slim tall body | approx. 39 × 39 × 183 mm | Fence posts, square-section frames, where you have height but not width |
| Compact block | approx. 108 × 89 × 36 mm | Shallow guard frames, sliding doors, restricted body length |
Measure the frame before ordering — a body-format mismatch is the most common reason a delivered switch cannot be fitted.
6. Ingress protection and materials
IP67 with a flame-retardant PA66 housing and stainless steel internals is the working standard. A metal (rather than plastic) head is worth paying for on any door that sees impact or vibration.
7. Emergency and auxiliary release
Any door large enough for a person to enter needs a way out. Specify a front emergency unlocking knob as standard and a rear unlocking kit where the release must be operated from outside the enclosure. Ensure the unlocking knob is reset before the machine returns to service — an unreset knob leaves the guard unsecured.
Standards you will be asked about
| Standard | What it covers |
|---|---|
| IS 16812:2018 / EN ISO 14119 | Design and selection of interlocking devices associated with guards |
| IS 16810 / ISO 13849-1 | Safety-related parts of control systems; Performance Level (PL a–e) |
| EN / IEC 60947-5-1 | Control circuit devices and switching elements, including positive opening |
| IS 16819 / ISO 12100 | Risk assessment and risk reduction |
An important point for buyers: the Performance Level is a property of the circuit, not of the switch. A PL e claim on a product datasheet alone is meaningless. PL is achieved through architecture, diagnostic coverage and the safety controller used — the machine builder validates it.
Common specification mistakes
- Using the switch as a door stop. It is a monitoring and holding device, not a mechanical end stop. Fit a separate stop.
- Wiring NO contacts into the safety circuit. NO contacts are auxiliary signalling only — they go to the PLC, never into the safety chain.
- Ignoring hinge radius. On side-hung doors, a hinge radius below about 300 mm will bind the actuator.
- Leaving spare actuators loose in the toolbox. A spare coded actuator is a ready-made defeat device. Control their issue and storage — EN ISO 14119 explicitly requires it.
- Using metal cable glands. Most plastic-bodied interlocks specify non-metallic glands to preserve the IP rating and insulation class.
Where guard locking is used in Indian industry
Power press and press brake guarding • injection moulding and blow moulding • robot and automation cells • packaging, printing and converting lines • CNC machining centres • conveyor and material handling access gates • perimeter fencing and isolation rooms • test enclosures.
If the machine has stored energy, rotating mass, or a run-down time longer than the time it takes a person to reach the hazard, guard locking is not optional — it is the control measure your risk assessment will point you to.
Ready-to-ship options from Voxintech
Voxintech stocks the Voxintech VXT-SS series of solenoid guard-locking interlocks — 1300 N locking force, 24 V DC solenoid, IP67, spring locking with solenoid unlocking, designed to EN ISO 14119 and EN / IEC 60947-5-1, with eleven interchangeable actuator keys and matching handle-and-bolt units.
- VXT-SS-W2-D-NCNO-L-NCNO — slim tall body, 4 contacts, three M20 entries, independent door and lock circuits.
- VXT-SS-W5-D-2NC1NO-L-2NC1NO — compact block, 6 contacts, two internally series-linked safety channels for dual-channel architecture from one device.
GST invoice on every order, delivery across India.
Frequently asked questions
What is the difference between a safety interlock switch and a guard locking switch?
A plain safety interlock switch only monitors whether the guard is closed. A guard locking switch also physically holds the guard shut until the machine is safe. If your machine has a run-down time, you need guard locking.
What locking force do I need?
1300 N covers the large majority of industrial guard doors and fence gates. Higher forces are only needed for unusually large, heavy or wind-loaded doors.
Is a guard locking interlock mandatory in India?
There is no blanket product-level mandate. The requirement comes from your risk assessment under IS 16819 / ISO 12100, read with the Factories Act duty to fence dangerous machinery. Where access is possible before the hazard has stopped, an interlock with guard locking is the recognised control.
Can I use one device for a Category 3 dual-channel circuit?
Yes, if it provides two independent NC safety channels. A six-contact device with internally series-linked door and lock contacts gives two “closed and locked” channels from a single unit.
What voltage are these switches available in?
24 V DC is the industry standard for the solenoid. Status LEDs are commonly available in 24 V DC or wide-range 10–115 V AC/DC executions.