Relays and Contact Closures in Plain English

Published August 12, 2026 · Revised September 6, 2026

How simple switches and relay outputs control nurse call, alarms, motors, and other medical equipment functions

Relays show up everywhere in medical equipment.

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What This Page Explains

This page covers:

The Simple Version

A relay has a control side and a contact side. Energizing the coil—or commanding a solid-state equivalent—changes the contact state, allowing a small control signal to switch another circuit. The two sides may be electrically isolated, which is why relays are common in alarm outputs, nurse-call interfaces, pumps, heaters, valves, and power-control circuits.

“Normally open” and “normally closed” describe the contact state when the relay is not energized, not necessarily the device's normal clinical condition. Troubleshoot by confirming the expected state, coil command, contact rating, wiring, and voltage on each side. A continuity check on an unplugged relay may find welded or open contacts, but intermittent resistance and failures under load can require the manufacturer's functional test.

Think of a Relay Like a Light Switch

A normal wall switch is controlled by your hand.

A relay is controlled electrically.

When the relay receives the correct control signal, an internal mechanism changes the position of its contacts.

Those contacts can then:

That is why relays are so useful.

The Coil

Many electromechanical relays contain a coil.

When voltage is applied to the coil, it creates a magnetic field.

That magnetic field moves the internal switch.

You may hear the relay:

Click.

When coil power is removed, the relay returns to its normal position.

Contacts

The contacts are the switching side of the relay.

Common labels include:

These labels are important.

COM

COM means:

Common

This is the contact that switches between the other positions.

NO

NO means:

Normally Open

“Normally” means the relay is not energized.

With the relay inactive:

COM and NO are not connected.

When the relay activates:

COM and NO connect.

So the circuit goes from:

Open

to:

Closed

NC

NC means:

Normally Closed

With the relay inactive:

COM and NC are connected.

When the relay activates:

That connection opens.

So the circuit goes from:

Closed

to:

Open

What “Normal” Actually Means

This causes confusion.

Normal does not mean:

Normal clinical operation.

It generally means:

The relay coil is not energized.

That distinction matters.

A device may intentionally energize a relay during normal operation and release it during an alarm.

Always check the manufacturer's design.

Contact Closure

You will hear this term constantly.

A contact closure simply means two electrical contacts become connected.

For example:

A ventilator goes into high-priority alarm.

Its nurse call relay closes COM to NO.

The nurse call system detects that closed circuit.

Then the nurse call system generates a notification.

The ventilator may not be sending any complicated data.

It may simply be:

closing a switch.

Dry Contact

You may hear the term:

dry contact

This usually means the relay contact itself does not provide the external circuit's operating voltage.

It simply opens or closes a path.

The receiving system provides whatever electrical signal is needed.

This is common in alarm interfaces.

Why Relays Are Useful

Relays can provide electrical separation between circuits.

For example:

A medical device has its own internal electronics.

A nurse call system has its own electrical system.

The device can use relay contacts to signal nurse call without directly connecting the two systems' internal power supplies.

That can simplify interfacing and provide isolation.

Relay Outputs Are Everywhere

Examples include:

The exact design varies, but the logic is often similar.

Testing a Relay Contact

Suppose the manual says:

Normal:

COM to NO = open.

Alarm:

COM to NO = closed.

With the device in the appropriate safe test condition, you can use a multimeter to verify whether that change occurs.

Normal state

Measure continuity:

COM to NO.

Expected:

Open.

Alarm state

Generate the appropriate alarm.

Measure again.

Expected:

Continuity.

If the contact never changes, the relay circuit deserves investigation.

Test the Contact, Not Just the Sound

A common mistake is:

I heard the relay click, so the relay is good.

Not necessarily.

The coil may activate and physically move.

But the contacts may still be:

A click proves movement occurred.

It does not prove the electrical contact is good.

High-Resistance Relay Contacts

Relay contacts can degrade.

Instead of a good closed contact measuring nearly zero resistance, it may measure much higher.

The circuit may:

Continuity mode might still beep.

Actual resistance may tell you more.

Relay Contacts Can Weld Closed

If excessive current passes through contacts, they can sometimes weld together.

Then the relay may remain electrically closed even after the control signal disappears.

Symptoms may include:

This can become a serious safety problem depending on the circuit.

Relay Contacts Can Fail Open

The opposite can happen.

The coil clicks.

The device appears to command the relay.

But the contacts never complete the circuit.

Possible symptom:

Ventilator alarms locally.

Nurse call never activates.

If the output relay is supposed to close during alarm but remains open, you have narrowed the failure.

The Coil Can Fail

The coil itself may fail.

Possible signs:

If manufacturer documentation allows, measuring coil resistance may help identify an open coil.

No Control Voltage

The relay may be perfectly good.

If the coil never receives the control signal, it cannot activate.

Possible causes include:

Again:

Do not replace the relay until you know whether it is being told to activate.

Relay Driver Circuits

A microprocessor usually cannot directly drive a large relay coil.

There may be an intermediate driver circuit.

