How Nurse Call Relay Outputs Work

How medical equipment uses dry contacts, relays, and simple state changes to signal nurse call systems

Many medical devices can send an alarm or status signal to a nurse call system.

Published August 28, 2026 · Revised September 6, 2026

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What This Page Explains

This page covers:

The Simple Version

When the medical device detects a qualifying alarm, an internal relay changes the state of isolated contacts. The nurse-call system senses that open-to-closed or closed-to-open transition and generates its own room or central notification. The interface may carry only a generic state—not the alarm text, priority, or patient data shown on the device.

Troubleshooting must separate alarm generation, relay operation, cable and pinout, wall interface, nurse-call input programming, and downstream notification. A working local alarm does not prove the relay changed, and a changing relay does not prove the hospital system received it.

Worked Example: Local Alarm Works but Nurse Call Does Not

Generate the manufacturer-approved test alarm and measure or observe the relay state at the device output using the correct pinout and electrical limits. If the contact does not change, check configuration, alarm eligibility, delay, connector, and relay circuit. If it changes, move the boundary to the approved cable and nurse-call input.

Coordinate with the nurse-call owner before testing live infrastructure and prevent nuisance clinical alarms. After repair, verify activation, clearing, priority behavior, delays, and failure response at both the medical device and receiving station.

What Is a Relay?

A relay is an electrically controlled switch.

It has two basic sides:

Control side

and:

Contact side.

The device controls the relay internally.

The contacts connect to the external nurse call system.

Why Use a Relay?

A relay allows two systems to interact without directly tying their internal electronics together.

This can provide useful electrical isolation.

Dry Contact

A:

Dry contact

means the relay contact itself does not supply voltage.

It simply acts like a switch.

The external system provides whatever sensing voltage or current it needs.

Dry Contact Does Not Mean No Electricity Is Present

This is an important distinction.

The medical device may not supply voltage on the contact.

But the nurse call system may apply voltage across it.

So do not assume:

Dry contact = electrically dead.

Normally Open Contact

A:

Normally Open, or NO,

contact is open in its normal state.

When the relay activates:

The contact closes.

Simple NO Example

Normal:

Open circuit.

Alarm:

Closed circuit.

The nurse call system interprets closure as:

Call active.

Normally Closed Contact

A:

Normally Closed, or NC,

contact is closed in its normal state.

When the relay activates:

The circuit opens.

Why Use Normally Closed?

A normally closed circuit can sometimes provide fail-safe behavior.

If the cable breaks:

Circuit opens.

The receiving system may recognize that as an abnormal condition.

The exact nurse call design varies.

Common Terminal

A relay may provide:

COM means:

Common.

The common terminal switches between the NO and NC contacts.

Relay at Rest

At rest:

COM ↔ NC usually connected.

COM ↔ NO usually open.

Relay Activated

When energized:

COM ↔ NC opens.

COM ↔ NO closes.

That is the basic relay behavior.

Medical Device Alarm Output

The medical device decides when to activate the relay.

Possible triggers include:

This is configurable on some equipment.

Not Every Device Sends Every Alarm

A nurse call complaint may actually be a configuration issue.

Example:

Low-priority technical alarms are not configured to trigger nurse call.

The system may be working exactly as designed.

Check the Intended Alarm Behavior

Before troubleshooting hardware, ask:

Which alarm is supposed to activate the nurse call relay?

Then test that exact condition.

Alarm Exists Locally but Not at Nurse Call

This is a common scenario.

The device:

but no nurse call appears.

Now separate:

Alarm generation

from:

Relay output.

Local Alarm Proves Only the Device Alarmed

It does not prove:

Relay Status

Some devices show internal status such as:

That is useful evidence.

But it still does not independently prove the physical contacts changed.

Multimeter Testing

A multimeter can often verify relay contact behavior if the manufacturer permits it.

Use:

with the interface disconnected from external voltage as appropriate.

Example

Normal condition:

COM to NO:

Open.

Generate alarm:

COM to NO:

Near zero resistance.

The relay contact changed correctly.

What Did That Prove?

You proved:

The device's physical relay contact closed during the tested alarm condition.

That is strong evidence the medical-device output is working.

Device Relay Works, Nurse Call Still Fails

Now move downstream.

Check:

Cable

A nurse call cable may have:

depending on the system.

Cable Continuity

Verify the appropriate conductor paths.

But again:

Static continuity does not prove the cable remains good under movement.

Intermittent Cable

A cable may:

Flex testing can help reproduce the failure.

