Safety door switches (safety interlock switches) are among the most widely used safeguarding components on industrial machinery. Choosing the wrong one can mean production downtime, failed CE/TÜV audits, or — worst case — an operator injury. This guide walks through the decision criteria in the order a machine builder or maintenance engineer should apply them.
1. Start with the Required Safety Level (PL / SIL)
Every safety function must achieve a target performance level (PL a–e, per EN ISO 13849-1) or safety integrity level (SIL 1–3, per IEC 61508). The required PL comes from a risk assessment of the machine, not from the switch catalogue. In practice:
- PL c — typical for simple guard position monitoring on low-risk machines
- PL d — the most common requirement for interlocked movable guards
- PL e — reserved for high-risk applications such as robot cells, presses and cutting machinery
The switch you select must be rated for the PL you need, and the wiring must match the required architecture — for example, dual-channel with positively driven, monitored contacts for PL d/e. A switch alone does not make a safety function; the whole chain (switch → safety relay/PLC → contactor) must be engineered to the same level.
2. Choose the Interlock Principle: Mechanical vs. Non-Contact
Per EN ISO 14119, interlocking devices for movable guards fall into two families:
| Feature | Mechanical (e.g. Schmersal AZ 16) | Non-contact magnetic (e.g. Schmersal BNS series) |
|---|---|---|
| Tolerance to misalignment | Low — actuator must align with the switch head | High — up to several millimetres of lateral offset |
| Wear and maintenance | Mechanical wear over cycles; requires periodic inspection | Wear-free — no moving parts in the sensing zone |
| Tamper resistance | Better with coded actuators (EN ISO 14119 coding levels) | Depends on coding — coded magnets give higher protection |
| Typical applications | Hinged doors, sliding guards on standard machinery | Sanitary/clean environments, vibrating machinery, robot cells |
3. Decide Whether You Need Guard Locking
A non-locking interlock switch only detects whether the guard is closed. A locking interlock (guard locking device) physically holds the guard closed until the hazardous machine motion has stopped — essential where the machine has residual energy or run-down time after the guard opens. Guard locking comes in two variants:
- Spring-locked, solenoid-unlocked — the guard stays locked if power fails; suitable for most safety applications (fail-safe)
- Solenoid-locked, spring-unlocked — the guard releases on power loss; used where access must be possible in an emergency
For robot cells and large process machines, an electromagnetic guard locking device such as the Schmersal AZM 170 series is the standard solution.
4. Check the Environmental and Electrical Conditions
- Ingress protection (IP rating) — IP67 is typical for washdown and dust-heavy environments; IP65 suits most indoor machine guarding. Check the exact rating on the datasheet, not just the marketing photo.
- Temperature range — most switches operate from −25 °C to +70 °C; verify for cold stores or furnace-adjacent applications.
- Contact configuration — safety contacts are normally-closed (NC) with positive break operation; auxiliary signalling contacts can be normally-open (NO). Count your inputs against the safety relay before ordering.
- Voltage and current — confirm the switching capacity matches your control voltage (24 V DC is standard, but 110 V AC and 230 V AC versions exist).
- Housing material — glass-fibre reinforced thermoplastic for standard use, stainless steel or die-cast versions for aggressive environments.
5. Consider Coding and Tamper Resistance
EN ISO 14119 defines coding levels (low, low-coded, high-coded) that determine how difficult it is to defeat the switch with a substitute actuator. In high-risk applications, choose coded switches and physically position them so they cannot be easily bridged.
6. Match the Series to Your Application
Within one manufacturer's range the naming usually encodes the function. Using Schmersal as an example:
- AZ 16 — mechanical safety door switches with optional interlock; the workhorse of machine guarding. See the AZ 16-03ZVRK and AZ 16-12ZVK as representative versions.
- BNS series — non-contact magnetic safety switches with coding options, ideal where hygiene or vibration rules out mechanical actuation (e.g. BNS 33-11Z).
- MZM 100 — non-contact magnetic interlock with guard locking, combining wear-free sensing with a locking function (e.g. MZM 100 ST-1P2PR-A).
- AZM 170 — electromagnetic guard locking devices for robot cells and high-risk machinery (AZM 170-02ZRKA 230VAC).
7. Budget for the Complete Chain, Not Just the Switch
Every interlocked guard needs a logic element to evaluate the switch signals — safety relay module or a safety PLC. If your existing cabinet already uses Schmersal SRB safety relays or a safety PLC, staying within the same brand simplifies wiring, documentation and spare-parts management.
8. Verify Certifications Before You Buy
For CE-marked machinery, the switch should carry a TÜV or equivalent assessment to EN ISO 13849-1, and the manufacturer should publish the PL/SIL claim, the safety contact data, and the wiring diagrams. Ask for the declaration of conformity and the datasheet — any supplier that cannot provide them is a red flag.
Buying Guide for Glorient Customers
Glorient supplies genuine Schmersal safety door switches, guard locking devices and safety relay modules with competitive pricing and worldwide delivery. To order the correct part:
- Identify the manufacturer's order number on your existing switch (e.g. 101189634 for AZ 16 models).
- Check the full range in our safety door switches category.
- Send the order number or a photo of the nameplate to our team via the quote request form — we will confirm the exact replacement within 24 hours.
Replacement of a safety component is not a place for guessing. If you are unsure which switch your machine needs, our engineers can help you match the model, the wiring and the required safety level before you commit.
Frequently Asked Questions
What is the difference between AZ, AZM and hinge safety door switches?
AZ switches are non-latching position switches that only monitor whether the guard is closed. AZM switches add a solenoid locking mechanism with holding force to prevent the door opening during machine run-down. Hinge switches mount at the door hinge and monitor the hinge angle rather than a separate actuator, ideal where a key slot would collect debris.
What holding force do I need for a solenoid-interlocked door?
The holding force must exceed the forces that could act on the guard. A common minimum is 1000 N for personnel doors on robot cells; 2000 N and above is typical for heavy sliding gates on press lines and machines with long run-down times. Always derive the value from the risk assessment.
Should I choose power-to-lock or power-to-unlock?
Power-to-unlock keeps the guard locked when power is removed and releases on energising - the convention for personnel safety, since a trapped operator can escape on power loss. Power-to-lock releases on power loss and is used for process protection where the hazard must stay isolated. Choose based on the escape-path analysis, not convenience.