A simplified path:

Processor

↓

Driver transistor

↓

Relay coil

↓

Relay contacts

If the processor commands the output but the driver fails, the relay never moves.

Understanding the signal path helps avoid replacing the wrong component.

Relays and Nurse Call

This is probably one of the most common biomed examples.

Device generates alarm.

Internal software decides the alarm should trigger external notification.

Relay changes state.

Nurse call cable carries that contact state.

Wall system detects it.

If nurse call fails, isolate:

  1. Did the medical device alarm?
  2. Did the relay output change?
  3. Is the cable good?
  4. Did the wall system detect the signal?

That is much better than immediately blaming the nurse call jack.

Normally Open vs Normally Closed Nurse Call

The receiving system may expect either behavior.

Normally Open

Normal:

Circuit open.

Alarm:

Circuit closes.

Normally Closed

Normal:

Circuit closed.

Alarm:

Circuit opens.

If the device and nurse call system are configured differently, you may get:

Configuration matters.

Fail-Safe Design

Sometimes normally closed circuits are used because a broken cable creates the same electrical condition as an alarm or fault.

For example:

Normal:

Circuit closed.

Cable breaks:

Circuit opens.

System notices.

That can make failures more visible.

But do not assume every NC circuit is automatically supervised or fail-safe.

The receiving system has to be designed to recognize that condition.

Relays and Motors

A device may use a relay to switch power to:

Suppose a compressor never starts.

Possible causes:

Do not immediately replace the compressor.

Find out whether power is actually being switched to it.

Relays and Solenoids

Solenoid valves are also commonly controlled by relays or similar switching devices.

Example:

Control system commands valve open.

Relay activates.

Voltage reaches solenoid.

Valve moves.

If the valve does not move, determine where the sequence stops.

Mechanical Relays vs Solid-State Switching

Not every relay-like output uses a mechanical clicking relay.

Some systems use:

There may be no click.

The troubleshooting principle is still similar:

Did the control signal produce the expected output change?

Intermittent Relay Problems

Relays can fail intermittently.

Possible causes:

A device may work when cold and fail later.

Or work when stationary and fail during movement.

If the complaint is intermittent, reproduce the relevant conditions.

Burned Relay Contacts

Switching current causes small electrical arcs at relay contacts.

Over time, this can damage them.

Signs may include:

Relays switching motors and inductive loads can experience significant stress.

Real-World Example: Nurse Call Does Not Activate

Patient monitor generates high-priority alarm normally.

Speaker works.

Visual alarm works.

Measure relay output:

Normal:

COM to NO = open.

Alarm:

COM to NO = still open.

The problem is now much more specific.

The device is detecting the alarm correctly.

The external relay output is not changing state.

Real-World Example: Nurse Call Stays Active

Alarm clears locally.

Nurse call remains active.

Measure relay:

COM to NO remains closed after alarm clears.

Possible causes include:

Now test which side retains the signal.

Real-World Example: Compressor Does Not Run

Device commands compressor.

Relay clicks.

Voltage measured before relay:

24 VDC.

Voltage after relay while activated:

0 VDC.

That strongly suggests the relay contacts are not passing power.

Hearing the click was not enough.

Real-World Example: Relay Never Clicks

Device should activate valve.

Relay remains silent.

Measure coil command:

No voltage.

Now the relay itself may not be the problem.

Look upstream toward the control circuit.

Common Mistakes

Assuming Click Means Good

Verify the contacts.

Replacing Relay Without Checking Control Signal

It may never be receiving the command.

Confusing NO and NC

Read the diagram.

Assuming Contact Closure Supplies Voltage

A dry contact may only act as a switch.

Testing Only Continuity

High resistance may matter.

Ignoring Configuration

Software may determine relay behavior.

A Useful Troubleshooting Framework

For a relay-controlled function, ask:

Should the relay be active right now?

Then:

Is the relay receiving the command?

Then:

Does the relay physically change state?

Then:

Do the contacts electrically change state?

Then:

Does the downstream device respond?

That breaks the problem into clear steps.

What Did You Actually Prove?

If you hear:

Click.

You proved:

Something mechanically moved.

If you measure:

COM to NO changes from open to 0.2 Ω.

You proved:

The contact electrically closed under those conditions.

Those are different levels of evidence.

Final Thoughts for Biomeds

Relays are not complicated once you stop thinking of them as mysterious electrical components.

They are switches.

Something tells the relay to change state.

The contacts open or close.

That change controls something else.

When troubleshooting, follow the sequence.

Control signal.

Relay.

Contacts.

Load.

Do not jump straight to the component at the end of the chain.

Sometimes the ventilator is fine and the nurse call relay is bad.

Sometimes the relay is fine and the cable is bad.

Sometimes the relay never received a command at all.

Find where the expected state stops changing.

That is the troubleshooting path.

— Jake

Important Note

Relay circuits may switch hazardous voltages, currents, motors, heaters, alarm outputs, and other safety-critical functions. Follow manufacturer service procedures, facility safety requirements, approved test methods, and your authorized service scope. Do not bypass interlocks, alarms, or protective relay functions.

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