Connector Pinout

The device connector may expose several pins.

Possible functions include:

Do not guess.

Use manufacturer pinout documentation.

Wrong Pin Pair

If the cable is wired to:

COM and NC

but nurse call expects:

COM and NO,

behavior may appear reversed.

Reversed Behavior

Possible symptom:

Nurse call active when device is normal.

Clears when device alarms.

That suggests the wrong contact pair may be used.

Nurse Call Input

The hospital nurse call system has an input that detects the relay state.

That input may apply:

to determine whether the circuit is open or closed.

Medical Device Usually Does Not Control the Nurse Call Voltage

It only changes the switch state.

This is why understanding the boundary matters.

Device Side vs Facility Side

Think:

Medical Device Relay

↓

Interface Cable

↓

Wall/Room Interface

↓

Nurse Call System

A failure can occur at any layer.

Failure Follows Device

Device A:

Does not trigger nurse call in Room 1.

Move to known-good Room 2:

Still fails.

Device B:

Works in both rooms.

Failure follows Device A.

Failure Follows Room

Device A:

Works in Room 1.

Fails in Room 2.

Device B:

Also fails in Room 2.

Failure follows room/infrastructure.

Failure Follows Cable

Original cable:

Fails.

Known-good cable:

Works.

Original cable fails with another device.

Failure follows cable.

Use Cross-Testing

This is often the fastest way to isolate nurse call issues.

Swap only one variable at a time.

Interface Adapter

Some systems use an adapter between:

The adapter may convert connector types or electrical behavior.

That adapter is part of the troubleshooting chain.

Proprietary Nurse Call Interfaces

Not every medical device uses a simple bare relay contact directly.

Some systems use:

Always confirm the actual architecture.

Relay Contact Ratings

Relay contacts have maximum allowable:

The receiving nurse call circuit must stay within those ratings.

Do Not Apply Arbitrary Voltage

Never inject voltage into nurse call contacts unless the manufacturer procedure specifically calls for it.

You can damage:

Isolation

Relay isolation helps protect one system from the other.

That matters because medical equipment and facility systems may have different electrical references.

Ground Is Not Necessarily Shared

A dry-contact interface does not require the two systems to share a signal ground.

That is one of its advantages.

Solid-State Relay

Some devices may use an electronic equivalent instead of an electromechanical relay.

The external behavior may still look like:

but internal operation differs.

No Audible Click

A relay output may work without an audible click.

Do not diagnose based on sound.

Audible Click Is Not Proof

Even with a mechanical relay:

Click means the armature moved.

It does not prove the contacts conducted properly.

Measure them.

Burned Relay Contacts

Contacts can become:

They may mechanically switch but show excessive resistance.

High Contact Resistance

Nurse call input may not recognize a closure if the relay contact resistance is too high.

Intermittent Contact

A worn relay may work sometimes and fail other times.

Repeated cycling during controlled testing may reveal it.

Relay Driver

The relay coil or solid-state relay is controlled by electronics inside the medical device.

If the alarm occurs but relay never actuates:

Possible causes include:

Control Signal vs Contact Output

You can separate:

Is the relay being commanded?

from:

Do the contacts actually change?

That is useful board-level reasoning.

Nurse Call Enable Setting

Some devices have a menu option such as:

If disabled:

No relay output occurs.

Configuration Lost

After:

configuration may return to default.

A previously working nurse call interface may stop because the setting changed.

Alarm Priority Configuration

Some devices let you choose which alarm levels trigger the relay.

Possible example:

High only

versus:

High + medium.

Test the Correct Alarm

If you generate a low-priority technical alarm but the relay is configured for high-priority alarms only:

No output is expected.

Alarm Delay

The nurse call relay may not activate instantly.

Some systems intentionally delay or filter alarm forwarding.

Know the device behavior.

Latching Behavior

The relay may:

depending on design.

Momentary Output

Some nurse call systems expect a brief closure rather than continuous state.

Do not assume every output is maintained.

Normally Energized Relay

Some fail-safe designs keep the relay energized during normal operation.

If device loses power:

Relay changes state.

This can allow loss of equipment power to create a nurse call condition.

Power-Off Behavior Matters

Check what the relay should do when:

according to documentation.

Alarm Silence

Silencing the local alarm may or may not clear the nurse call relay.

The exact behavior varies.

Alarm Reset

Some systems require the alarm condition itself to clear before the relay resets.

Nurse Call vs Central Monitoring

These are separate systems.

