Safety Relay Module Symbol
Definition: The Safety Relay Module symbol represents a dedicated, certified relay module that monitors dual-channel safety inputs (e-stops, interlock switches) and provides forced-guided output contacts for machine safety functions, depicted as a rectangular block with coil supply pins A1 and A2, normally-open output contacts NO1 and NC1 (NC1 used for feedback monitoring), conforming to IEC 60947-5-1 (force-guided contacts) and certified to IEC 62061 / ISO 13849-1 for SIL 2 / Performance Level d applications.
Also known as: safety relay, pilz relay, schmersal relay, guard monitoring relay, e-stop monitoring relay, dual-channel safety relay.
What the Safety Relay Module symbol means
The Safety Relay Module symbol in a circuit diagram represents a compact, self-contained safety function module that continuously monitors the state of one or two safety input devices (emergency stop buttons, guard door switches, two-hand controls, light curtains) and provides certified forced-guided output contacts to enable or inhibit machine operation. Unlike a standard relay, the safety relay module has internal cross-channel monitoring, a start-up test requirement, and positive-guided contacts that allow detection of welded contacts on the next safety cycle.
In machine safety schematics, the safety relay symbol appears between the safety input devices (connected to A1/A2 coil terminals and the enable circuit) and the power contactors or drive-enable circuits (connected to NO1 output contacts). The NC1 contact provides feedback monitoring, confirming the output contacts have physically opened before allowing a restart.
How to identify the Safety Relay Module symbol
The Safety Relay Module symbol is drawn as a rectangle labelled 'Safety Relay' with coil supply pins A1 (positive) and A2 (negative) at the top edge, and output contact pins NO1 and NC1 at the bottom edge. A1 is the +24 V coil supply through the safety input chain; A2 is 0 V return. NO1 is the normally-open safety output contact that enables the machine drive; NC1 is the normally-closed auxiliary contact used for EDM (external device monitoring) feedback. The symbol resembles a standard relay but is distinguished by the 'Safety Relay' label and the A1/A2/NO1/NC1 pin nomenclature specific to safety relay conventions.
Function in a circuit
A safety relay module monitors its A1 and A2 coil input circuit, which includes the safety input devices (e-stop buttons, door interlock switches) wired in series in the enable circuit. When all safety inputs are closed, the A1–A2 circuit is complete, the safety relay's internal logic energises the output relay coil, and NO1 contact closes, enabling the machine. When any safety input opens (e.g., e-stop pressed), the A1 circuit opens, the safety relay de-energises, NO1 opens within milliseconds, and machine power is removed. The NC1 contact is wired back to the safety relay's reset input for EDM — the relay verifies that NO1 has physically opened before allowing the operator reset button to re-energise the output.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-5-1 Annex L defines positive-guided (force-guided) contact requirements used in safety relay output stages. IEC 62061:2021 and ISO 13849-1:2015 define the SIL/PL requirements and diagnostic coverage achievable with safety relay modules. EN ISO 13849-1 Category 3/PLd is the most common certification level for dual-channel safety relays. |
|---|---|
| ANSI/IEEE 315 | ANSI/UL 508A governs industrial control panel components including safety relays in North America. ANSI/NFPA 79 references IEC 60947-5-1 for contact ratings. Safety relay products sold in North America carry UL508 or UL61508 listing in addition to CE/TÜV certification. |
| Key difference | IEC and ANSI/UL standards are technically aligned for safety relay modules. IEC uses SIL (IEC 62061) and PL (ISO 13849-1) metrics; ANSI/UL applications typically reference both. Pin labelling (A1, A2, NO, NC) is identical under both frameworks. |
Terminals / pins
| Pin | Name |
|---|---|
| a1 | A1 |
| a2 | A2 |
| no1 | NO1 |
| nc1 | NC1 |
Typical values
Coil supply: 24 V DC (most common), 115 V AC or 230 V AC variants available. Output contact rating: 6 A at 250 V AC / 6 A at 24 V DC (positive-guided). Response time (de-energise to safe state): <20 ms typical. Mechanical life: ≥10⁶ operations. Achievable SIL/PL: SIL 2 / PLd (dual-channel with EDM); SIL 3 / PLe possible with additional architecture.
