Omron G9SP to ABB ACS850 STO Wiring With and Without Safety Relay

Overview

The Omron G9SP series of safety controllers is commonly paired with the ABB ACS850 industrial drive to implement the Safe Torque Off (STO) safety function. The G9SP provides safety-rated semiconductor outputs that can be wired directly to the ACS850 STO input, or routed through an interposing force-guided safety relay when the application demands galvanic isolation, contact multiplication, or expansion of the STO loop into additional circuits.

This reference documents both wiring topologies, the applicable functional safety standards, the STO electrical interface of the ACS850, the output characteristics of the G9SP, and the test-pulse behavior that must be considered when omitting the relay. The goal is to give the controls engineer a single document that supports the safety circuit design decision, the cabinet build, and the commissioning verification.

Applicable Functional Safety Standards

The STO function on the ACS850 family (which includes the ACSM1, ACS850, and ACQ810 product lines) is implemented per the following standards, as documented in the ABB Safe torque off function for ACSM1, ACS850 and ACQ810 application guide:

Standard	Title	Scope for STO
EN 61800-5-2:2007	Adjustable speed electrical power drive systems – Part 5-2: Safety requirements – Functional	Defines STO as a stop category 0 function that prevents torque-producing current to the motor
EN 62061:2005/AC:2010	Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems	SIL claim limits and PFHd calculation for the STO subsystem
EN ISO 13849-1:2008/AC:2009	Safety of machinery – Safety-related parts of control systems	Performance Level (PL) and Category assessment

Verify the latest revision of each standard against your local jurisdiction before signing off on the safety design. The ABB application guide is the authoritative source for ACS850 STO claim limits.

Omron G9SP Output Characteristics Relevant to STO

The G9SP-N10S, G9SP-N20S, and G9SP-N30S (and the later G9SP-N__D where applicable) are configurable safety controllers. Output modules include solid-state outputs and relay outputs. The solid-state outputs are the preferred interface for direct STO drive wiring because they are short-circuit protected, monitored, and pulsed-tested internally by the G9SP.

Parameter	Typical G9SP Solid-State Output Value
Output type	PNP sourcing, short-circuit and overload protected
Output voltage	24 VDC nominal (operating range 19.2 to 28.8 VDC)
Continuous current per output	0.5 A typical (verify exact module in the G9SP datasheet)
Test pulse width	Internal diagnostics; consult G9SP Configurator help for the active configuration
Safety rating (typical dual-channel use)	SIL 3 / PL e / Category 4, application dependent

Because the outputs are rated for direct safety use, adding an intermediate relay is only justified when (a) the receiving device requires a dry contact or a different voltage level, (b) the safety loop must be expanded into additional actuators, or (c) galvanic isolation is mandated by the cabinet design. The ABB ACS850 STO input accepts a 24 VDC signal; therefore an interposing relay is normally not required.

ACS850 Safe Torque Off Electrical Interface

The ACS850 STO is implemented through a dedicated two-channel input on the drive control board. Both channels must be de-energized to remove torque-producing current from the motor. When 24 VDC is removed from either channel, the drive firing commands are blocked within the time defined by the standard.

STO Input Parameter	Specification
Input voltage range	19 to 28 VDC
Typical input current	10 to 30 mA per channel (see drive manual for exact frame)
Channel count	2 (STO1 and STO2), both must drop to remove torque
Response time	≤ 20 ms typical from input de-energization to torque removal
Restart behavior	Drive latches a fault; requires explicit reset per start/stop logic
Diagnostic feedback	Optional STO status word over fieldbus for confirmation

Wire each G9SP safety output channel to a dedicated STO input on the ACS850. Do not parallel a single output to both STO inputs; doing so defeats the dual-channel diagnostic coverage required for SIL 3 / PL e.

Method 1: Direct Wiring (No Interposing Relay)

Direct wiring is the preferred topology when the G9SP solid-state outputs are within their ratings for the cable run length and the receiving drive input current draw. The two-channel output of the G9SP maps directly to the two-channel STO input of the ACS850.

Wiring Topology

• Route a dedicated 24 VDC safety power supply (typically PELV/SELV) to the G9SP safety outputs and the ACS850 STO common.
• Connect G9SP safety output channel A to ACS850 terminal STO1.
• Connect G9SP safety output channel B to ACS850 terminal STO2.
• Tie the 24 V return (0 VDC) to the ACS850 STO common terminal at the drive end only, to avoid ground loops.
• Use shielded twisted-pair cable if the run exceeds 10 m or passes near VFD output cables.

Acceptance Criteria

Before adopting direct wiring, the following checks must be completed in the G9SP Configurator project file and in the cabinet documentation:

1. Confirm in the G9SP output configuration that the test-pulse interval and width are compatible with the receiving drive. The ABB ACS850 STO input tolerates brief interruptions of typically up to a few milliseconds; the G9SP test pulse is well within this tolerance, but the engineer must validate the exact pulse width selected in the project.
2. Confirm the output current rating of the G9SP module exceeds the sum of the STO input currents for all wired drives plus any other loads on the same channel.
3. Confirm the cable resistance does not drop the loop voltage below 19 VDC at the drive terminals when the worst-case supply (20.4 VDC) and maximum load are applied.
4. Document the safety chain PFHd calculation including the G9SP output module and the ACS850 STO subsystem per the values in the manufacturer manuals.

Method 2: Wiring Through a Force-Guided Safety Relay

An interposing force-guided (mirror-contact) safety relay is used when the application requires dry contacts, contact multiplication for multiple downstream devices, or when the receiving device cannot tolerate the test pulse from the G9SP solid-state output.