A patient monitor may communicate with a central station over:

while nurse call uses:

Both can exist simultaneously.

Central Works, Nurse Call Fails

That tells you:

Network communication works.

It says nothing about relay output.

Nurse Call Works, Central Fails

Now the relay interface works.

The network problem is separate.

Nurse Call Is Usually Low Information

A simple relay often cannot tell the nurse call system:

It may only say:

Attention needed.

Device Integration Is Different

Modern digital integrations can transmit much richer data.

Do not confuse those systems with simple relay nurse call.

Bed Exit Nurse Call

Hospital beds often use relay outputs for events such as:

The same contact-closure principles can apply.

Ventilator Nurse Call

A ventilator may provide remote alarm relay output.

Again, the nurse call system may only know:

Ventilator alarm active.

Infusion Pump Nurse Call

Some pumps may support external alarm interfaces depending on model and system.

The architecture can vary substantially.

Real-World Example: Device Alarm but No Nurse Call

Device:

High-priority alarm active.

Relay tested at connector:

COM-NO closes correctly.

Known-good cable:

Still no room call.

Facility nurse call input found disabled.

Medical device is functioning correctly.

Real-World Example: Failure Follows Cable

Relay output:

Works.

Room input:

Works with another device.

Original nurse call cable fails continuity intermittently near strain relief.

Cable is the failure.

Real-World Example: Reversed Nurse Call

Room call active during normal operation.

Clears during alarm.

Cable wired to:

COM-NC

instead of required:

COM-NO.

Real-World Example: Configuration Lost

Monitor main board replaced.

Local alarms normal.

Nurse call no longer triggers.

Remote alarm output found disabled in configuration after replacement.

No relay repair required.

Real-World Example: Relay Clicks but No Output

Alarm generated.

Relay audibly clicks.

Continuity across COM-NO remains open.

Relay contacts failed.

The click did not prove the output worked.

Real-World Example: Problem Follows Room

Ventilator triggers nurse call normally in Room 1.

Fails in Room 2.

Second known-good ventilator also fails in Room 2.

Failure is downstream infrastructure.

Common Mistakes

Assuming a Local Alarm Proves Nurse Call Output Worked

It only proves the local alarm system activated.

Assuming a Relay Click Proves Contacts Closed

Measure the contacts.

Ignoring Configuration

The output may simply be disabled.

Guessing Connector Pins

Use the manufacturer pinout.

Applying External Voltage to a Dry Contact Without Documentation

Dry contact does not mean any voltage is safe.

Replacing the Medical Device Before Testing the Room or Cable

Follow the failure.

Confusing Nurse Call Relay With Networked Central Monitoring

They are different systems.

A Useful Troubleshooting Framework

For a nurse call problem, ask:

Does the medical device generate the expected local alarm?

Then:

Is nurse call output enabled and configured for that alarm?

Then:

Do the relay contacts physically change state?

Then:

Does the interface cable pass the signal?

Then:

Does the room nurse call input detect a known-good contact closure?

That separates:

Another Useful Question

Ask:

Where does the contact change stop?

That is the core of relay-interface troubleshooting.

What Did You Actually Prove?

If the medical device displays:

Nurse Call Active

you proved:

The device believes the remote alarm condition is active.

You did not prove:

The external relay contacts actually changed.

If your meter confirms:

COM-NO changes from open to closed during the required alarm condition,

you proved:

The device's physical relay output worked during that test.

If the nurse call still does not activate:

Move downstream.

Do not keep troubleshooting a relay you already proved works.

Final Thoughts for Biomeds

Nurse call relay interfaces are usually easiest to understand as a simple chain:

Medical Device Alarm → Relay → Cable → Nurse Call Input.

Do not make the system more complicated than it is.

First confirm:

Did the device actually alarm?

Then:

Did the relay change state?

Then:

Did that state reach the nurse call system?

Use the meter.

Use a known-good cable.

Use another room or device when possible.

And remember:

The relay usually is not sending detailed information.

It is often doing something much simpler:

Opening or closing a switch.

Once you identify where that switch action disappears, the nurse call problem becomes much easier to isolate.

And as always:

What did you actually prove?

— Jake

Important Note

Nurse call interfaces, connector pinouts, relay contact ratings, alarm-priority behavior, normally open/closed logic, isolation methods, and facility nurse call architectures vary by medical-device and nurse-call manufacturer. Follow current manufacturer service documentation and facility procedures, do not apply undocumented voltage to relay contacts, and verify all required local alarms and remote nurse call behavior after service.

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