Every safety function on a machine has three layers: an input (safety switch, light curtain, emergency stop), a logic element that evaluates the input signals, and an output (contactor, drive, valve) that stops the hazardous motion. The safety relay module sits in the middle — and choosing it correctly is just as important as choosing the switch. This guide explains what safety relays do and how to select one.
1. What Makes a Safety Relay Different from a Standard Relay?
A standard relay is a single switching element. A safety relay is engineered for fault tolerance:
- Internal redundancy — two independent channels evaluate the same safety signal, so a single internal fault cannot silently defeat the safety function.
- Force-guided (positively driven) contacts per EN 50205 — the normally-closed and normally-open contacts are mechanically linked, so if one welds shut, the others cannot close, and the fault is detected.
- Self-monitoring — the relay performs internal tests and checks external wiring (e.g. contactor feedback) before allowing the machine to restart.
This is why a standard relay can never replace a safety relay in a safety circuit — the failure modes are fundamentally different.
2. When Do You Need a Safety Relay Module?
If your machine uses safety components — safety door switches, emergency stop buttons, light curtains, two-hand controls — and the risk assessment requires PL c or higher per EN ISO 13849-1, you need a certified logic element. The safety relay module:
- Evaluates the dual-channel signals from the safety devices
- Provides the required performance level (PL d/e) for the circuit
- Handles reset logic (automatic or monitored manual reset)
- Monitors external contactors through a feedback loop
For very complex machines with many safety functions, a safety PLC may be justified instead; for single guards and standard cells, a safety relay is simpler, cheaper and easier to commission.
3. Key Selection Criteria
3.1 Input Type
Match the relay to the safety device feeding it:
- Safety switches / interlocks — relay accepts NC contacts from mechanical or non-contact switches
- Emergency stop — relay with approved E-STOP input and monitored reset
- Light curtains / photoelectric sensors — relay with OSSD input compatibility (pulsed PNP signals)
- Two-hand controls — relay with simultaneity monitoring
3.2 Number of Input Channels
Single-channel wiring is allowed only for lower PL requirements (PL c and below with appropriate structure). For PL d/e, dual-channel wiring with short-circuit detection is the norm — ensure the relay supports it and the wiring follows the datasheet.
3.3 Output Contacts
Count the safety outputs you need: two NO (normally-open) safety contacts are typical (one for the contactor, one spare or for signalling). Verify the contact rating (e.g. 6 A / 250 V AC) against your contactor coil and the auxiliary NC contacts for feedback.
3.4 Feedback / External Monitoring Loop
If the relay switches a contactor or drive, the contactor's own NC contacts should be wired into the feedback loop. This detects a welded contactor so the machine cannot restart into a dangerous state.
3.5 Reset Mode
Automatic reset restarts the machine as soon as the guard closes — convenient but riskier. Monitored manual reset requires the operator to press a reset button after the guard closes, and the relay verifies the button releases before re-enabling. For most production machinery, monitored manual reset is the safer, recommended choice.
3.6 Expansion and Delay Modules
For larger cells, expansion modules add output contacts, and time-delay modules extend the stop delay after the guard opens. Check that the expansion modules are approved by the same manufacturer for the same safety level.
3.7 Certification and PL/SIL Rating
The relay must carry a TÜV or equivalent assessment to EN ISO 13849-1 (PL) and IEC 61508 / IEC 62061 (SIL). Verify the claimed performance level in the datasheet — a PL e claim means the whole wiring and architecture must match the documented structure.
4. The Schmersal SRB Family as a Reference
Schmersal's PROTECT SRB range covers most standard needs, and the naming encodes the function:
- SRB-E series — modular safety relay modules with configurable inputs; e.g. the SRB-E-322ST is a versatile two-channel module for safety switches, E-stops and magnetic interlocks.
- SRB-NA series — compact relay modules for standard single-function applications; e.g. SRB-NA-NF-24VDC for safety switch circuits.
- SRB 3xx / 4xx series — dedicated modules with specialised input logic, such as the SRB303SQP and SRB402NE.
See the full safety relays & safety PLC category for the complete range.
5. Buying Guide for Glorient Customers
Glorient supplies genuine Schmersal safety relay modules with competitive pricing and worldwide delivery. To select the right module:
- Identify your safety inputs (switch, E-stop, light curtain) and the required PL/SIL level from your risk assessment.
- Note the number of output contacts and whether you need expansion or delay modules.
- Send your circuit details via the quote request form — our engineers will confirm the correct module, wiring structure and reset mode within 24 hours.
A safety relay is only one link in the chain — but it is the link that makes the whole circuit fail-safe. If you are unsure about your architecture, send us the circuit diagram and we will help you select the right module.
Frequently Asked Questions
What is the difference between a safety relay and a standard relay?
A standard relay is a single switching device with no fault detection. A safety relay has dual-channel inputs, cross-monitoring and self-diagnostics, so a single fault (welded contact, broken wire, short circuit) is detected and forces a safe shutdown rather than silently disabling the safety function.
What is dual-channel monitoring?
Dual-channel monitoring uses two independent input circuits that must both change state together. If the two channels disagree - for example one contact welds - the safety relay detects the discrepancy and prevents restart. This redundancy is what allows Category 3/4 architectures.
What safety level can a safety relay achieve?
A properly wired safety relay can achieve up to Category 4 / PL e / SIL 3 per EN ISO 13849-1 and IEC 61508, depending on the architecture of the connected safety devices and the diagnostic coverage.