Technical Quick Guide: Safety Switches – One-Channel vs Two-Channel Configurations

1. Introduction

Safety switches are essential components in industrial automation and machinery safety systems. They are used to detect the opening of guards, doors, or gates and to initiate a stop command to remove potential hazards to personnel. The design and configuration of the safety circuit—especially the use of single or dual channels—significantly impact the system’s integrity, reliability, and compliance with safety standards such as ISO 13849-1 and IEC 62061.

2. One-Channel Safety Switch Systems

2.1 Description

A one-channel (single-channel) safety switch system uses a single Normally Closed (NC) contact wired into the safety input of a controller (e.g., safety relay or PLC).

2.2 Operation

• The NC contact remains closed when the safety guard is in position.
• When the guard opens, the contact opens, signaling the controller to stop the machine.

2.3 Advantages

• Simple wiring and low cost.
• Suitable for low-risk applications (Performance Level a or b, according to ISO 13849-1).

2.4 Disadvantages

• No fault detection for contact welding or short circuits.
• Does not meet higher safety requirements (Performance Level c, d, or e).
• No redundancy.

3. Two-Channel Safety Switch Systems

3.1 Description

A two-channel system typically uses:

• Two NC contacts (redundant and monitored separately), or
• One NC + one NO contact configuration.

3.2 One NC + One NO Contact Configuration

• Channel 1: NC contact
• Channel 2: NO contact

This combination allows for cross-monitoring between the two channels to detect faults like:

• Contact sticking (welding)
• Cross-short between wires
• Mismatched switching timing (discrepancy)

3.3 Advantages

• Redundancy: Two channels improve system reliability.
• Fault detection: Enables controller to detect wiring or contact failures.
• Required for applications needing Performance Level c to e.
• Enhanced diagnostics.

3.4 Disadvantages

• More complex wiring
• Slightly higher cost
• Requires safety-rated logic devices for proper monitoring

4. Comparison Table: One-Channel vs Two-Channel Safety Switches

5. Types of NO Contacts in Safety Switches

The NO (Normally Open) contact in a safety switch can be one of two types:

5.1 Mechanically Linked NO Contact

• Physically actuated by the same mechanism as the NC contact.
• Ensures safe and predictable timing.
• Used in traditional electromechanical switches.

5.2 Electronic or Semiconductor NO Contact

• Uses transistors or solid-state outputs.
• Can provide faster switching and higher lifetime.
• Common in RFID or coded non-contact switches.

Note: Electronic NO contacts must meet safety standards for diagnostic coverage and fail-safe operation if used in safety circuits.

6. Conclusion and Recommendation

In safety-critical applications, two-channel safety switches with 1xNC + 1xNO contacts are the preferred solution due to their fault detection capability, redundancy, and compliance with higher Performance Levels. For basic or low-risk installations, single-channel systems may suffice, but should be used with caution and under proper risk assessment.

Recommendation: Use two-channel systems for all machinery where safety function reliability is critical. Prefer mechanically linked contacts unless high-speed or non-contact switching is needed, in which case ensure the electronic contacts are safety-rated.