Spring Lock or Solenoid Lock? Choosing the Right Guard Locking Principle
Power-to-lock or power-to-unlock — how the two locking principles behave on power failure, and how to choose the right one for Indian plants.
Two guard-locking interlocks can look identical on the shelf, carry the same 1300 N locking force and the same 24 V DC coil — and behave in exactly opposite ways the moment the power goes out. One holds the guard shut. The other lets it swing open.
Getting this choice wrong is one of the most consequential specification errors in machine safety. Here is how to get it right.
New to guard locking? Start with our pillar guide: Guard Locking Safety Interlock Switches — A Complete Buyer's Guide for Indian Machine Builders.
Safety Interlock Switches In Stock
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Browse the full range on our brand pages: Allen-Bradley, Schmersal, Omron, Telemecanique and Banner Engineering.
The two locking principles
Spring locking with solenoid unlocking (power-to-unlock)
A compression spring drives the lock bolt into the trapped actuator. The bolt stays extended by mechanical force alone. To release the guard, the safety controller applies 24 V DC to the solenoid terminals, which retracts the bolt against the spring.
Behaviour on power failure: the guard stays locked.
Solenoid locking with mechanical release (power-to-lock)
The solenoid must remain continuously energised to hold the bolt extended. Remove the supply and the bolt retracts under spring return, releasing the guard.
Behaviour on power failure: the guard opens.
Why this matters: run-down time
Most industrial hazards do not stop the instant you press the stop button. A press flywheel, a centrifuge, a large fan, an injection screw, a robot axis under load — all continue to move after the drives are disabled. That interval is the run-down time.
If an operator can reach the hazard before the run-down finishes, the guard must remain locked for the whole of that interval. And critically, it must remain locked even if the plant loses supply during run-down, because the hazard does not stop just because the electricity did.
This is why spring locking with solenoid unlocking is the default choice for the large majority of machines, and it is a particularly important consideration in Indian plants, where supply interruptions and voltage dips are a routine part of the operating environment. A power-to-lock device in a factory with unreliable supply will release guards every time the incomer trips.
When is power-to-lock the correct choice?
Power-to-lock is not wrong — it is a specific answer to a specific risk assessment outcome. Use it where the greater risk is being trapped inside rather than reaching in, and where there is no dangerous run-down. Typical cases:
- Walk-in enclosures and isolation rooms where personnel could be inside during a power failure, and free egress is the priority.
- Machines with no stored energy and effectively instantaneous stopping.
- Process rooms where emergency evacuation drives the design.
Even then, a walk-in enclosure fitted with spring locking should be provided with an escape release or a rear unlocking kit so that a person inside can always get out — that is generally the safer combination.
Side-by-side comparison
| Spring locking / solenoid unlocking • Solenoid locking / mechanical release | |
| Also called | Power-to-unlock, mechanical locking • Power-to-lock, electromagnetic locking |
| Bolt held by | Compression spring • Energised solenoid |
| On power failure | Guard stays locked • Guard releases |
| Solenoid duty | Energised only to release • Continuously energised |
| Coil heating | Minimal — brief energisation • Continuous; expect housing temperature rise |
| Suits machines with run-down | Yes • No |
| Suits walk-in enclosures needing egress | Only with escape release fitted • Yes |
| Typical share of applications | ~80–90% • Specialist cases |
The practical consequences people forget
Coil temperature
A power-to-lock device holds its coil energised for the entire production shift. A 4.8 W coil at 100% duty will raise the housing surface roughly 25 °C above ambient. In a Chennai or Ahmedabad summer, in an enclosed guard frame, that stacks up. Spring-locking devices energise only for a few seconds per door opening.
Nuisance releases
Every brownout, every generator changeover, every RCBO trip releases the guard on a power-to-lock installation. Operators stop trusting the system, and the safety relay resets become a constant irritation.
Standstill confirmation is still your job
Spring locking guarantees the guard cannot be opened without a deliberate unlock command. It does not guarantee that the machine has stopped. The unlock command must be gated by a standstill monitor, a run-down timer sized to the worst case, or a zero-speed relay. The interlock is one element of the safety function — never the whole of it.
Manual release must be reset
Both principles are supplied with a front emergency unlocking knob. After any manual unlocking operation, reset the knob before the machine returns to service. An unreset knob can prevent the bolt from engaging and leaves the guard permanently unsecured — a defect that is easy to miss on a walk-round.
How to specify it on your order
Locking principle is usually a suffix or a separate model code, not a field-settable option. It is decided at the factory. Confirm it in writing on your purchase order, and confirm it again against the rating label before commissioning.
The Voxintech VXT-SS series is supplied as standard in the spring locking / solenoid unlocking execution — the guard remains locked when de-energised, including on power failure — with a power-to-lock execution available as a catalogue variant where a risk assessment calls for it. Contact arrangement and all other technical data are identical between the two executions.
- VXT-SS-W2-D-NCNO-L-NCNO — slim body, 4 contacts, 1300 N, 24 V DC.
- VXT-SS-W5-D-2NC1NO-L-2NC1NO — compact body, 6 contacts, dual safety channels.
Frequently Asked Questions
Which is safer, spring locking or solenoid locking?
Neither is universally safer — it depends on the hazard. Spring locking is safer where reaching into a running-down machine is the risk. Power-to-lock is safer where being trapped inside is the risk. Your risk assessment decides.
What happens to a spring-locking switch during a power cut?
The lock bolt stays extended and the guard remains locked. It can only be opened with the front emergency unlocking knob or the optional rear unlocking kit.
Can I convert a spring-locking switch to power-to-lock on site?
No. The locking principle is built into the device. Order the correct execution.
Does the solenoid need to stay energised on a spring-locking device?
No. It is energised only long enough to release the bolt. Do not leave it energised while the guard is manually unlocked.
What voltage is the solenoid?
24 V DC ±10% is the standard, drawing around 200 mA / 4.8 W. Observe polarity — E1 is positive on most devices.