Where the Safety Relay Module symbol is used
- Emergency stop circuit monitoring — detecting e-stop button actuation and de-energising the main drive contactor within 20 ms
- Guard door interlock monitoring — verifying door closure via dual-channel interlock switch signals before permitting machine cycle start
- Two-hand control monitoring — ensuring both operator hands are on the control buttons simultaneously before the hazardous motion begins
- Light curtain OSSD output monitoring — reading the dual OSSD channels and translating them into forced-guided relay contacts for the machine power circuit
- Enabling device (dead man's handle) monitoring — verifying the enabling switch is in the held position for collaborative robot operation
- Safety mat and bumper monitoring — detecting contact on pressure-sensitive safety mats at robot cell perimeters
Example
In a pillar-drill machine safety circuit, the Safety Relay Module symbol shows A1 connected to +24 V through the series chain of the e-stop NO contact and the guard door interlock NO contact, and A2 connected to 0 V; NO1 output drives the main motor contactor coil; NC1 feeds back to the safety relay's Y1 EDM input so the relay verifies contact opening before the operator's reset button on S3 can re-energise the circuit.
Key facts
- Safety relay modules use positive-guided (force-guided) output contacts per IEC 60947-5-1 Annex L — NO and NC contacts are mechanically linked so a welded NO contact holds the NC contact open, making the fault detectable at the next cycle.
- External Device Monitoring (EDM) uses the NC1 feedback contact wired back to the safety relay reset input to verify that the output contacts have physically opened before a restart is permitted — preventing restart on a welded contact.
- Dual-channel input monitoring compares two independent safety input signals (e.g., two NO contacts of a dual-channel e-stop button); if the channels disagree for more than a few seconds, the safety relay latches off and requires a manual reset.
- Popular safety relay module brands include Pilz PNOZ series, Schmersal SRB series, Phoenix Contact PSR series, and Siemens 3SK series — all conform to IEC 62061 / ISO 13849-1 for SIL 2 / PLd as a minimum.
- The reference designator for safety relay modules in schematics is K or KS (KS for safety relay) followed by a number; Pilz relay terminals are standardised as A1 (+24 V in), A2 (0 V return), S11/S12/S21/S22 (input channels), 13/14 (NO1), 21/22 (NO2), 31/32 (NC1).
- Manual vs. automatic reset: a manually-reset safety relay requires deliberate operator action to re-energise after a safety stop; automatic reset re-energises when inputs return to the safe state without operator action (not permitted for certain safety functions per ISO 13849-1).
- A single safety relay module typically achieves Category 3 / PLd (SIL 2) when used with a dual-channel input device; achieving Category 4 / PLe (SIL 3) requires redundant safety relay modules with cross-monitoring.
Frequently asked questions
What does the safety relay module symbol mean in a circuit diagram?
The safety relay module symbol represents a certified relay that monitors dual-channel safety inputs (e-stops, door switches) via its A1/A2 coil circuit and provides forced-guided NO1 output contacts to enable or inhibit machine power. When any safety input opens, the relay de-energises and NO1 opens, removing power from the machine drive within 20 ms.
What does a safety relay symbol look like?
The safety relay module symbol is a rectangle labelled 'Safety Relay' with coil input pins A1 (+24 V) and A2 (0 V) at the top, and output contact pins NO1 (normally-open enable contact) and NC1 (normally-closed feedback/EDM contact) at the bottom. The symbol is similar to a standard relay block but carries the safety-specific labelling and additional EDM contact.
What is the difference between a safety relay and a standard relay?
A safety relay uses positive-guided (force-guided) contacts (IEC 60947-5-1 Annex L), has dual-channel input monitoring with discrepancy detection, and requires External Device Monitoring (EDM) to verify contact opening before restart. A standard relay has no positive guidance, no dual-channel monitoring, and no EDM, so it cannot be used for SIL 2 / PLd safety functions.
What is EDM on a safety relay?
EDM (External Device Monitoring) is a function where the safety relay's NC1 auxiliary contact is wired back to the relay's Y1 input. After a safety stop, the relay checks that NC1 is closed (confirming NO1 has physically opened) before accepting the operator's reset signal. This detects welded or stuck NO1 contacts and prevents restart on a fault.
What SIL or Performance Level does a safety relay achieve?
A dual-channel safety relay module in a Category 3 circuit achieves SIL 2 / PLd per IEC 62061 / ISO 13849-1. Achieving SIL 3 / PLe requires redundant safety relays with cross-monitoring (Category 4). The exact PL/SIL depends on the PFHD of the complete safety function including the input devices and output actuators.
What standard defines safety relay requirements?
Safety relay contact requirements are defined by IEC 60947-5-1 Annex L (positive-guided contacts). Application requirements and achievable SIL/PL are defined by IEC 62061:2021 and ISO 13849-1:2015. Safety relay products must be certified by an independent body (TÜV, SGS, UL) to these standards.
What are the pin designations A1, A2, NO1, NC1 on a safety relay?
A1 is the positive coil supply terminal (+24 V, connected through the series safety input chain). A2 is the coil return (0 V). NO1 is the normally-open safety output contact that closes when the relay is energised, enabling the machine. NC1 is the normally-closed auxiliary contact used for EDM feedback monitoring and HMI status indication.
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