Douglas Krantz - Technical Writer - Describing How It Works

What's that Normally Open 24-Volt Wire for?

By Douglas Krantz | Maintenance

What's that Normally Open 24-Volt Wire for?


What's that Normally Open 24-Volt Wire for?


Greetings Douglas,

I am performing an inspection in a 2-story house turned into 3 small retail units.

The fire alarm panel has a monitoring alarm relay but there is no alarm monitoring information or security system anywhere. The building owner does not know if the building is monitored as he is new to the building.

How can I measure if this relay is active or changes state? The common and 24v aux power are jumped together, and the normally open wire goes out into the field. Could this be a regular relay in the building or in fact an alarm relay to a monitoring company.

Thank You, DM

Relay contacts are Normally Open or NO, Normally Closed or NC. and Common or C. An electromagnet throws the switch.



See "How does a Relay Work" at: How Does a Relay Work?

A relay is a switch. A switch has contacts that, when closed, short wires together. In your picture, the alarm relay shorts the blue wires together that are on the screw terminals labeled "ALARM RELAY C" to the blue wires on the screw terminals labeled "ALARM RELAY NO".

The relays, themselves, are in the black boxes below the terminal strip labeled "ZETTLER".

The blue wire carries positive 24 volts that is switched on and off by the alarm relay. The brown wire is the ground-return wire for the circuit.



When the panel goes into alarm, the ALARM RELAY in activated, providing the control panel's "24 V" to the blue wire leaving the panel.

There is a ground return for the circuit, it's the brown wire attached to the screw terminal labeled "COM (-)".

There is a high probability that the 24 volts provided on the blue wire is to activate something in the building when there is an alarm. I'd have to be on site, and follow wires to find out what is being activated when there is an alarm.

Measuring the Relay

Use your voltmeter to determine if the relay activates when the panel goes into alarm. Your voltmeter has two probes. For all measurements, the black probe of the meter should be on the "COM (-)" screw terminal that has a brown wire attached. That is negative point of the panel's power supply; that is your minus-voltage measuring point.

  1. When the panel isn't in alarm, use the red probe and measure the voltage at the "ALARM RELAY C" screw terminal. That voltage should be the same as "24 V" screw terminal.

  2. When the panel still isn't in alarm, use the red probe and measure the voltage at the "ALARM RELAY NO" screw terminal. This is the voltage that is normally sent out of the panel on the blue wire.

  3. Set the panel into alarm. Use the red probe and measure the voltage again at the "ALARM RELAY NO" screw terminal. Inside the panel, the relay contacts short the "ALARM RELAY C" screw terminal to the "ALARM RELAY NO" screw terminal.

If, when the panel goes into alarm, the NO terminal change from zero volts to 24 volts, and, when the panel is returned to a normal condition, the NO terminal changes back to zero volts, the relay is good.

Whatever the 24 volts is operating has to be checked out separately.

Probably Not Monitored

When trying to determine if the fire alarm system is outside-monitored by a monitoring company, ask the owner if the monitoring is being paid for. If the monitoring is not being paid for, there is no monitoring.

Many times, older systems are not monitored. Monitoring costs money, and the requirements for monitoring were more lax back then. Also, almost every time the system is being annually-tested, whoever does the testing will write down the account number phone number for the monitoring company. That's just so anyone else testing the system next year doesn't have to look up those numbers.

In the photograph you sent, I don't see any wires for a properly wired monitoring system. That's another indication that there probably is not an outside monitoring company.

As a very last resort and just to make sure, on their non-emergency number, call the local fire department to let them know exactly what you are doing, and that they should not send the trucks. Then put the panel into alarm and do all your other testing.

Afterwards, call the fire department non-emergency number again. Ask them if they received a call from a monitoring company to dispatch the trucks. If they didn't receive a call, the system is not monitored.

Don't forget to thank them.

Douglas Krantz


Mr. Krantz

Greetings Douglas

Thank you for this amazing description, Douglas. I appreciate you getting back to me. I will do the inspection and will let you know how it goes.

Thank You, DM




facpdoug@douglaskrantz.com
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