Wiring Topology

• G9SP solid-state safety output energizes the coil of the force-guided relay (typically a 24 VDC coil).
• The force-guided relay's normally-open contacts switch 24 VDC to the ACS850 STO1 and STO2 inputs through separate conductors.
• The force-guided relay's normally-closed mirror contact is wired back to a G9SP input for EDM (External Device Monitoring) to confirm the relay has not welded.

Selection Criteria for the Relay

Parameter	Requirement
Contact type	Force-guided / mirror contact per EN 50205
Safety rating	Must meet or exceed the safety level claimed for the overall subsystem (e.g., SIL 3 / PL e / Cat 4)
Coil voltage	24 VDC to match G9SP output
Contact rating	Must switch the STO input current at the applied voltage with margin
Response time	Add relay response time to overall safety stop time when calculating the safety distance

Common force-guided relay families used in this role include Omron G7SA, G7S, and G6S series, Phoenix Contact PLC-OSC series, and Siemens SIRIUS 3SK series. Verify the exact catalog number against the application requirements and the safety subsystem calculation.

Test Pulse Considerations

The G9SP performs periodic test pulses on its solid-state outputs to detect short circuits to 24 V, short circuits to 0 V, and cross-channel faults. A receiving device that interprets the test pulse as a stop command will cause nuisance trips. Drives are particularly sensitive to this because an STO input drop is treated as a stop demand.

The ABB ACS850 STO input is designed to tolerate the short test pulses produced by standard safety controllers. However, the engineer must:

• Verify in the G9SP Configurator that the configured test-pulse width is within the ACS850 STO tolerance window.
• If the drive nuisance trips during commissioning, switch to the force-guided relay method where the relay coil integrates the test pulses and the dry contacts present a stable signal to the drive.
• Document the chosen pulse width in the safety validation report.

ATEX and Motor Heating Considerations

For motors installed in potentially explosive atmospheres, the STO function alone may not satisfy the requirement for safe disconnection. The ATEX-certified safe disconnection function for ACS850 drives application guide documents that the STO function prevents torque-producing current but does not isolate the drive from the intermediate DC bus; in a short-circuit fault at the drive output, the DC link can continue to feed the motor and cause heating.

For ATEX applications, additional measures are required:

1. Use an ATEX-certified motor with thermal protection integrated into the safety chain.
2. Implement a certified line contactor upstream of the drive or an ATEX-certified output filter that isolates the motor from the DC bus in the safe state.
3. Follow the exact wiring diagram from the ABB ATEX guide; do not improvise.

Commissioning and Verification

After wiring is complete, execute the following verification steps before placing the machine into production:

1. With the drive disabled, force the G9SP safety outputs OFF and measure 0 VDC at the ACS850 STO1 and STO2 terminals.
2. Enable the G9SP safety outputs and measure the configured 24 VDC at both STO terminals.
3. Run the drive at low speed, then trigger the safety stop from the G9SP. Confirm the drive removes torque within the time specified in the safety calculation.
4. Force a single-channel fault (e.g., disconnect only STO1) and confirm the drive enters a safe state and reports a fault per the configuration.
5. If the EDM relay is used, force a welded contact condition by simulating the fault and confirm the G9SP detects it and prevents restart.
6. Verify the safety stop time (input drop to motor torque removal) is included in the safety distance calculation per EN ISO 13855.
7. Sign and date the safety validation report with the wiring diagrams, the G9SP project file checksum, and the drive parameter list.

Troubleshooting Matrix

Symptom	Likely Cause	Corrective Action
Drive trips into STO fault on every power-up	24 VDC not present at STO inputs at the moment the drive boots	Verify G9SP outputs are enabled before drive run command; sequence the start chain in the G9SP logic
Drive trips intermittently during normal operation	G9SP test pulse width exceeds ACS850 STO tolerance, or cable noise	Reduce test pulse width in G9SP Configurator; switch to force-guided relay; check shielding and grounding
Safety stop does not remove torque	STO wiring reversed (single channel only), or drive parameter disables STO	Verify both STO1 and STO2 are wired and driven; confirm the STO function is enabled in the drive parameter set
G9SP reports output fault on safety stop	Short circuit on the cable or at the drive terminal	Disconnect drive and measure cable; replace damaged cable; confirm correct terminal assignment
EDM loop does not detect welded contact	Mirror contact not wired back to G9SP input, or wrong NC contact used	Wire the force-guided NC mirror contact to a dedicated G9SP EDM input and configure the EDM logic block

Decision Path: Relay or No Relay

The default recommendation, supported by both the ABB STO application guide and standard safety engineering practice, is direct wiring when the G9SP solid-state outputs meet the electrical requirements of the ACS850 STO input. Insert a force-guided safety relay only when one of the following applies:

• The receiving drive is not on the ABB compatibility list and is known to misbehave with test pulses.
• The safety chain must be expanded into multiple downstream actuators sharing one output channel.
• Dry contacts are required by the cabinet specification.
• EDM is required and the drive does not provide an integrated EDM loop.

When in doubt, use the relay. The added cost and response time are small compared to the cost of an undetected STO fault.

Additional Resources

For the authoritative electrical interface, STO claim limits (SIL, PL, PFHd), ATEX wiring schemes, and parameter assignments, refer to:

• ABB Safe torque off function for ACSM1, ACS850 and ACQ810 – Application Guide
• ABB ATEX-certified Safe Disconnection Function for ACS850 Drives
• ABB Functional Safety Options for Drives